Skip to content

# Weed Control in Turf: A Review of the Basics and Recent Updates

## GTBOP Green & Commercial — November 17, 2017

Speaker: Dr. Patrick McCullough, Weed Scientist, University of Georgia Moderator: N/A (pre-recorded presentation) Duration: 1:10:00

## Introduction

Patrick McCullough: Good morning, everybody. This is Patrick McCullough. I have pre-recorded this for the meeting. Sorry I could not be with you today. I'm recording this on Friday, November 10th. And this presentation is going to cover weed control topics in turfgrass management. We're going to review the basics and also get some recent updates on some current trends and some new products that are coming out in the turfgrass industry for pre- and post-emergent weed control.

## Weed Identification Fundamentals

### Identification Resources

Just to start off, going over some of the basics here. Books for weed identification are very important for turfgrass managers to have. We recommend two specific books for turfgrass managers to have to help identify weeds in their turf. One is the Color Atlas of Turfgrass Weeds. This is a hardback book published by the Golf Course Superintendents Association of America. The other is Weeds of Southern Turfgrass. This is a UGA publication that you can get to the Athens bookstore. Both of these books are available online. You can buy them through the Amazon site, eBay, and various other online sites and vendors. But weed identification is very critical. We need to first identify the weed species before we can select appropriate control options. And having a good book to reference can help you key out a weed species as you see new species emerging in your turf.

### Weed Categories

Weed identification books are typically broken down into two main categories. The first is grassy weeds versus broadleaf weeds. Weeds are typically classified as either grasses or broadleaf weeds, monocots or dicots. There's also a third category that we often find in weed identification books, sort of a miscellaneous weed species section, and this is grass-like weeds. This includes everything from sedges, wild garlic, wild onion, plants like Star of Bethlehem. These plants typically don't fall under the category of grasses or broadleaf weeds, but they are in that third category of grass-like weeds.

### Identification by Seed Heads

Some of the key characteristics that can help you identify weed species, starting with seed heads. This is usually the first characteristic that I look for on a weed sample when we are trying to identify the species. Most grassy plants have a very distinct seed head that is indicative of a certain species. You can see here on this slide where we've got various seed heads on the top left there. That is dallisgrass where it's a group of alternating spikes on the seed head structure. The bottom left you can see crowfootgrass where the spikes join together at the main point at the end of the seed head stem there. So these are all very good characteristics that can help you quickly identify a weed species.

Typically in turfgrass we don't always have seed heads present, especially during the growing season when we're constantly mowing off shoot growth and the seed head formation. But this is typically the best characteristic to quickly key out a weed species that you may have in your turf. Here's a good example looking at two Paspalum species here. You can see they have a very similar seed head spike there between bahiagrass and dallisgrass. These are two warm season perennial weedy species that have very different selective control options. We can get very good control of bahiagrass using herbicides like metsulfuron and various warm season species. Whereas dallisgrass, we need to have very specific application programs and certain turfgrass species. So getting the identification of those species keyed out can be critical. You can see bahiagrass has a B-shaped seed head where the spikes join at the base of the seed head versus dallisgrass that again has the alternating spike. So if you don't have that seed head structure present, it may be difficult to key out those two weed species in your turf. So a good example there where seed heads can help you identify the species of the weed.

Here's a look at the Poa annua seed head. We're going to start seeing a lot more this here in the winter and the springtime. The panicle inflorescence where it's got multiple branches and this is also a great characteristic to determine Poa annua versus some of the other grassy weeds we may have present in turf.

### Identification by Ligules

Grassy weeds also have a very distinct structure on the base of the leaf blade called ligules. This is a structure that is found at the base of the leaf where it joins the stem. Grassy weeds have typically a very distinct ligule that can help you determine the species if you do not have a seed head present. Ligules can be tall, fleshy, white structures. They can be smooth there on the margins. Some weeds like barnyardgrass on the top right of this slide do not have a distinct ligule where that structure is absent from the plant. So if you do not see a ligule, very good chance it could be barnyardgrass in the summertime versus crabgrass that has a very similar appearance but has a fleshy ligule often with a fringe of hairs there at the base of the leaf. So ligules are distinct structures on grassy plants only, broadleaf weeds and sedges. We are not looking for ligules at the base of the leaf, but good characteristic to help identify grassy weed species.

### Identification by Flowers and Leaf Characteristics

Broadleaf weeds often have distinct flowers. They can have colorful showy flowers like you see there on the top left with weeds like oxalis. Other weeds like henbit, purple dead nettle, can have very small flowers but can be very colorful, pink to purplish in color as the plant matures. We can also use flowers to determine one species from another based on the color of the petals on the flower. A good example here are the two day flowers where the species on the left has two blue petals and one white petal versus spreading day flower on the right that's got three blue petals. So good examples there where flowers can help you determine the species on broadleaf weed and the colors and the size of the flower also can be a good characteristic to help you identify a broadleaf weed in your turfgrass.

Broadleaf weeds, we can also take a look at how the leaves are arranged on the stems. Some broadleaf weeds can have sort of the alternate arrangement of leaves there on the stem. Some can also have the opposite arrangement where the leaves sort of join together at the base of the stem of the plant. So something else to consider as you are trying to key out weed species. This is how are the leaves arranged on the stem of the plant.

### Identification by Pubescence

Some weeds have dense hairs all over the leaves and the stems like you see there on the left with sticky chickweed. And then some plants have a few hairs or are smooth like you see with common chickweed on the right. Common chickweed typically has soft hairs on the margin of the leaf versus sticky chickweed, which is generally covered with hairs on the leaves and stems.

Another good example where you can take a look at the hairs on the plant to help key out a grassy weed would be crabgrass. We have various crabgrass species that are found in turfgrass in Georgia. Smooth crabgrass gets its name because it is hairless. Large crabgrass is covered with hairs all over the stems and all over the leaves. And then southern crabgrass typically has hairy stems, hairy stolons, and has smooth leaves. So another good example where the hairs on the plant can help you identify the species of the weed.

### Identification by Leaf Markings

And then of course leaf markings, other characteristics to key out broadleaf weeds. White clover on the left with the white markings on the base of the leaf versus spotted burr clover on the right that has the purplish dot there in the center of the leaf. This is important because we're looking at two different clover species. One is a cool season perennial white clover versus the winter annual spotted burr clover. Looking at characteristics, identifying the plant can help you determine when those weeds will emerge, how they're going to grow, and how we can plan weed control programs around their lifecycle.

## Weed Lifecycles

### Winter and Summer Annuals

Right now we're seeing many winter annual weeds begin to germinate. We're already seeing species like henbits, weeds like annual bluegrass, bittercress, and hop clovers. These are true winter annual weeds that are germinating in the fall. They are starting to grow actively in landscapes and in turfgrass. These plants go to seed in the springtime and then they will complete their life cycle and then die out in the summertime. And this is a good example of the annual life cycle that these weeds will grow for one year and then go to seed and then die out.

The advantage of this life cycle is that it is predictable. We know when annual bluegrass begins to germinate in turfgrasses. We know when crabgrass begins to germinate in the late winter and springtime. And therefore, we can plan management programs around when these weeds begin to emerge in turfgrass. And we can apply pre-emergent herbicides to prevent their establishment based on when these weeds begin to germinate in the soil.

Summer annual weeds include species like goosegrass, crabgrass, foxtail, sandbur. Some of the broadleaf weeds that are true summer annuals include species like spotted spurge, doveweed, purslane, these are all weeds that germinate in the springtime. They resume active growth throughout the summertime. They go to seed in the fall and then they transition out and die out in the wintertime. So they complete their lifecycle during the warm season of the year.

### Simple Perennials

Perennial weeds are much more difficult to control in turfgrass than the annuals because these plants can germinate from seed, but they can also regrow from tap roots and below ground plant parts, stolons, rhizomes, and tubers. Simple perennials can primarily be reproduced by a seed. We can partially control these plants by hand pulling and digging them out of the ground. Pre-emergent herbicides can be effective, but they are often providing erratic levels of control because these plants can also emerge from below ground vegetative structures. So perennials are less predictable on their establishment and their growth. They are much more difficult to control than the annual weeds that we have in turfgrass.

A good example of a simple perennial that is starting to emerge in turf in the fall. Weeds like wild garlic, wild onion, they are emerging from not only seed, but they are also starting to establish from below ground bulbs that stay dormant during the summertime. As temperatures cool down, these plants will then begin to reemerge and wild garlic can be a very troublesome weed and dormant turfgrasses during the wintertime. So typically, pre-emergent herbicides do not control plants that are emerging from below ground vegetative structures, such as below ground bulbs like we see with wild garlic.

### Complex Perennials

Complex perennials are the most difficult weeds to control in turfgrass because they are going to survive multiple years. They primarily reproduce and spread through asexual reproduction, which includes stolons, rhizomes, tubers such as with the sedges. These weeds include species such as white clover, Canada thistle, ground ivy, bermudagrass, yellow nutsedge, purple nutsedge. These are all perennial weeds that are going to survive multiple years in our lawns and landscapes.

Management implications here, we can hand pull and dig some of these plants out of the ground, but it's often not a long-term control strategy. Because these plants can spread laterally, because they can produce runners, those plants can then create daughter plants and trying to dig those plants out of the ground, we often leave behind some of the stems and stolons that are present in a turf. And when they're growing in patches, sometimes the best way to control these species, especially in turf, is to come in with non-selective options such as Roundup and just treating the patch and then treating the area around the patch to ensure that you're getting all the surrounding stems and stolons that may have been created from that main patch of the weed.

A good example here of a complex perennial is bermudagrass. This of course is a major warm season turfgrass species, but if you had Bermuda growing in the middle of centipedegrass, zoysiagrass, or fescue, or various other turf species, it can be a long-term invasive weed species and be very competitive with other turf species. And if it's not controlled early, bermudagrass is going to spread from lateral stems and it will eventually have significant competition that can lead to the need to renovate a lawn because selective control of bermudagrass is very difficult in many warm and cool season turfgrass species.

## Scouting, Early Detection, and Cultural Practices

So it's important to routinely scout your turf. You know, identify weeds that may warrant control, but also note new weed species that may be present. Early detection is very critical, especially with perennial weeds. We want to get on top of these species as soon as possible, get them removed, hand pull them out, treat herbicides if needed, and prevent their spread populations because most of the time if a perennial weed is left uncontrolled it's going to over time spread, reproduce, and create a long-term problem for us. So detecting these weeds early on can be very critical. A good example would be something like purple nutsedge where if you have a few small plants it's important to get those controlled, get them removed because there will be significant reproduction below ground with tuber chains and that weed will continue to spread and be a very severe infestation over time if it's left uncontrolled.

Also, as you are identifying new weed species present, it's important to evaluate turfgrass cultural practices that may need to be adjusted. Oftentimes, when we see weeds that are starting to emerge or new weed species, they are taking advantage of the lack of competition from turfgrass growth. And if we can make adjustments in mowing programs, fertility, modifying how much we irrigate, this all can enhance turfgrass competition to reduce the overall spread and the population of weeds present in our turfgrass.

### Mowing Height and Crabgrass Competition

A good example of a cultural practice that will influence the population of a weed species in a lawn is mowing height on crabgrass. This is very important in tall fescue where during the summertime, tall fescue typically declines due to summer stress and crabgrass becomes very competitive. But making a simple adjustment in the height of cut of a tall fescue lawn can significantly increase the competition of tall fescue with crabgrass in the summertime.

So this is a look at a study that was conducted in North Carolina where they looked at four different mowing heights of tall fescue on the percent cover of crabgrass in that lawn. And as you can see when they raised the mowing height from one to four inches, they cut the crabgrass population down from 95% cover to basically 0%. So as they increased the height of tall fescue, it became more competitive. It was able to shade out crabgrass and they were able to basically prevent the emergence of crabgrass because the fescue was so competitive.

So mowing height and mowing frequency can be very critical. This will affect the competitive growth of turfgrasses, can help cut down on weed populations and which over time can of course help cut down on the need to apply herbicides and various other management inputs. So depending on the species that you're managing, there is an appropriate mower and height of cut and mowing frequency to prevent scalping. So typically we want to remove no more than one-third of the total leaf area with the mowing. And based on the turf species that could be every five to seven days, five to ten days for grass like centipede that doesn't grow quite as quickly as some of the other warm season grasses. So something to consider is just make sure you are mowing at the appropriate height and the appropriate frequency during active growth. And this will just help promote the recovery of a lawn from a mowing operation and should help with promoting competition with weed species in your turf.

### Irrigation and Weed Populations

Weed populations are influenced by irrigation, how much we water, how frequently. Typically weed species thrive in areas that remain wet for extended periods of time. A good example there is weeds like dollarweed. This is a slide that shows the effects of watering programs either daily, conditionally, or as or when the grass showed severe wilt on the x-axis there on percent dollarweed cover. And this was a three-year field study in Florida and you can see there where they watered every single day. They had about five to six fold increase in dollarweed cover compared to when they watered as the grass needed it.

So how much we water will certainly influence the pressure and the growth of weeds like sedges, which thrive in wet soils. White clover likes to have wet feet as well, so poorly drained, high irrigation programs will certainly favor and encourage the growth of those types of weeds in our turf. For weeds in the wintertime, Poa annua likes also to have wet soils. It's going to thrive in poorly drained areas. Making modifications in the frequency of the watering program, improving drainage, and also trying to relieve compaction, trying to promote the health of the grass by core aerification and minimizing compaction can also help reduce weed populations in your turfgrass as well.

### Seed Quality and Planting Material

Here's a picture of a tall fescue lawn that has a significant infestation of broadleaf dock. You would think that looking at this picture, there's a major problem with this weed species in this area. However, as we pull back from this picture, you can see that only that center plot, that center rectangle has broadleaf dock present. Whereas all the other rectangles of different tall fescue cultivars are weed-free. And this is a picture of a tall fescue NTEP trial where that specific seedlot that they seeded had seedlot contamination with broadleaf docks. So they brought that seed in when they planted that particular plot. And this is just a great example of the importance of planting high-quality certified seed so that we're not bringing in new weed species when we plant.

And this is a look at a fescue lawn there where they bought basically the low quality seed and this can be an issue with a lot of the cheap seed that is sold in the big box retailers that may have noxious weeds present or unwanted species such as ryegrass or carpet grass present in the seed bag. So just taking a look at the percent seed, what's in the bag that you're purchasing, and just make sure that you are making a good investment when you are planting turfgrasses, not only from seed but sod as well. Looking at the sod before you purchase it, making sure that there's no weeds present can certainly be important so you're not bringing in weeds when you're planting a new field or new lawn.

## Pre-Emergent Herbicide Science

### How Pre-Emergent Herbicides Work

Pre-emergent herbicides are some of the most important tools that we have to prevent the establishment of weed species and turfgrass. Pre-emergent herbicides are applied prior to weed seed germination. So we need to get these herbicides out in high enough concentration before we see the emergence of winter and summer annual weeds.

Pre-emergent herbicides are applied to the soil. They are tightly bound to the upper half inch or so of the soil profile and they are not readily mobile in the soil. So they are bound and they typically stay put once they finally bind to the soil. Pre-emergent herbicides are concentrated in the upper layer of the soil profile, which is important because that is where the weed seed bank is in the soil. Weeds are going to germinate and the young roots and shoots will absorb that pre-emergent herbicide out of the soil. It will then absorb that product through the soil water solution. And most pre-emergent herbicides are going to tie up cell division. So weed seedlings that germinate first, taking the herbicide will fail to establish a healthy root system and they will die out.

Pre-emergent herbicides do not prevent weed seed germination. So the weeds must first germinate. They must take in the herbicide through the roots and shoots. And that is how we control weeds prior to establishment with the use of a pre-emergent herbicide in a lawn and landscape.

### Effects on Established Turfgrass

Pre-emergent herbicides typically do not inhibit the root growth of well-established turfgrasses. Usually turfgrasses that are mature have a deep and healthy root system that penetrate below the layer or where the presence of that pre-emergent herbicide is in the soil. A lawn that has a three to four inch root depth on it typically will not be affected by pre-emergent herbicides concentrated in the upper half inch of the soil profile.

Where we run into trouble with pre-emergent herbicides and turfgrass rooting is when we have winter kill, when we have thinned out grass, when we have disease, and that grass is trying to re-root into treated areas. When it's spreading a lateral stem or a stolon, and it's trying to tack down a new root on that lateral stem, that's where we see the greatest potential to inhibit turfgrass rooting is when it's trying to produce a new root on a lateral stem when it's trying to peg down in a bare ground situation. But generally speaking, well-established lawns, there's limited to no risk on the health of the root system using pre-emergent herbicides at appropriate label use rates.

### Product Overview and Formulations

We have a wide variety of pre-emergent herbicides that are available to turfgrass managers. They're sold under a wide variety of trade names and they can also be found in various formulations, either sprayable or spreadable formulations. Some of these can be impregnated on a fertilizer with a weed and feed type of application. So some of these herbicides like Prodiamine and Pendimethalin, these are widely used for pre-emergent crabgrass control in the late wintertime and in the spring. We can also use these herbicides in late summer and fall to prevent the establishment of annual grassy weeds. So we can control annual bluegrass with a timely application of those herbicides as well in the fall.

However, pre-emergent herbicides generally do not provide acceptable levels of post-emergent weed control. So once the weed has established, these products generally are not effective for controlling established weeds present in our turf. There are some active ingredients like isoxaben or Gallery which is very strong on broadleaf weeds specifically but a little bit weaker on grassy weeds versus some products like prodiamine which is very strong on grassy weeds but can be weak on broadleaf weeds. So these all have strengths and weaknesses, depending on the weeds that you're targeting with that application.

We are using pre-emergent herbicides to control summer and winter annual weeds only. Again, these are weeds that establish from seed. And pre-emergent herbicides are most effective for controlling weeds that are germinating from seed only. We are not targeting perennial weeds with pre-emergent herbicides. We are also not targeting weeds that are currently present in our turf, generally speaking with most pre-emergent herbicides, because they are not effective once the plant has been established and is healthy and actively growing.

### Herbicide Activation: Sprayable vs. Granular

One of the concerns that we have is we have to get pre-emergent herbicides activated. We need to irrigate behind these treatments to get them off of the shoots of turfgrass and get them activated in the soil so that they will be available for absorption by weed seedlings. So one of the concerns is that if you cannot irrigate and you don't have timely rainfall, going out with a sprayable formulation could lead to potential failures because it's not getting into the soil and it's not getting activated as readily as a dry granule formulation.

We can have herbicide losses with a sprayable formulation where it fails to get to the soil, either through photo degradation or breakdown by sunlight, volatilization where it's lost through a gas form, and then of course clipping collection and traffic. If we actually physically remove that herbicide from the area, of course it's not going to be in high enough concentration to provide effective weed control when the product gets into the soil.

Dry granular formulations, spreadable products are much better if you do not have irrigation or we're in a drought situation. These products can get to the soil much better and they are not moved and there's much less potential for losses using a granular product compared to a sprayable product if you cannot water behind the treatments.

### Spring Application Timing

Typically, we are looking at pre-emergent herbicide applications in March in most parts of the state of Georgia. The application timing is going to depend on soil temperatures. So in the springtime, we typically get our pre-emergent herbicide applications out when soil temperatures reach the low 50s in the upper two inches of the soil profile. And this is going to vary based on where you are in the state. So obviously the southern part of Georgia is going to warm up much earlier than the central and northern part of the state. And these dates are just general guidelines for when we should be targeting getting those applications out for pre-emergent weed control.

A very good website to track local soil temperatures, air temperatures, and growing degree days is georgiaweather.net. GeorgiaWeather.net has many different weather stations scattered throughout the state, and it is a very good resource to get local soil temperatures to help time management inputs such as pre-emergent herbicide applications in the spring and the fall. So georgiaweather.net, you can go in, type in your location and it will give you the closest weather station to your area and a great way to track local growing conditions so that you can more effectively time pre-emergent herbicides in the spring.

### Residual Activity and Product Selection

Pre-emergent herbicides used in turfgrass all have different lengths of residual activity. Some of these herbicides are going to last longer in the soil at labeled use rates compared to products that have a moderate or short activity such as products like Siduron, Tupersan, pendimethalin, dithiopyr. These typically are going to last a few months in the soil based on labeled use rate. Products like prodiamine, oxadiazon at labeled rates are going to last four to six months depending on the conditions, soil temperatures, and factors that are going to influence the residual effects of a herbicide and the degradation of the herbicide in the soil.

But something to have an appreciation for is that there are pre-emergent herbicides that you can use that will provide four to six weeks of residual weed control. And that may be all you need if you need to come in and seed or sod in a treated area. There are some products that will not last quite as long. And then there may be cases such as in lawn care where you want the longest control possible. So going with a Prodiamine treatment may be more appropriate there where you're trying to extend the length of residual control throughout the growing season.

### Fall Pre-Emergent Timing

Winter annual weeds, we typically target getting pre-emergent herbicides out in September in most areas in the state. South Georgia, typically we start looking at pre-emergent herbicide applications around the first week of October. This is when soil temperatures start to dip below 70 degrees. So as we cool down, winter annual weeds begin germination and we need to get our pre-emergent herbicides out before those weeds start to emerge. Weeds like annual bluegrass, henbit, we can see them germinate in mid-September, late September, depending on where you are in the state. So this is just a general reference and a guideline to get fall pre-emergent herbicides out at various locations in the state.

### Split Applications

One of the ways that we can extend the length of pre-emergent weed control is to apply split applications of a pre-emergent herbicide. So instead of putting out all the product at once, we can make multiple applications at a six to eight week interval. A good example here is instead of applying Barricade or Prodiamine to one pound active ingredient per acre, putting that application into half a pound active applied in March and come back around late May or June with another half pound active per acre has shown to extend residual control greater than just putting out all the product at once. We are able to get better control of the late season flushes of crabgrass and other annual weeds by just splitting that application and going with a split program. Helps provide a fresh supply of that herbicide to the soil and can extend the length of control compared to just a single treatment of that total application rate all with one shot.

### Fall Products for Poa annua

Pre-emergent herbicides that we can use this time of year for controlling winter annual weeds such as Poa annua. There are many different products on the market. Many of the herbicides that control crabgrass and goosegrass also can provide pre-emergent control of weeds like annual bluegrass. So crabgrass preventer herbicide can also be used in the fall to control weeds like annual bluegrass. So we have products like prodiamine, pendimethalin. One of the most popular herbicides now in lawn care in Georgia and in golf course turf in parks as well is Specticle. The active ingredient indaziflam is very good, very active on Poa annua. And it also provides a different mode of action to the dinitroanilines that we are using for controlling crabgrass and other weeds with that different mode of action.

## Herbicide Resistance

### The Growing Problem

Some of the concerns that we have right now especially with annual bluegrass is herbicide resistance. We are seeing pictures like this where turf managers are telling us that control is just not what it used to be using the same product year after year. And typically what we're seeing with weeds like annual bluegrass that have received the same herbicide in an area for multiple years is segregation in the population. We are seeing a shift where we are seeing resistant biotypes emerging that are not responding to a herbicide that may have been used exclusively for a certain period of time. And this resistance issue is increasing with annual bluegrass, goosegrass, and other weeds in turfgrass. And something to have an appreciation for is that if you use the same products or the same herbicide mode of action year over year, you can cause a shift in the weed population. We're seeing this right now, especially with annual bluegrass and turfgrass throughout the state of Georgia where we're seeing resistance issues that are increasing in lawns, golf courses, sod farms, and various turfgrass areas.

### Selection Pressure and Target-Site Resistance

Herbicide resistance occurs through selection pressure. This graph shows in year one where all the green plants present are controlled by a certain herbicide. However, that one plant in orange survives that treatment. It is a naturally resistant biotype that does not respond to that specific herbicide. That one plant in year one could be one in a thousand, it could be one in a million, but over time with selection pressure, using the same herbicide over and over, that one plant will spread, it will go to seed. And over time, year two, year three, four, and five, we are shifting that population. And we are giving the opportunity of that resistant biotype to spread, reproduce, and it is not being controlled by the use of the same product over and over. And then by year five, you've got a very serious problem where you are now dealing with a weed population that is resistant to that specific herbicide. And this is something that we are finding in turfgrass throughout the state. We're seeing more and more weeds with this issue that are not responding to a pre or post-emergent herbicide.

Typically what is happening here is resistant weeds have an altered target site where the herbicide simply just does not bind the way it normally does to a susceptible population. So the target site where that herbicide normally binds on the right of this slide, that herbicide is obviously not going to bind properly and therefore it is not controlled. And this is the most common form of herbicide resistance in a weed. It is a naturally occurring trait in that specific biotype. So we are not causing a change in the plant by using a herbicide, but what we are doing is selecting for biotypes that have that mutation present that prevents that herbicide from binding properly to get effective control.

### Poa annua Resistance in Georgia

This is a big problem right now for us in Georgia with Poa annua. Annual bluegrass is one of the most difficult weeds to control, probably the most troublesome weed for us in turfgrass, especially in the wintertime. We are seeing widespread resistance to dinitroaniline pre-emergent herbicides. This includes some of the active ingredients like pendimethalin and prodiamine. Some of the other herbicides that are group three mitotic inhibitors include products like Dimension. This is widely used for crabgrass but it has a similar mode of action to the dinitroanilines and what we're seeing in Georgia is that Poa annua populations that are resistant to pendimethalin and prodiamine are also cross-resistant to Dimension as well. So that is a concern because these are very popular pre-emergent herbicides.

Here's a look at Barricade resistant Poa and some of our field research where we went out with Barricade at the standard timing. This is Prodiamine. And you can see we're getting very significant failures in control with that pre-emergent treatment of Barricade there where it just looks like we sprayed water. There's just no response at all. What we are doing is testing plants to confirm resistance in the field. So what we are doing is growing these plants out hydroponically and exposing them to various concentrations of a pre-emergent herbicide. And what we're typically doing is coming in and cutting the roots off the plants and then sticking them in the tanks that have hydroponic solutions with or without the pre-emergent herbicide present. And if a weed is resistant to dinitroaniline herbicides like Prodiamine, it will grow a nice healthy root system in the presence of that herbicide in the hydroponic tank. And this is what we're seeing where weeds are growing right through pre-emergent herbicides like Prodiamine. We're growing a nice healthy root system there versus the susceptible biotypes on the right that are completely controlled by that treatment, which is showing very susceptible root systems there, growing in the presence of that pre-emergent herbicide.

### Fall Post-Emergent Timing and Sulfonylurea Programs

Fall is a great time of year also to come out with post-emergent herbicides to control annual bluegrass. That is when winter annual weeds, poa, henbit, hop clovers, that's typically when we get the best control. When these plants are at the seedling stage, they are most susceptible. Certain products like Katana, Monument, some of the sulfonylureas can provide very good post-emergent control early in the fall and then it has enough residual to get through peak winter annual weed germination periods. Here's a look at Katana, which is applied in the fall and this picture was taken in March. You can just see the pressure of the annual bluegrass that sort of surrounds that plot there. So this can be a very good treatment to control seedling winter weeds at that fall timing. Typically getting these treatments out around mid-November, sometime around Thanksgiving or so, can get very good post-emergent control of the seedling weeds and get you through that peak germination period that will hold throughout the season.

However, again, we are seeing sulfonylurea resistance like this, where we come in and we're getting just segregation. You come in and you get complete control of some plants and other plants are growing right through those treatments. And this is a widespread problem now in turfgrass throughout the southeast, specifically with sulfonylurea herbicides, triazine herbicides, and the dinitroaniline pre-emergent herbicides as well.

What we're seeing is sort of classic target site resistance with sulfonylurea herbicides with annual bluegrass. Here's a look at a resistant biotype that we tested in the greenhouse on the top versus the local Griffin biotype on the bottom there, susceptible population to Monument. You can see where we started these rate titrations. With the resistant biotype, the standard labeled use rate is half an ounce of product per acre. We went up to about a 300x rate and there was simply just no response there from the plant. So this is a classic target site resistance where no matter how much herbicide you apply to the plant, because there's an altered mutated target site, it is just not going to bind properly and there's no response from the plant.

### Resistance Management: Golf Course Trials

So what we did was looked at some resistance management programs for controlling annual bluegrass last fall. These were done on three different golf courses that had expressed concerns over herbicide resistance issues. And I think this kind of tells a nice story to show some of the plans and programs that you could use in warm season grasses to combat herbicide resistant Poa this fall and winter in your turf.

At these three golf courses we applied Barricade at the standard pre-emergent timing. At golf course one and two, we saw that we had resistance problems where we were not getting good control. At golf course three, we had susceptible Poa, so we had actually very good control at that third golf course. However, when we switched from Barricade to a different mode of action, which is Specticle, we got basically good to excellent control at all three locations. So rotating modes of action, rotating chemistries from a dinitroaniline to a different chemistry, Specticle was a great tool to get that dinitroaniline-resistant Poa control at golf course 1 and 2.

We also found simazine resistance at golf course one and three. However, golf course two did have simazine susceptible Poa annua. We also tested Revolver, which is a sulfonylurea herbicide. At golf course one and three, we had susceptible Poa to that mode of action. However, at golf course two, where simazine was working, we had resistance issues there using Revolver. And what we found was that when we came in with a combination of the Revolver with Simazine, having those two modes of action gave us complete control of Poa at all the golf courses that we tested.

Really the take home message here is that rotating modes of action can be important, but for post-emergent Poa control in Georgia, especially in bermudagrass and zoysiagrass turf, coming in with two modes of action, combining a sulfonylurea herbicide with a triazine herbicide, two different chemistries that offer different modes of action, can help control biotypes with suspected resistance issues to one of those modes of action. So the combination, this is a fairly cost-effective way to help combat resistance issues and help prevent the spread of biotypes with herbicide resistance as well.

### Poa annua Control Programs by Turfgrass Species

So for controlling Poa annua in centipedegrass, mid-November, early December, timings of Katana, which is a sulfonylurea herbicide, can work very well with Simazine. We can also use rimsulfuron, which is TranXit, in non-residential areas with Simazine at a quart per acre. Having that simazine in there is a nice kicker to using sulfonylureas. It is about a $5 per acre treatment and can provide a second mode of action in the mix with a sulfonylurea to help combat resistance. Curb can also be used in non-residential turfgrass at that late November timing. One and a half pounds active ingredient per acre can also help control Poa annua at that timing, and it provides a different mode of action than most other post-emergent herbicides used in turfgrass. And then pre-emergent control, obviously, in September, the first week of October, depending on where you are in the state, dinitroanilines, rotating Specticle as possible in lawns and landscapes is a very good program to prevent the establishment of Poa in turf.

Here's a look at centipedegrass that was treated with the tank mixtures and you can see using simazine with Katana, using it with rimsulfuron, it has shown to be safe at that application time again in late fall. So as the grass is slowing down we can use those combinations safely as long as the centipede is healthy and is not growing under pressure from disease or other stresses. Those tank mixture combinations can be very effective for controlling Poa and can be safe in that species.

For controlling Poa in bermudagrass and zoysiagrass, again sulfonylurea herbicides applied with simazine is a great program. We've got a lot more sulfonylureas that are safe for use in these two turf species, products like Revolver, Monument, Katana, Tribute Total, those are all sulfonylurea, ALS inhibiting herbicides, but having that second mode of action in there with Simazine can make a big difference whether or not you're successful for controlling Poa. Curb also again non-residential sites one and a half pounds active ingredient per acre and then pre-emergent control if possible in the fall is going to set you up for success for controlling Poa in those lawns.

### Sedge Resistance

We're also seeing resistance issues now with sedges. This is a look at Cyperus compressus annual sedge, which is a true summer annual, goes to seed. We are seeing problems with multiple populations found throughout the state of halosulfuron-resistant sedge, and it's also cross-resistant to other sulfonylureas. So rotating modes of action there, coming in with sulfentrazone or Dismiss, having Basagran as a tank mix partner can also help combat resistance issues with ALS resistance sedges that we're seeing in the state.

There is also other sedge species that have confirmed resistance to post-emergent applications of sulfonylureas. Products like halosulfuron or SedgeHammer, we have seen resistance with yellow nutsedge and various other sedge species that we can also find in turfgrass. These have been reported in various cropping systems throughout the southeast.

Ways to combat resistance with sedges, again using Dismiss, which is a different mode of action. Sulfentrazone, 10 to 12 ounces of product per acre in a tank mixture with sulfonylureas can help control resistant biotypes and help delay the spread of resistance in sedge populations. Basagran can also be used. And we do have some pre-emergent herbicides that will control certain sedge species. Products like Ronstar in non-residential turf. Products like Echelon and Dismiss that contain sulfentrazone going out in late springtime with those treatments can provide pre-emergent control of sedges, such as annual sedge, yellow nutsedge, and certain kyllinga species as well. And then we do have other pre-emergent herbicides that are labeled in warm season turfgrasses like Pennant, Tower, and Freehand that also have very good activity for pre-emergent control of sedge species.

### Other Resistant Weed Species

Other herbicide resistant weeds that we have found in turf include goosegrass, which has — we found multiple populations with resistance to pre and post-emergent herbicides that are popular in turfgrass. Spotted spurge, ryegrass, plantain, southern crabgrass. So this is an issue that will continue to be a problem for us in turfgrass. Something that turf managers need to have an appreciation for is how weeds develop resistance to herbicides and what we can do to be proactive in preventing resistance from becoming a long-term problem. And sometimes we don't have alternative herbicide chemistries that are safe and selected for controlling resistant biotypes. And that creates great concern for us. But we are now seeing more and more weed species with resistance issues. It's just something that you need to have an appreciation for, especially for weeds like Poa annua, where it is just so difficult to control now. We need to have multiple programs and rotation of herbicides and the modes of action that are going out in weed control programs.

## New Herbicides for 2018

### Halauxifen: A New Synthetic Auxin

All right, new herbicides that are coming out here in 2018. The first one is a new active ingredient from Dow called halauxifen. This is a synthetic auxin herbicide group 4 broadleaf product that provides post-emergent control of annual and perennial broadleaf weeds. It has very fast activity. Halauxifen is a fast active ingredient. We typically see the response in susceptible broadleaf weeds within about five to seven days. Very rapid browning and necrosis of the tissue on the plant. It has favorable characteristics for turfgrass and also sod production where it has rapid degradation. There's no composting issues so it has a favorable characteristic for us in lawns and landscapes.

### RELZAR (Halauxifen + Florasulam)

The first product that's going to be released from Dow that contains halauxifen is going to be called RELZAR. This is a combination with broadleaf herbicide with a different mode of action, florasulam, which is found in the product Defender. This will be released as a WG formulation in the second quarter of 2018. It's going to be used in all major warm and cool season turfgrasses for post-emergent broadleaf weed control. And they're going to keep it simple. It's going to have one labeled use rate, which is 0.72 ounces of product per acre. That's going to be the standard use rate for all labeled turfgrass species. And this will be a good tool, I think, for us in Georgia, because it's going to include centipede, bermuda, zoysiagrass, fescue. So if you're managing lawns with mixed species or if you need to make up a tank and spray multiple lawns of various turf species, this product will have a nice fit for that use.

Again, this product has very rapid activity. This is four weeks after treatments for controlling common chickweed in the winter time where many broadleaf weed products are slow with cold temperatures there in the winter. Very rapid knockdown control with RELZAR applied in the winter for controlling common chickweed. Very active on broadleaf weeds as well in the summertime, weeds like buttonweed, matchweed here in St. Augustinegrass and it should have a good fit for use in the summertime for controlling weeds like doveweed. Some activity on common lespedeza, but it's not going to be a standalone product. But there's going to be some weeds like plantain and dandelion, which can provide very rapid knockdown control with a single treatment.

### Game On (Halauxifen + 2,4-D Choline + Fluroxypyr)

Game On is another new product from Dow that's going to contain halauxifen. This is a three-way combination with 2,4-D choline and fluroxypyr. It will also be released in 2018. They're going to primarily target Game On for use in cool season grasses. Most warm season species like centipede and St. Augustine are going to have sensitivity issues to 2,4-D. So it's going to be a cool season grass, tall fescue, Kentucky bluegrass, ryegrass product. We can use it in bermudagrass and zoysiagrass and this product provides very rapid knockdown control of perennial broadleaf weeds and I think it's going to be a strong new combination product for us in turfgrass.

Here's a look at dandelion four days after treatments with Game On. Very rapid necrosis discoloration there of the plant and we see broadleaf weeds like dandelion and plantain twist up within about seven days. So very rapid control of susceptible broadleaf weeds to these active ingredients. There can be some bermudagrass injury from Game On, which can last two weeks or so at labeled use rates. There can be some off-coloring, especially during spring transition, but this treatment will probably have a good fit for mid-summer applications after the bermudagrass has resumed active growth, it's greened up, and it's healthy, should have good tolerance to those treatments.

RELZAR and Game On have shown very good activity for controlling doveweed, which is a very problematic weed for us in Georgia right now. You can see here the percent cover with RELZAR on the left and Game On there in the middle, compared to Celsius. Very comparable levels of activity to Celsius and therefore controlling doveweed in the summertime. And you can see what the non-treated is doing on the far right where the population is going to increase over time. So new tools in the toolbox for controlling problem weeds, especially doveweed, which I think is going to be one of the most promising uses of these new products coming out from Dow here next year.

### Switchblade (Halauxifen + Dicamba + Fluroxypyr)

The next new product that contains halauxifen that's going to be released in late 2018 is called Switchblade. This is a three-way combination product similar to Game On, except they replaced the 2,4-D choline with dicamba. So it's going to contain the new active ingredient, halauxifen, has a rapid knockdown control, dicamba and fluroxypyr. This will be labeled in most major warm season turfgrasses including centipedegrass and cool season grasses. It will also have a St. Augustinegrass use on the label as well. And again, very good three-way combination for controlling many troublesome perennial broadleaf weeds and turfgrass.

### Vexis (Pyrimisulfan + Penoxsulam)

The next new active ingredient that's going to be released in 2018 in turfgrass is called pyrimisulfan. This is going to be a combination product with penoxsulam and sold as a trade name Vexis. This will be released again in 2018. This product is a combination of two ALS inhibiting herbicides. So the same mode of action as sulfonylureas. And it will come out as a granular product. So it's going to be a spreadable product. Likely going to have a fertilizer carrier on it. The potential uses is going to be for warm season grasses. This combination product, Vexis, will control broadleaf weeds. It does have good activity on many sedges as well, but it's weak on crabgrass and goosegrass and some of the grassy weeds that we have in the summertime.

But the advantage of using Vexis is going to be the root uptake. You don't have to have dew on the plant for it to stick and provide control. So you can go out on dry turf, spread it, and both of these active ingredients have significant root uptake and you can still get very good weed control with this product without dew present.

Here's a look at some of our plots where we've researched Vexis over the years with the fertilizer carrier. We have seen very good control of winter annual broadleaf weeds in our plots. Weeds like parsley-piert, cudweed, some of those types of weeds that are starting to emerge right now. Vexis has very good activity on those species. And then of course the fertilizer can give a little greening effect following those treatments. We have seen some erratic levels for controlling annual bluegrass. So I don't think this is going to be a very good Poa herbicide, but the strength is going to be primarily broadleaf weeds in warm season turfgrasses, especially winter annual weeds with those fall treatments.

Here's a look at Prodiamine in those plots where we got very good Poa control, but basically released lawn burweed. We've seen that Barricade and Prodiamine when we select for annual grassy weeds, we can actually enhance the establishment of some of the weeds that are not susceptible to that mode of action. Lawn burweed, as you can see in those plots, is not controlled by Barricade applications. So using a product like Barricade with Vexis or different broadleaf herbicide is going to be important if you're targeting both grassy and broadleaf weeds in the fall.

One of the advantages with Vexis is that it has very good activity for controlling sedges. Here's just a look at some of our research in the greenhouse where we looked at the control of biotypes that are susceptible and resistant to sulfonylurea herbicides. And Vexis is there on the right with its experimental code number there. Very good activity on susceptible biotype to sulfonylureas. It also has some activity for controlling resistant biotypes there on the right. You can see the activity on the ALS resistance sedge, and this product is showing good activity for partial suppression. I think multiple applications may have a nice fit with other chemistries for controlling the sedge populations with resistance issues to sulfonylurea. Although this is the same mode of action as the sulfonylurea, Vexis has one of the active ingredients from a different chemical family and that difference in the binding is giving partial control of resistant biotypes to sulfonylurea herbicides. So another tool in the toolbox, I guess, for managing resistant weeds in turfgrass.

### Solero (Mesosulfuron)

The next new product is a trade name called Solero. The active ingredient is mesosulfuron. This is a product being sold by Nufarm that was released about a year or two ago. This was developed by Valent over the years. It's now sold by Nufarm. It is labeled for all major warm and cool season turfgrasses. It controls annual and perennial sedges, comparable levels of control to Monument and Certainty for controlling perennial sedges and kyllinga. It also does have some activity for controlling broadleaf weeds. So just something to have an appreciation for. Solero is out there. It has comparable levels of use or use patterns as SedgeHammer, halosulfuron, for use in both warm and cool season turfgrasses. It has very good activity for controlling many different sedge species in turf.

### Dismiss NXT (Sulfentrazone + Carfentrazone)

Dismiss NXT is another new product that was released in the turfgrass industry this year. This was brought to the market by FMC in their line of sulfentrazone products. This is a combination of sulfentrazone, which is the active ingredient in Dismiss, with carfentrazone, which is the active ingredient in Quicksilver. And the benefits of using carfentrazone in the mix with this product is the speed of control. We are getting rapid control of sedges, kyllinga with Dismiss NXT. It's labeled for most major warm and cool season turfgrasses. And we are using Dismiss NXT on the same spectrum of weeds that we use Dismiss for. So the sedges and kyllinga, certain broadleaf weeds as well, does have some activity on goosegrass. But Dismiss NXT provides rapid control of kyllinga, seven days after treatments like you see there, very fast takedown and response of Dismiss NXT.

However, we are not seeing a significant difference in the levels of control, long-term control, with Dismiss NXT compared to straight Dismiss. So the speed of control with Dismiss NXT, I think, is the major advantage here, but in terms of it being better than Dismiss, we just do not have data to support that claim. But still, good product, rapid control, and sometimes getting that response from the weeds can make your clients happy, can make homeowners happy, and that rapid activity is certainly nice to see after you make those applications.

### Coastal (Simazine + Imazaquin + Prodiamine)

And finally, we are testing a new three-way combination product that should be released next year from a company called Sipcam. This is a three-way combination product for simazine, imazaquin, and prodiamine. A product called Coastal. This is going to be simazine and imazaquin, which is going to have post-emergent activity for controlling broadleaf weeds and sedges. They will also control Poa annua. So having two chemistries in there for post-emergent Poa control is nice. It has two different modes of action. And then Prodiamine, which is Barricade for the residual pre-emergent control of weeds in there as well. So it's a pre-emergent plus post-emergent treatment.

We have seen very good control of winter annual weeds like clover, Poa annua, and various other winter annual broadleaf weeds in our test plots. As you can see here from Coastal and with the Prodiamine in the mix with those spring treatments we can get very good crabgrass control as well throughout the year. So I think we're going to start seeing a lot more of these combination products that are combining multiple chemistries that provide both pre and post-emergent weed control, different modes of action as well there to combat resistant weeds and Coastal is going to be the first of many of these types of combinations that will be coming out for the turfgrass industry.

## Conclusion

So with that, I conclude my presentation. Thank you for your attendance and hope to see you guys sometime soon. Thank you so much.

Transcript processed for UGA Center for Urban Agriculture / GTBOP Archives Source: Corrected SRT (Stage 1) — GTBOP_Transcript_2017-11-17_WeedControlTurf.srt (649 blocks)