# GTBOP Webinar Archive
## Weed Control in Ornamentals for the Nursery and Landscape

**Webinar Date:** July 13, 2023
**Speaker:** Dr. Chris Marble, Associate Professor, Ornamental and Landscape Invasive Weed Management, Mid-Florida Research and Education Center, University of Florida
**Moderator:** Dr. Shimat Joseph, UGA Turfgrass Entomologist
**Duration:** 50:38
**Series:** Green & Commercial
**CEU Categories:** Category 10 (Private), Category 21, Category 23, Category 24, Category 27, Category 31, Category 32, Category 35

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## Summary

Dr. Chris Marble, an associate professor specializing in ornamental and landscape invasive weed management at the University of Florida's Mid-Florida Research and Education Center, presented a comprehensive guide to developing herbicide programs for nursery and landscape ornamentals. His presentation addressed both production and landscape settings, emphasizing that effective weed control depends on building a complete program rather than relying on individual products.

Marble began by identifying poor calibration as the number one reason herbicides fail, noting that his team's field measurements across multiple Southeast locations found some applicators off by 50 to several hundred percent of their target rate. He highlighted free calibration and mixing calculators developed at UF to address this problem, available for both granular and liquid applications. Turning to herbicide timing, he explained that pre-emergent herbicides must be applied before target weed germination and that over-the-top applications should avoid periods of tender new growth, using a SureGuard (flumioxazin) trial on holly to illustrate how young foliage is more susceptible to injury than hardened growth. For post-emergence herbicides, he stressed treating weeds while small and actively growing, and ranked the factors affecting post-emergence performance: herbicide rate and efficacy first, followed by weed size, environmental conditions, adjuvants, and time of day.

The core of the presentation was a three-step process for selecting herbicides: determine what is labeled and safe for the target ornamental, identify the primary and secondary weed species, and combine those into a year-round rotation using different modes of action. Marble walked through a detailed example using container-grown gardenia, narrowing granular pre-emergent options using the 2017 Southeast Pest Control Guide's efficacy and safety charts, then building a seasonal rotation targeting spotted spurge and eclipta in warm months and bittercress, annual bluegrass, and oxalis in cool months. He presented research showing that combining pre-emergent herbicides like Specticle or SureGuard with post-emergence treatments reduced total herbicide use by 40 to 60 percent and costs by up to 30 percent compared to post-only programs. Marble also reviewed post-emergence alternatives to glyphosate — including glufosinate, acetic acid products, and other desiccant-type herbicides — noting that while they provide fast initial burndown, most require follow-up applications for lasting control. He concluded by highlighting underutilized selective options such as graminicides for grass control in ornamental beds and products like Basagran, Lontrel, and Certainty for sedge and broadleaf management.

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## Video Chapters

0:00 Introduction and Speaker Credentials
0:58 Overview: Weed Control in Nurseries and Landscapes
1:36 Why Herbicides Fail: Calibration and Application
4:34 Calibration Tools and Mixing Calculators
6:35 Pre-emergent Herbicide Timing
8:20 Avoiding Plant Injury During Application
9:29 SureGuard Holly Trial: New Growth vs. Hardened Foliage
10:42 Post-emergence Herbicide Timing
11:30 Environmental Factors Affecting Efficacy
13:18 Stressed Weeds and Mowing Impacts
14:28 Ranking Factors That Impact Post-emergence Performance
15:38 Importance of Developing a Program, Not Just Products
17:17 Herbicide Rotation and Resistance Prevention
18:48 Three Steps to Choosing Herbicides
19:17 Pre-emergence Options Color-Coded by Mode of Action
20:48 The 2017 Southeast Pest Control Guide
21:00 Step 1 Example: Labeled Options for Container-Grown Gardenia
22:02 Grouping Herbicides by Mode of Action
23:01 Step 2: Targeting Primary and Secondary Weed Species
25:20 Step 3: Building a Year-Round Rotation
27:03 Year-Round Nursery Rotation Plan by Month
27:34 Research: Pre-emergent + Post-emergent vs. Post-Only Programs
30:44 Cost Savings and Herbicide Reduction Results
31:55 Application Interval Considerations
32:09 Landscape-Specific Rotation Planning
33:52 Timing Examples: One, Two, or Three Applications Per Year
35:20 Generic Landscape Rotation Example by Season
37:01 Post-emergence Alternatives to Glyphosate in Landscape Beds
38:08 Glufosinate (Finale/Cheetah) as an Alternative
38:43 Non-Selective Alternatives: Desiccant-Type Herbicides
40:12 Acetic Acid Trial: Burndown and Recovery Results
42:23 Selective Post-emergence Options for Landscape Beds
43:04 Graminicides: Underutilized Grass-Selective Herbicides
43:55 Basagran, Lontrel, Certainty, and Scepter
44:44 UF Mid-Florida REC Resources and Contact Information
45:52 Q&A: Signal Words and PPE Requirements
48:02 Q&A: Wind Drift and Reducing Spray Drift
49:08 Q&A: Marengo for Nursery Gravel Areas

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## Questions & Answers

**Q: What is the number one reason herbicides fail to provide expected weed control or cause plant injury?**
A: According to Dr. Marble, the number one reason is poor calibration and application practices. His team measured applicator accuracy at multiple Southeast locations and found that while most people were within 10 to 20 percent of the target rate, some were off by 50 to several hundred percent. When the rate is that far off, the herbicide will either fail to control weeds or cause significant plant injury.

**Q: When should pre-emergent herbicides be applied in nursery versus landscape settings?**
A: In nurseries, pre-emergent herbicides should be applied shortly after potting once plants are watered in, and then reapplied approximately every 8 to 12 weeks throughout the production cycle. In landscapes, application can occur soon after planting for woody ornamentals, and two to three applications per year are generally sufficient. In both cases, the soil should be settled before application to prevent root exposure to the herbicide.

**Q: Why should applicators avoid treating during periods of tender new growth?**
A: Plants are most susceptible to herbicide injury during bud swell and when young foliage is developing. Dr. Marble demonstrated this with a SureGuard trial on holly, where the newest growth was severely affected while older, hardened foliage showed no injury at all. High temperatures further increase the risk. While plants typically recover, the delay can affect sale timing in nurseries and upset clients in landscapes.

**Q: How should a professional select pre-emergent herbicides for a specific ornamental crop?**
A: Dr. Marble outlined a three-step process. First, determine which herbicides are labeled and safe for the ornamental species in question. Second, identify the primary and secondary weed species at each time of year. Third, match herbicides that are effective on those weeds — rated "good" for the primary species and at least "fair" for secondary species — and arrange them in a rotation that uses different modes of action across the year.

**Q: Why is rotating herbicide modes of action important, and what can happen if you don't?**
A: Relying on a single herbicide or mode of action allows uncontrolled weed species to take over and increases the risk of resistance development. Dr. Marble shared an example of a nursery that switched to Gallery (isoxaben) for bittercress control and used it exclusively. Because Gallery is only effective on broadleaves, grass species that were not controlled exploded in population. Weed populations increase exponentially, not linearly, so the problem can escalate rapidly.

**Q: How much can pre-emergent herbicides reduce overall herbicide use and cost compared to a post-emergence-only program?**
A: Research conducted by Dr. Marble's team compared post-only programs using glyphosate, Finale, or Reward against programs that combined those post-emergence herbicides with Specticle or SureGuard as pre-emergents. The pre-plus-post programs reduced the total amount of herbicide active ingredient applied by 40 to 60 percent and lowered total costs by 3 to 30 percent over 12 months, while also requiring far fewer follow-up applications.

**Q: What are the main alternatives to glyphosate for non-selective post-emergence weed control in landscape beds?**
A: Glufosinate (sold as Finale or Cheetah) is the most common alternative, offering broad-spectrum activity on broadleaves, grasses, and sedges, though it is primarily contact-acting and doesn't provide the same systemic control as glyphosate. Other alternatives include desiccant-type products such as Scythe, Reward, Axxe, Finalsan, and FireWorxx. These work fast — burndown can be visible within 30 minutes — but because they are not translocated, they often require follow-up applications, especially on larger weeds. Some products are OMRI-certified for organic use. Signal words and PPE requirements vary significantly among these alternatives.

**Q: How do desiccant-type herbicides perform compared to glyphosate on common landscape weeds?**
A: Dr. Marble's research showed that at two weeks after treatment, an acetic acid product provided 60 to 90 percent burndown on crabgrass, spotted spurge, and bittercress. However, by four to eight weeks, control dropped dramatically as crabgrass and spurge recovered from incomplete coverage. Two applications were generally needed to achieve control comparable to a single glyphosate application on annual weeds. For large perennials, multiple applications are required and control remains difficult compared to systemic herbicides.

**Q: What selective post-emergence herbicides can be applied over the top of ornamentals in landscape beds?**
A: Graminicides such as sethoxydim (Segment), clethodim (Envoy), fluazifop (Fusilade), and fenoxaprop (Acclaim) can be applied over the top of hundreds of broadleaf ornamental species and even some monocots like liriope and mondo grass. Dr. Marble considers them underutilized. For sedge control, Basagran (bentazon) and Certainty are options, with Certainty also labeled for over-the-top use on Asian jasmine, liriope, and certain junipers. Lontrel (clopyralid) is effective on Asteraceae family weeds and legumes. Scepter provides additional broadleaf options in select ornamentals.

**Q: What are the three signal word levels on pesticide labels, and why do they matter for landscape operations?**
A: The three levels are caution (lowest toxicity), warning (intermediate), and danger/poison (highest toxicity), based on routes of exposure including skin and eye contact. Higher signal words require more PPE for applicators, and products may become restricted use with stricter record-keeping requirements. In Florida, limited-license holders can only use caution-label products in non-turf landscape areas, which restricts their product options.

**Q: How can applicators reduce herbicide drift during spraying?**
A: Dr. Marble recommended using coarser nozzle tips so that water particles are larger and fall more readily, and lowering sprayer pressure. The worst scenario for drift is high pressure combined with fine nozzle tips, which produces very fine water particles that travel off-target. While most herbicide labels recommend application when wind speeds are below 5 miles per hour, conditions are variable in real-world situations, so equipment adjustments are the most practical way to minimize drift.

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## Additional Resources

- **2017 Southeast Pest Control Guide for Nursery Crops and Landscape Plantings** — Compiled by Dr. Marble, Jeff Derr, and Joe Neal. Available for free download or approximately $20 for a hard copy from North Carolina State University. Contains herbicide efficacy and ornamental safety charts.
- **UF Mid-Florida Research and Education Center** — Dr. Marble's faculty page includes publications, herbicide calibration calculators (granular and liquid), weed identification resources including a weed ID by flower color tool, and guides on using free plant identification apps effectively.
- **Contact:** Dr. Chris Marble — email and contact information available through his UF faculty page.

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*Part of the Getting the Best of Pests (GTBOP) Green & Commercial Webinar Series, hosted by the University of Georgia Center for Urban Agriculture.*

*Processed for UGA Center for Urban Agriculture / GTBOP Archives*
