Skip to content

# GTBOP Webinar Archive Summary

## Weed Control in the Landscape & Nursery

Webinar Date: November 18, 2021 Speaker: Dr. Mark Czarnota, Associate Professor of Horticulture (Weed Science), University of Georgia Griffin Campus Moderator: Dr. Shimat Joseph, Turfgrass Entomologist, UGA Griffin Campus Duration: 50:05 Series: Green & Commercial CEU Categories: Category 10 (Private Applicator), Category 21 (Plant Agriculture), Category 22 (Animal Agriculture), Category 23 (Forestry), Category 24 (Ornamental and Turf Pest Control), Category 27 (Right-of-Way Pest Control), Category 31 (Public Health Pest Control), Category 32 (Regulatory Pest Control), Category 35 (Industrial, Institutional, Structural and Health Related)

## NARRATIVE SUMMARY

Dr. Mark Czarnota, a weed scientist at the University of Georgia Griffin Campus with expertise in ornamental, nursery, small fruit, and Christmas tree weed management, presented a comprehensive overview of weed control strategies for landscape and nursery professionals. Drawing on his background in weed science from Virginia Tech and Cornell University and his experience in the commercial nursery and chemical industries, Czarnota organized his talk around three primary control approaches: physical removal, physical barriers, and chemical weed control.

Czarnota began by defining weeds and outlining six characteristics that predict whether a plant will become problematic, including vegetative reproductive structures, abundant seed production, rapid population establishment, seed dormancy, adaptive reproduction, and ability to colonize disturbed sites. He illustrated real-world challenges using examples of Marchantia (Marchantia spp.) infesting propagation houses, purple nutsedge (Cyperus rotundus) penetrating plastic mulch in blackberry plantings, and weeds emerging from container weep holes — situations where chemical options remain limited.

The presentation covered physical control methods including tillage, flame weeding, and emerging automated weeding technology, as well as physical barriers such as landscape fabrics and mulches. Czarnota emphasized maintaining a two-to-four-inch mulch layer and noted that bare soil will always produce weed growth. He discussed biological control successes including grass carp for aquatic weed management, goats for kudzu suppression, and the thistle weevil for musk thistle control, along with the bioherbicide DeVine for strangler vine in citrus.

The bulk of the presentation focused on chemical weed control, covering the distinction between pre-emergent and post-emergent herbicides, selective versus non-selective products, and granular versus sprayable formulations. Czarnota highlighted underutilized products including Dimension (dithiopyr) and Marengo (indaziflam), which can provide up to 16 weeks of pre-emergent control in established woody ornamentals. He reviewed modes of action for key herbicides including glyphosate's inhibition of the EPSP synthase pathway and the dinitroaniline herbicides' disruption of microtubule formation. Czarnota also discussed glyphosate safety, citing a 2017 Journal of the National Cancer Institute study of nearly 45,000 licensed applicators finding no association between glyphosate use and cancer incidence above national background rates. The session concluded with audience questions on torpedograss control using Drive (quinclorac), Virginia buttonweed management with atrazine, and selective bermudagrass control in ornamentals using clethodim (Envoy).

## YOUTUBE TIMESTAMPS

0:00 Introduction and Speaker Background 3:02 What Is a Weed? Definitions and Weediness Predictors 5:13 Challenging Weed Problems in Nursery Production 9:03 Weed Identification and Plant Life Cycles 11:55 Weed Control Methods: Physical Removal and Barriers 15:44 Biological Control: Grass Carp, Goats, Thistle Weevil 19:50 Chemical Weed Control: Impact of Major Herbicides 21:07 Pre-Emergent vs. Post-Emergent Herbicides 25:16 Trade Names, Active Ingredients, and Cost Savings 26:20 Herbicide Modes of Action 29:42 Pre-Emergent Herbicides for Landscape and Nursery 31:17 Post-Emergent and Combination Herbicides 35:16 Recommended Products: Marengo, BroadStar, SureGuard 36:40 Best Practices and Common Application Errors 40:21 Herbicide Fate, Organic Options, and Glyphosate Safety 43:40 Glyphosate Damage, Adjuvants, and Future Technologies 44:54 Q&A: Torpedograss, Virginia Buttonweed, Bermudagrass Control

## QUESTIONS & ANSWERS

Q: What are the most important characteristics that make a plant likely to become a weed problem? A: According to Czarnota, six characteristics consistently predict weediness: the presence of a vegetative reproductive structure (like nutsedge tubers), abundant seed production (pigweed can produce 5,000–10,000 seeds per plant), rapid population establishment, seed dormancy allowing long-term survival in soil, the ability to reproduce both vegetatively and by seed, and the capacity to colonize disturbed sites.

Q: Why is maintaining a mulch layer so important for weed management in landscape beds? A: Czarnota emphasized that bare soil will always produce weed growth. A two-to-four-inch layer of composted organic mulch prevents weed seed germination, maintains soil temperature and moisture, and adds organic matter. This layer should be refreshed once or twice per year. Organic mulch should be composted and contain less than 10% white wood to avoid nitrogen tie-up.

Q: What is the difference between pre-emergent and post-emergent herbicides? A: Pre-emergent herbicides are applied to bare soil or mulch before weed seeds germinate. They need a rain event of about half an inch to one inch to move into the top quarter inch of soil where seeds germinate, and they provide roughly 8–12 weeks of control. Post-emergent herbicides are applied after weeds have emerged and need a period of dryness after application to be absorbed into the plant.

Q: Which pre-emergent herbicides does Czarnota recommend most for landscape use? A: Czarnota highlighted Dimension (dithiopyr) as one of the most underutilized pre-emergent herbicides, safe for use on most turfgrasses and landscape situations including broad woody ornamentals and some perennials and annuals. He also recommended Marengo (indaziflam) as one of the best products he has tested, providing up to 16 weeks of weed control in established woody ornamentals, though it does not control nutsedge. BroadStar (flumioxazin) as a granular and SureGuard as its sprayable form were also recommended.

Q: Why is glyphosate considered to have low toxicity to humans and animals? A: Glyphosate inhibits the EPSP synthase pathway, which produces three amino acids — tryptophan, phenylalanine, and tyrosine. This pathway does not exist in humans or any animals; we must obtain these amino acids through our diet. Because glyphosate targets a biochemical process absent from animal biology, it is one of the reasons the herbicide has low mammalian toxicity. Czarnota also cited a 2017 study from the Journal of the National Cancer Institute reviewing nearly 45,000 licensed applicators that found no association between glyphosate use and cancer above background rates.

Q: What are the advantages and disadvantages of granular versus sprayable herbicide formulations? A: Granular formulations fall through the plant canopy to the ground, making them safer for desirable plants, but they are more expensive per unit of active ingredient — most granulars are only about 2% active ingredient. Sprayable formulations are cheaper because the concentrate can be up to 90% active, but they contact plant foliage directly, increasing the risk of damage. Combination granular products like Snapshot and FreeHand offer both pre-emergent control and some burndown activity from oxyfluorfen.

Q: How should glyphosate be applied to avoid damaging desirable plants? A: Czarnota advised spraying glyphosate only on actively growing plant material, never applying it to the trunk or base of thin-barked trees, and ensuring the application dries before rain. He noted that repeated applications near ornamentals can cause "bud blasting" — sublethal damage where plants emerge with distorted growth. For stump control or selective removal, glyphosate or triclopyr can be painted directly onto cut stumps, which kills the plant about 90% of the time without affecting surrounding plants.

Q: What product works best for controlling torpedograss? A: Czarnota recommended Drive (quinclorac) as a product that controls torpedograss very well. He noted that Drive is not yet labeled for over-top use on ornamentals but has been used in blueberry and blackberry weed control. His university research has found it to be fairly safe on most woody ornamentals, though he cautioned it would not be a labeled application and recommended growers conduct their own trials.

Q: How can bermudagrass creeping into ornamental beds be selectively controlled? A: Czarnota recommended clethodim (Envoy) as the product giving approximately 10–15% better activity than alternatives, providing three to four months of bermudagrass suppression before retreatment is needed. Other effective grass herbicides include Segment, Fusilade DX (also sold as Grass-B-Gon), though all perform similarly. He noted that glyphosate also controls bermudagrass well at high rates but cannot be used selectively — it would kill both the bermudagrass and any desirable plants.

Q: What common mistakes reduce the effectiveness of herbicide applications? A: Czarnota listed several frequent errors: failing to irrigate after pre-emergent applications (they need rain within 72 hours), getting rain too soon after post-emergent applications, poorly calibrated equipment, poor herbicide selection for the target weed, and applying pre-emergent herbicides after weeds have already germinated. He recommended two to four applications per year for field situations and four to six for container production, using at least two different products to broaden the spectrum of control.

Q: What biological control methods have proven effective for weed management? A: Czarnota described several successful biocontrol examples. Grass carp stocked at 8–10 fish per surface acre effectively control submerged aquatic weeds. Goats can rapidly clear kudzu — in a demonstration at UGA's ag forestry field day, goats cleared a fenced kudzu area overnight, after which the site could be treated with herbicides on the regrowth. The thistle weevil successfully reduced musk thistle over a two-to-three-year period by larvae feeding on developing seeds, preventing reproduction. The bioherbicide DeVine, developed with the University of Florida, effectively controlled strangler vine in citrus.

## ADDITIONAL RESOURCES

- Speaker Contact: Dr. Mark Czarnota offered to answer additional questions via email (to be distributed to attendees)
- Label and SDS Resources: CDMS.net — contains approximately 80% of pesticide labels and safety data sheets
- Turfgrass Weed Control Referral: Dr. Patrick McCullough, UGA, for turfgrass-specific weed control questions (referenced during Virginia buttonweed discussion)

Processed for UGA Center for Urban Agriculture / GTBOP Archives Source: Corrected SRT (Stage 1) — GTBOP_Transcript_2021-11-18_WeedControl.srt (786 blocks)