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# GTBOP Webinar Archive

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

Webinar Date: November 17, 2017 Speaker: Dr. Patrick McCullough, Weed Scientist, University of Georgia Duration: 1:10:00 Series: Green & Commercial CEU Categories: Category 24 (Ornamental and Turf Pest Control) Format: Pre-recorded presentation (no live Q&A)

## SUMMARY

Dr. Patrick McCullough, weed scientist at the University of Georgia, delivers a comprehensive review of turfgrass weed management fundamentals paired with updates on herbicide resistance issues and new product introductions for the 2018 season. The presentation covers the full scope of practical weed science knowledge that turfgrass managers need for effective control programs.

McCullough begins with weed identification principles, recommending two key reference books — the Color Atlas of Turfgrass Weeds and UGA's Weeds of Southern Turfgrass — and walking through diagnostic characteristics including seed heads, ligules, flowers, leaf arrangement, pubescence, and leaf markings. He illustrates these with specific examples such as distinguishing bahiagrass from dallisgrass by seed head structure, differentiating crabgrass species by hair patterns, and separating white clover from spotted burr clover by leaf markings. Understanding weed lifecycles — winter annuals, summer annuals, simple perennials, and complex perennials — is essential because each group requires different management strategies and timing.

Cultural practices receive substantial attention. McCullough presents research showing that raising tall fescue mowing height from one to four inches reduced crabgrass cover from 95% to essentially zero, and a three-year Florida study demonstrating that daily irrigation increased dollarweed cover five- to six-fold compared to as-needed watering. He emphasizes planting certified seed to avoid introducing weed species, citing an NTEP trial where seedlot contamination introduced broadleaf dock into a single plot.

The core of the presentation covers pre-emergent herbicide science — how these products bind in the upper soil profile, inhibit cell division in germinating seedlings, and require timely activation through irrigation or rainfall. McCullough details application timing by Georgia region, residual activity differences among products, and the advantages of split applications for extending seasonal control.

Herbicide resistance emerges as a major theme. McCullough presents field and greenhouse data showing widespread Poa annua (Poa annua) resistance to dinitroaniline and sulfonylurea herbicides across Georgia golf courses, sod farms, and lawns, along with halosulfuron-resistant sedge (Cyperus compressus) populations. He demonstrates how combining modes of action — such as pairing a sulfonylurea with simazine — controlled resistant biotypes at all three golf course test sites. The presentation concludes with updates on new 2018 products: RELZAR, Game On, and Switchblade (all containing the new active ingredient halauxifen from Dow); Vexis (pyrimisulfan + penoxsulam); Solero (mesosulfuron from Nufarm); Dismiss NXT (sulfentrazone + carfentrazone from FMC); and Coastal (simazine + imazaquin + prodiamine from Sipcam).

## VIDEO CHAPTERS

0:00 Introduction 0:46 Weed Identification Books and Resources 2:00 Weed Categories: Grassy, Broadleaf, and Grass-Like 2:43 Identification by Seed Heads 3:57 Comparing Bahiagrass and Dallisgrass 5:13 Poa annua Seed Head Identification 5:34 Using Ligules for Grassy Weed Identification 6:50 Broadleaf Weed Flowers and Day Flower Species 7:52 Leaf Arrangement and Hair Characteristics 9:34 Leaf Markings: White Clover vs. Spotted Burr Clover 10:16 Weed Lifecycles: Winter and Summer Annuals 12:05 Perennial Weeds: Simple and Complex 16:02 Scouting, Early Detection, and Cultural Practices 17:08 Mowing Height Effects on Crabgrass Competition 20:07 Irrigation Influence on Weed Populations 22:05 Seed Quality and Seedlot Contamination 24:07 How Pre-Emergent Herbicides Work 27:00 Pre-Emergent Product Overview and Formulations 29:00 Pre-Emergent Targets and Limitations 29:38 Herbicide Activation: Sprayable vs. Granular 31:07 Spring Application Timing by Georgia Region 32:52 Residual Activity and Product Selection 34:48 Fall Pre-Emergent Timing for Winter Annuals 35:11 Split Applications for Extended Control 36:21 Fall Products for Poa annua Management 37:31 Herbicide Resistance: Overview and Mechanisms 41:01 Poa annua Resistance in Georgia 43:34 Fall Post-Emergent Timing and Sulfonylurea Programs 48:55 Poa annua Control Programs by Turfgrass Species 51:38 Sedge Resistance to Sulfonylureas 53:44 Other Herbicide-Resistant Weed Species 55:01 New for 2018: Halauxifen Products (RELZAR, Game On, Switchblade) 1:01:22 New for 2018: Vexis (Pyrimisulfan + Penoxsulam) 1:05:31 Solero (Mesosulfuron) from Nufarm 1:06:27 Dismiss NXT (Sulfentrazone + Carfentrazone) 1:08:19 Coastal: Three-Way Pre + Post Combination from Sipcam 1:09:53 Conclusion

## QUESTIONS & ANSWERS

Q: What are the three main categories used to classify weeds in turfgrass? A: Weeds are classified as grassy weeds (monocots), broadleaf weeds (dicots), and grass-like weeds. The grass-like category is a miscellaneous grouping that includes sedges, wild garlic, wild onion, and plants like Star of Bethlehem that don't fit neatly into the other two categories.

Q: Why is it important to distinguish bahiagrass from dallisgrass when planning herbicide treatments? A: Although both are warm-season perennial Paspalum species with similar seed head spikes, they have very different selective control options. Bahiagrass can be effectively controlled with metsulfuron and various warm-season species herbicides, while dallisgrass requires very specific application programs. Misidentifying one for the other could lead to ineffective treatments.

Q: How does mowing height affect crabgrass populations in tall fescue? A: Research conducted in North Carolina showed that raising the mowing height of tall fescue from one inch to four inches reduced crabgrass cover from 95% to essentially 0%. The taller fescue was able to shade out crabgrass and prevent its establishment through increased competition.

Q: How do pre-emergent herbicides work, and do they prevent weed seed germination? A: Pre-emergent herbicides do not prevent germination. They bind to the upper half inch of the soil profile and are absorbed by the roots and shoots of germinating seedlings. Most pre-emergent herbicides inhibit cell division, so seedlings that take in the herbicide fail to establish a healthy root system and die. Well-established turfgrass with roots penetrating below the treated zone is generally not affected.

Q: Why are split applications of pre-emergent herbicides recommended over single applications? A: Splitting a pre-emergent application — for example, applying half a pound of prodiamine per acre in March and the other half in late May or June — provides a fresh supply of herbicide to the soil and extends residual control beyond what a single full-rate application achieves. This approach improves control of late-season flushes of crabgrass and other annual weeds.

Q: What is herbicide resistance, and how does it develop in weed populations? A: Herbicide resistance develops through selection pressure. A naturally occurring resistant biotype — perhaps one in a thousand or one in a million plants — survives treatment because of an altered target site where the herbicide cannot bind properly. When the same herbicide is applied repeatedly over multiple years, susceptible plants are killed while the resistant biotype reproduces and spreads, eventually shifting the population toward resistance. This is not a change caused by the herbicide, but a selection of pre-existing genetic traits.

Q: What herbicide resistance problems is Georgia currently experiencing with Poa annua? A: Georgia is seeing widespread Poa annua resistance to dinitroaniline pre-emergent herbicides (pendimethalin, prodiamine) with cross-resistance to Dimension (dithiopyr). Sulfonylurea post-emergent resistance is also prevalent. McCullough's greenhouse testing showed a resistant biotype survived rates up to 300 times the labeled rate of Monument, demonstrating classic target-site resistance. These resistance issues are increasing on lawns, golf courses, sod farms, and other turfgrass areas throughout the state.

Q: What resistance management strategy does McCullough recommend for Poa annua control in bermudagrass and zoysiagrass? A: McCullough recommends combining two modes of action — a sulfonylurea herbicide (such as Revolver, Monument, Katana, or Tribute Total) with a triazine herbicide (simazine) at a quart per acre. In field trials across three golf courses with different resistance profiles, this tank mixture provided complete Poa control at all locations, even where individual products had failed. Simazine adds a second mode of action for approximately $5 per acre.

Q: What is halauxifen, and what new products will contain it? A: Halauxifen is a new synthetic auxin (Group 4) active ingredient from Dow with very rapid broadleaf weed activity — typically five to seven days to visible response. Three products containing halauxifen were set for 2018 release: RELZAR (halauxifen + florasulam) for all major turfgrass species; Game On (halauxifen + 2,4-D choline + fluroxypyr) primarily for cool-season grasses and bermudagrass; and Switchblade (halauxifen + dicamba + fluroxypyr) for warm- and cool-season grasses including centipedegrass and St. Augustinegrass. All three showed promising activity on doveweed.

Q: How does Dismiss NXT differ from standard Dismiss? A: Dismiss NXT combines sulfentrazone (the active ingredient in Dismiss) with carfentrazone (the active ingredient in Quicksilver). The primary advantage is speed of control — Dismiss NXT provides rapid knockdown of sedges and kyllinga, with visible results within seven days. However, McCullough's research did not show a significant difference in long-term control levels compared to straight Dismiss. The rapid visual response can be valuable for client satisfaction.

Q: What is the Coastal combination product, and why is it significant? A: Coastal is a three-way combination from Sipcam containing simazine, imazaquin, and prodiamine. It provides both post-emergent activity (simazine and imazaquin controlling broadleaf weeds, sedges, and Poa annua with two different modes of action) and pre-emergent residual control (prodiamine for grassy weeds). McCullough sees this type of multi-chemistry combination product as a model for future turfgrass herbicide development, particularly for managing resistant weed populations.

## RESOURCES

- GeorgiaWeather.net — Weather station network for tracking local soil temperatures to time pre-emergent herbicide applications
- Color Atlas of Turfgrass Weeds — Published by GCSAA, available through Amazon and other retailers
- Weeds of Southern Turfgrass — UGA publication, available through the Athens bookstore and online retailers

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