Weed control in newly established cool season grass stands

The weather in many areas this past fall has been conducive for excellent growth on late summer or early fall seeded stands of cool season grasses, such as novel endophyte tall fescue. Most extension guidelines in the mid-Atlantic region recommend establishing perennial cool season grasses during this period to avoid the competition of summer annual weeds with a young stand of grass.

However, winter annual broadleaf weeds can also prove a hazard to young cool season grass seedlings. Dealing with these weeds in the fall may be difficult as most broadleaf herbicide labels state that young grasses must start tillering or have a secondary root system prior to application. The weather may turn too cold to spray before the grasses can tolerate their first broadleaf herbicide application. Waiting too long to spray the weeds also proves problematic as they may mature and produce seed as early as April and May.

With good planning and timely spraying, winter annual broadleaf weeds may be effectively killed with an early spring spray while most grasses seeded the year before will have matured enough to withstand a selective broadleaf herbicide.

The common selective broadleaf herbicides rely on growth hormones to disrupt growth of the weed to the point that it can no longer survive. The key for an effective kill on broadleaf weeds early in the year is to time the spray while the weeds are young and actively growing.

Occasionally, we’ll get three or four days of warm weather in early spring (February-March) where temperatures reach over 55 °F during the day without a hard frost at night. Spraying on the second day of such a window of weather will help the weeds absorb the herbicide while they are already growing profusely, inducing uncontrolled growth and eventual death. This is the minimum weather window in which to spray. Warmer and longer windows increase the chance of kill, but applications should take place prior to weeds flowering and as soon as feasible to release the grass from the weedy competition.

I had an opportunity to utilize the spray-wait-spray method taught in the Alliance for Grassland Renewal workshops to replace five acres around my home of wildtype tall fescue with Martin2 with Protek. (Other great novel endophyte cultivars of tall fescue are available, including BarOptima plus E34, Estancia with Arkshield, Jesup MaxQII, Lacefield MaxQII, Texoma MaxQII, and Tower with Protek.)

Left: A steep and rocky hillside was broadcast seeded with a novel endophyte tall fescue variety in the fall of 2020. Right: Despite good grass growth, winter annual weeds are present and threaten the establishment. Photos by Dr. Gabe Pent.

After controlling tall fescue seedheads in the spring through managed grazing, I sprayed the stand with glyphosate in July. In order to plant this year’s crop of tall fescue seed, I had to wait until late September before I could spray the area again with glyphosate and then broadcast the tall fescue seed. (The ground is too steep and rocky and there are too many trees to use a tractor and seed drill, which otherwise is preferred.)

Although broadcast seeding is risky and soil moisture was moderately low in late September, the tall fescue started germinating in 10-14 days. The stand now (December) looks acceptable, but there is a substantial presence of winter annual weeds (henbit and purple deadnettle) and wild garlic. The grass isn’t ready to withstand a broadleaf herbicide now, but it should be by February or March. I’m looking forward to utilizing this young stand of grass lightly next summer.

Select and use all pesticides carefully. Before using any pesticide, read the instructions printed on the label of its container; follow those instructions, heed all cautions and warnings, and note precautions about residues. Store pesticides in their original containers. Store them where children and animals cannot get to them — away from food, feed, seed, and other materials that could become harmful if contaminated. Dispose of empty pesticide containers in the manner specified on their labels. See your doctor if symptoms of illness occur during or after use of any pesticide. Commercial products and/or services are named in this publication for information purposes only. Virginia Cooperative Extension, Virginia Tech, and Virginia State University do not endorse or warrant these products and/or services, and they do not intend or imply discrimination against other products and/or services that also may be suitable.

~ Dr. Gabriel Pent, Superintendent, Virginia Tech Shenandoah Valley AREC and Dr. Michael Flessner, Extension Weed Specialist, Virginia Tech School of Plant and Environmental Sciences

The Alliance for Grassland Renewal is a national organization focused on enhancing the appropriate adoption of novel endophyte tall fescue technology through education, incentives, self-regulation and promotion.  For more resources or to learn more about the Alliance for Grassland Renewal, go to www.grasslandrenewal.org

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