Effect of Waterlogged Soil on Preemergence Herbicides in Turfgrass Systems

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Written by Travis Gannon, Edited By Minda Daughtry

Beyond the establishment of dense, healthy turfgrass, one of the most effective means of maintaining weed-free turfgrass is utilizing well-timed preemergence herbicide application(s). Applying a preemergence herbicide prior to crabgrass germination is critical to achieving maximum efficacy. Crabgrass germinates when 24-hour mean soil temperatures average about 54-55°F for 3-4 consecutive days. In recent years, these target soil temperatures have occurred earlier than in previous years. For this reason, earlier preemergence herbicide applications have been recommended in the last several years. Preemergence herbicides must receive rainfall or irrigation prior to germination and emergence to provide optimum efficacy.

Current timing recommendations for late winter preemergence herbicide applications for summer annual species range from late January to early February for the southeastern portion of the state, through early April for higher elevations in western North Carolina. Under normal growing conditions, common commercial

Crabgrass Seedlings

Crabgrass seedlings – Cira Sims CC0

preemergence herbicides provide effective weed control when applied at these earlier recommended application dates. This is particularly true as early preemergence applications are often associated with low soil temperatures when microbial activity, one of the primary sources of preemergence herbicide degradation, is low.

2020 ended as the 3rd wettest year on record in Raleigh and many parts of NC and 2021 is off to a wet start with over 6” rainfall in each January and February. As a result, soils in many areas have been saturated much of the winter. What are the impacts of this on preemergence herbicides? What happens when soils are saturated from a historic year of winter and early spring rainfall? Saturated soils may adversely affect the length of control provided by certain preemergence herbicides in anaerobic (flooded, waterlogged) conditions. As soils become waterlogged, oxygen is depleted, creating an anaerobic environment. These anoxic soils (depleted of dissolved oxygen) can facilitate accelerated anaerobic herbicide dissipation, opening the possibility for weed breakthrough after preemergence herbicides may have prematurely degraded below effective concentrations.

THE SOLUTION(S): This serves as a reminder to remain vigilant in scouting historically weedy, weak, and wet areas for newly emerging weeds following these wet seasons. If you typically split preemergence herbicide applications, consider increasing the active ingredient rate for the sequential application if your area has received excessive rainfall and soils have been saturated for prolonged periods of time. How much to increase the rate is an educated guess and depends on site-specific conditions but increasing the rate by 25% on the second preemergence application is certainly reasonable. Furthermore, if a split application was not planned and the entire herbicide was applied at the initial application, consideration of applying another application may be warranted. Additionally, be prepared to utilize postemergence herbicides if/when weed breakthrough occurs (it’s most likely not an issue of “if” but “when”…). As always, read and follow product label (the label is the law).

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Minda Daughtry is the Horticulture Agent for North Carolina Cooperative Extension in Lee County.