Control for Summer Annual Weeds in Lawn & Turf
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Summer weeds are getting ready to get going in your lawn, so its time to head them off at the pass with preemergent weed control before they can even get started. The old adage “An ounce of prevention is worth a pound of cure” applies to weeds as well so, the best time to control them is before they come up with preemergent. The winter annuals like deadnettle, chickweed, cress and henbit that are up now can be treated with post emergent herbicides at this point, but last fall was the best time to deal with them.
Preemergent herbicides offer a great option for certain annual grass and broadleaf weed control in warm- and cool-season turf. A Preemergent is activated in the top layer of soil, where the weed seeds sprout and then grow through the herbicide-treated barrier or zone. The herbicide is absorbed by emerging shoots and/or roots, then disrupts the plant’s vital processes. This keeps the plant from developing and results in death of susceptible plant, breaking the cycle of constant reseeding of new weeds. Usually, the chemical is activated by applying water at the correct time, in the correct amount. It can remain effective in the soil for several weeks up to several months, depending on the ingredient used and the concentration of it. Keep that effective-chemistry timeline in mind when re-seeding preferred grass seed. Read the herbicide label carefully to make sure you do kill the unwanted weed shoot but don’t kill the grass plant you want to grow.
As with any product it makes sense to understand how it gets the job done. In this case: how the herbicide affects the plants tissues or cells to create enough system failure and injury that the weed plant dies. This is the herbicide’s mode of action. Herbicides, insecticides, fungicides, etc.. are grouped according to mode of action – how the product gets the job done. This is very useful information when planning an herbicide rotation schedule to prevent resistance and to avoid experiencing undesired side effects of a particular mode of action. With preemergent herbicides, application timing, application coverage, and single versus split applications will impact the results obtained from the herbicide you apply.
Preemergent herbicides are typically applied late winter for control of many summer annual weeds, Application timing is critical with these products to obtain desired results. The temperature of the soil as it starts to warm up in the spring is going to be the primary indicator of the pending arrival of various weed species. For example, smooth and large crabgrass germinate when 24 hour mean soil temperatures (four inch depth) reach 55 degrees F. Goosegrass germinates when 24 hour mean soil temperatures reach 60 degrees F. Since these herbicides control susceptible species as they grow through the herbicide treated zone, the herbicide barrier must be in place before the weed seeds germinate. In most areas in NC, this occurs in mid- to late-March so products need to be applied late February, early March.
Common preemergent herbicides labeled for use in turfgrass areas include benefin, dithiopyr, oxadiazon, oryzalin, pendimethalin, prodiamine, and trifluralin (these are common names and are often sold under various trade names). Because weed species absorb the herbicide as it grows through the treated zone, uniform application coverage is imperative for acceptable weed control. No holes in the herbicide’s blanket of coverage, please.
Although an herbicide product may be an excellent one, we are working with a living, dynamic environment under changing conditions. Weeds are vigorous rivals against our grass plants and no herbicide is perfect all the time. The ideal status of a healthy, thick and strong turf requires a management plan for out competing the weeds. Turfgrass managers must make timely herbicide applications, select the proper herbicide, and always read and follow product label instructions.
For more information on how to identify the grass plants you have control weeds for your particular lawn review the following turf management guides at http://www.turffiles.ncsu.edu:
Minda Daughtry is Horticulture Agent for North Carolina Cooperative Extension in Lee County.