What?! Summer weeds, I have weeds right now! The winter annuals like chickweed and henbit are up and like many weeds, the best time to control them is before they come up ie. Preemergent. These winter annuals would have been best controlled with herbicide applications in the fall. Next week I will talk about postemergence weed control.
It’s time to think about preemergent herbicide applications for summer annual weed control in turfgrass environments. Preemergent herbicides offer a great option for select annual grass and broadleaf weed control in warm- and cool-season turf. Preemergent herbicides are commonly used for crabgrass and goosegrass control but also control other grass and broadleaf weeds propagated from seed. As with any herbicide, one must be mindful of the herbicide mode of action. Specifically, with preemergent herbicides, application timing, application coverage, and single versus split applications among other factors are crucial to the results obtained.
Preemergent herbicides are typically applied late winter for control of many summer annual weeds, particularly annual grasses including crabgrass and goosegrass species. Application timing is critical with these products to obtain desired results. Specifically, smooth and large crabgrass germinate when 24 hour mean soil temperatures (four inch depth) reach 55 degrees F whereas 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 established prior to weed seed germination. In most areas in NC, this occurs in mid- to late-March so products need to be applied prior to this time, in other words, NOW. If you are not able to track 24 hour mean soil temperatures on-site, you can visit http://www.nc-climate.ncsu.edu/products/wx and find a site in your geographic region to track 24 hour mean soil temperatures. I checked the average soil temperatures in Raleigh and they are 47 degrees F and in Wallace 51 degrees F.
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). Additionally, indaziflam is a preemergent herbicide recently registered by Bayer Crop Science. Although each of these products offers acceptable control, one should consider the herbicide mode of action. Most of these products, including benefin, oryzalin, prodiamine, pendimethalin, and trifluralin are members of the dinitroaniline (DNA) herbicide family. These herbicides inhibit mitosis, or plant cell division. Unlike the name “preemergent” suggests, preemergent herbicides do not actually prevent weed seed germination. Instead, as weed seed germinate and grow through the herbicide-treated barrier or zone, the herbicide is absorbed by emerging shoots and/or roots resulting in death of susceptible species. Because weed species absorb the herbicide as it grows through the treated zone, uniform application coverage is imperative for acceptable weed control.
Herbicide family, or mode of action, is important because select herbicide families may negatively impact turf species. Specifically, in areas that experience high traffic or are continually being “grown in”, select herbicides can hinder the recovery or lateral spread of warm-season turfgrasses. Examples of these high traffic areas include cart path entranceways on golf courses or areas of concentrated traffic on athletic fields. Specifically, because DNA herbicides and dithiopyr inhibit plant cell division of turf and weed species alike, DNA herbicides can cause “clubbed” rooting of turfgrasses on stoloniferous turfgrasses including bermudagrass, zoysiagrass, centipede, or St. Augustine. With “clubbed” roots, the warm-season stolons are not able to “peg” down and may prevent recovery of thin or damaged turf. Symptoms of “clubbed” roots are easy to observe in field environments as stolons of the affected turf will pull up easily.
For turfgrass areas that are not established or experience high volumes of traffic, oxadiazon may be an alternative to DNA herbicides for annual grass control. Oxadiazon belongs to a different family of herbicides and is only absorbed by emerging shoots. Although it is not labeled for use in home lawns, it is labeled for golf courses and athletic fields. Oxadiazon offers great control of summer annual grasses without inhibiting the lateral spread or recovery of warm-season turfgrasses. If one chooses to use oxadiazon, application timing is extremely crucial as oxadiazon is only absorbed by emerging shoots; therefore, if an application is made after weed seed germination, it will not be effective. It is therefore recommended that oxadiazon be applied about 2 weeks earlier than DNA herbicides. Additionally, with oxadiazon, the full labeled rate should be applied in a single application as opposed to split applications with DNA herbicides.
Additionally, most preemergent herbicides must receive rainfall or irrigation to work well. In summary, preemergent herbicides offer a great option for weed control in turfgrass environments. However, turfgrass managers must make timely herbicide applications, select the proper herbicide, and always read and follow product label instructions. I would like to reiterate although a herbicide may be good to excellent, no herbicide is perfect 100% of the time. We would like to offer the following suggestions to improve annual weed control with premergent herbicides:
- With oxadiazon, we recommend it be applied 2 – 3 weeks before typical preemergent timings.
- Make certain preemergent herbicides are irrigated or receive rainfall soon after application.
- See table below.
Active Ingredient Use Split Application ( Y or N) Sequential Application Timing
Dithiopyr Yes 6 – 8 weeks after initial
Pendimethalin Yes 8 weeks after initial
Prodiamine Yes 8 weeks after initial
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Brenda Larson is the Horticulture Agent for North Carolina Cooperative Extension in Lee County.