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Annual Weed Control
Late-Summer
By Dave Han, Ph.D., Auburn University & Alabama Extension
Al ab ama Tu rf Tim e s > >> Spr ing 201 6
S
ome of the most frustrating weeds to control are the annual weeds that seem to take over in the late summer. You’ve seen them before: spurges, chamberbitter (or gripeweed) and doveweed are among the biggest pains for turf managers from June through September. Why is that so? In a nutshell, it’s because their seeds don’t stop germinating until the fall. With some weeds — like crabgrass and goosegrass — late-winter and spring applications of preemergence herbicides can provide good control because their seeds germinate only for a few months in the spring. That’s why a traditional spring pre-emergence herbicide program consists of an application in late January or early February, followed by a second application in April. But even the longest-lasting preemergence herbicides provide only two to three months of control in the summer. By July, any herbicide applied in April will be long gone, and that’s
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when weeds that can germinate in very warm soils become problems. Although many post-emergence herbicides can kill summer annual weeds, many of them do not give longterm residual control — they will kill what is present when they are sprayed, but then new weed plants germinate from the seeds that are in the soil. For example, if you are a lawn care operator who cannot get back to respray a given lawn within four weeks, then clients will probably notice weeds returning before you do, when you use standard broadleaf herbicides. However, recent work done by Dr. Scott McElroy’s team at Auburn suggests that herbicide mixtures containing sulfentrazone — for example Surge (sulfentrazone, 2,4-D, dicamba, and MCPP) and Blindside (sulfentrazone and metsulfuron) — can give longer lasting control. Sulfentrazone is known to have pre-emergence activity, and these products seem to be able to give at
least six weeks of control of doveweed, sometimes more. So, if you are looking for a product to knock down summer annuals post-emergence, but also have a long enough residual to carry through until you can re-treat, products with sulfentrazone could be an attractive option. Another option is to use a third preemergence application in June, but many turf managers shy away from this because many popular pre-emergence herbicide inhibit root growth, and turf managers are concerned about suppressing the turf’s ability to spread and grow new roots all summer long. Some preemergence options do not inhibit root growth and have general labels, including home lawns, such as indaziflam (Spectacle) and dimethenamid (Tower), but they may not be cost-effective for all users. Note that oxadiazon (Ronstar) is not labeled for use on residential properties, but it could be an option on other turf. v