Scout now to prevent summer weeds from being a problem next year

October 25, 2022 -  By
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Purslane (Photo: PBI-Gordon)

As summer turns to fall, turf managers turn their attention to scouting for weeds, such as purslane, knotweed, foxtail, plantain and spurge. Turf managers must take steps now to prevent them from being a problem next year.

To help prevent the emergence of these weeds next spring, Eric Reasor, Ph.D., southeast research scientist with PBI-Gordon, and Gil del Rosario, western turf and ornamental district leader and market development specialist with Corteva Agriscience, share some management tips.

Scout now

Reasor and del Rosario say the best management strategy for purslane, knotweed, foxtail, plantain and spurge is to observe where infestations are prevalent in the turf in the summer/fall and develop a preemergence-postemergence herbicide treatment program based on that.

“The fall is a great time for LCOs to carefully observe the properties they manage to locate where these weeds are most prevalent,” says del Rosario. “This approach will enable the development of a good spring control program.”

Get ready for spring

“Purslane, knotweed, foxtail and spurge are annual weeds that emerge in the spring and summer months,” del Rosario says. “Even though it is perennial, plantain produces seeds that will emerge in spring.”

Both del Rosario and Reasor say that a preemergent application this fall may not last through the winter to be effective in the spring.

“Fall application of preemergence herbicides does not provide sufficient residual to control summer annual weeds or perennial weeds,” says Reasor.

He suggests applying a broad-spectrum preemergence herbicide in the spring as the most effective approach.

Del Rosario agrees, noting that “a preemergence herbicide may not have the expected control of these weeds because residual effects cannot last until spring when these species emerge.”

Best preemergence herbicides

Reasor says these annual summer weeds start to germinate soon after soil temperatures reach 50 F in the spring. This is the best time to think about a preemergent application.

To achieve the best control, Reasor cautions LCOs to bear in mind that most preemergence products work better on grasses than on broadleaf weeds.

“When the application of preemergence herbicides is in early spring, control of escapes is possible if a follow-up application is within 30 to 60 days,” he says.

Reasor suggests herbicides containing dithiopyr, pendimethalin and prodiamine for cool- and warm-season turfgrass. On warm-season turf, he suggests preemergent herbicides containing indaziflam.

Del Rosario suggests a targeted approach to active ingredients for each weed.

“Preemergence herbicides containing isoxaben will provide good control of purslane, knotweed and spurge,” says del Rosario. “Preemergence herbicides containing dithiopyr will control foxtails as well as crabgrass and small-seeded broadleaf weeds. The residual control of these active ingredients depends on temperature, moisture and soil type.”

Del Rosario reminds LCOs that a full rate of isoxaben or dithiopyr applied in spring should provide enough residual to protect turfgrass into summer.

Follow-up treatments

According to del Rosario, a good management plan for control of purslane, knotweed, plantain, foxtail and spurge should include applying an effective broad-spectrum postemergence herbicide in early summer.

“Postemergence herbicides containing one or more of the following active ingredients — 2,4-D, clopyralid, fluroxypyr meptyl, halauxifen-methyl or triclopyr — will provide good control of these weed species,” he says. “LCOs must pay close attention to the dominant turf type in the area to avoid turfgrass injury.”

Reasor says depending on location, late spring may also be a good time to apply a postemergent herbicide. Other active ingredients to consider include carfentrazone-ethyl, dicamba, dichlorprop, metsulfuron, MCPA, MCPP-p, mecoprop-p, sulfentrazone or triclopyr.

“Many postemergence products contain two or more active ingredients,” he says. “Applicators need to read the label, observe all precautions and follow directions carefully.”

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