How to slam summer weeds

L to R: Doveweed, knotweed, purslane (Photos: Bayer, Dean Mosdell, Ph.D., Bayer)

Summer is almost here. With the warmer weather comes some difficult-to-control weeds.

“A healthy, dense turf is the first line of defense against weeds,” says Todd Lowe, Bayer Green Solutions Team technical service manager for the Florida region. “So, optimize irrigation, fertilization and mowing.” In the event that the first line of defense has broken down, four experts gave us their tips to control and defeat some pesky weeds, including prostrate knotweed, purslane and doveweed.

Prostrate knotweed

“Prostrate knotweed is a summer annual broadleaf weed in the smartweed (Polygonaceae) family,” says Dean Mosdell, Ph.D., technical manager, Syngenta Professional Solutions. “It is one of the earliest germinating summer annuals and is often missed by preemergent herbicides targeting crabgrass.” He adds that key identification characteristics are matted prostrate growth, pink-white flowers in leaf axils, alternate leaf growth and blue-green leaf color.

“Prostrate knotweed can outcompete turfgrass in compacted soil and is considered an indicator weed of soil compaction,” says Eric Reasor, Ph.D., research scientist, PBI-Gordon. “If knotweed is a persistent problem, decreasing compaction with aeration should be considered.”

Reasor says that postemergent control is typically much easier when herbicides are applied earlier in the season rather than later. The dense canopy and thick stems associated with mature plants can make control difficult in late summer. Typical 3- and 4-way herbicide mixtures containing 2,4-D, dicamba, carfentrazone, fluroxypyr or triclopyr can provide acceptable postemergent control. Mosdell adds that preemergent herbicides such as (prodiamine) or isoxaben products applied in late fall are effective or combined with Mesotrione for early postemergent activity at crabgrass timing. Postemergent herbicides that contain a higher percentage of dicamba or clopyralid are also effective. He notes that 2,4-D is less active on this weed species.

Purslane

Count common purslane as a summer annual that is relatively easy to control. However, it often rears its head at the most inopportune times, says Aaron Hathaway, technical services manager, T&O for Nufarm.

“(Purslane) becomes most problematic during spring seedings and can quickly germinate and mature before seeded plants can. It’s during establishment that annual broadleaf weeds like common purslane can be a challenge — newly germinated seedling plants are often too sensitive to treat with an herbicide at all,” Hathaway says. “When purslane pops up in these seedings, a broadleaf herbicide with a protox inhibitor can be a great tool to quickly burn down common purslane before seeding or before seeds have germinated to remove competition that could stifle a new seeding project.”

Doveweed

Doveweed is a grasslike summer annual with fleshy, creeping stems that root at the nodes. “Its lance-shaped narrow leaves have parallel veins and alternate branching,” Lowe says. “Leaf sheaths have short hairs on the upper margins. Doveweed produces an open cluster of purple flowers throughout summer. Seeds germinate later than other summer annual weeds, such as crabgrass and goosegrass.”

Unlike purslane, Lowe says doveweed can be difficult to control, and a combination of preemergent and postemergent herbicides is most likely needed. “Apply a preemergent herbicide in areas historically infested with doveweed when soil temperatures at the 4-inch depth approach 70 degrees in spring,” Lowe advises. “If targeting just doveweed, application timing must be delayed relative to traditional preemergent application timings that target crabgrass and goosegrass to provide maximum length of residual control. To target doveweed, in addition to other summer annuals like crabgrass and goosegrass, the best approach would be a split-application preemergent program, combined with a postemergent program.”