One of the interesting quandaries of having turf infested with prostrate knotweed is that by the time you notice it in the spring, it’s already too late.
Prostrate knotweed, Polygonum aviculare, has been around much longer than Ron Calhoun has, but he has learned a lot about it during the last 15 years. Calhoun, Michigan State University Cooperative Extension environmental turfgrass specialist, is responsible for weed control trials at the university and says knotweed stands out because it is usually the first weed to emerge in the spring, and it has one overriding cause.
“The key to encouraging prostrate knotweed turned out to be high traffic,” Calhoun says of his observations in both controlled weed plots and in commercial facilities.
The way to correct and/or prevent knotweed is to keep soil in good health. The plant, which is low-lying and sends out runners with dull blue-green leaves, may seem to be shallow-rooted when it is pulled by hand, but it sends out a cobweb of extremely fine roots that can penetrate hard soils to a depth of 4 feet or more. It propagates by seed and appears in the spring as the snow melts, long before other weeds. Its tiny, pinkish flowers are distinctive, and it doesn’t look like a broadleaf at first emergence.
“It looks like grass at first. Then you’ll get a couple of nice days, and those leaves will unroll,” Calhoun says. Regardless of cultural control or chemical treatment, it must be done prior to spring emergence. The long-term solution is to renovate the soil, though simple aeration can help, and redirect traffic away from compacted areas.
Calhoun says postemergence control is often not effective because the herbicides don’t translocate well during the summer in knotweed. However, liquid herbicides containing carfentrazone (Quicksilver) have been effective in burning back knotweed during the growing season. Pyraflufen-ethyl (Octane) was not in his trials, but he has seen it work on similar broadleaf weeds. Premixes such as Speed Zone, as well as several consumer products, also give some degree of control, but more than one application may be necessary, although it can be spot-sprayed from a backpack sprayer.
Calhoun recommends using preemergent herbicides (applied in the fall) in tandem with soil conditioning to prevent knotweed from germinating in the spring. In order to maximize the amount of preemergent control in the spring, application should be delayed as long as possible in the fall.
The most effective preemergent herbicide in his trials has been isoxaben (Gallery), a dry, flowable that is mixed with water and irrigated in after application. Calhoun notes that some preemergent herbicides can be used to control both crabgrass and knotweed when they grow alongside each other. Isoxaben is not one of them, but it does keep knotweed from germinating in the spring.
Prodiamine (Barricade, ProClipse and others) can be utilized in a dispersible granule or loaded onto a fertilizer granule used in a fall fertility program. It has excellent activity on knotweed, and also has the longest residual action of any of the preemergent products.
Dithiopyr (Dimension EW) is a sprayable liquid, and can also be loaded onto a dry fertilizer product.
Another product available as a liquid or on a fertilizer granule is pendimethalin (Pre-M, Pendulum and others). Calhoun says this is a common ingredient in efficacious consumer products used primarily to control crabgrass, but it is also effective on knotweed.
“They all inhibit root formation,” he says of the preemergent herbicides. Calhoun says that even large areas of prostrate knotweed can be converted back to desirable grasses.
Don Dale resides in Altadena, Calif., and is a frequent contributor to Turf. He has covered the green industry for more than 10 years.