Knotweed thrives in compacted soils, so a thorough aeration is the first
step in control. This weed will not compete in a healthy lawn.
Chemically, there are two options. Knotweed is an annual that germinates in
late February or early March, so a preemergence herbicide can be used in the
late fall (about now). Pendimethalin (Scotts Halts), Surflan (Weed Impede),
Barricade, Dimension and XL are labeled for knotweed.
(Note: Pendimethalin, Barricade and Dimension can be used on all Kansas
turfgrasses, while Surflan and XL can only be used on tall fescue and
warm-season grasses). The other option is to use a combination postemergence
product such as Trimec, Weed-Out, Weed-B-Gon or Weed Free Zone after the
knotweed has germinated in the spring but is still young.
If spring seeding is planned, your options are more limited. Buctril can be
used on commercial sites and has a very short residual. It must be used on
very young knotweed to get control. Trimec and others require a month before
seeding. Obviously, don’t use a preemergence herbicide if you are trying to
get new seed established. For homeowners seeding in the spring, tilling will
control knotweed adequately without using a herbicide. If seeding without
tilling (e.g., overseeding using a slicer-seeder), then use a combination
product such as one mentioned above just after the knotweed comes up in the
spring, and be sure to wait at least a month before seeding.
By: Ward Upham