Foliage Friday: Knotweed

This week’s plant is knotweed, a.k.a. Polygonum bohemicum (Bohemian knotweed) or Polygonum cuspidatum (Japanese knotweed).

Knotweed is a flowering perennial with bamboo-like stalks, broad leaves, and clusters of tiny white-to pink flowers plant. It’s often found along roadways or near streams and grows 4-13 feet tall. Knotweed is not native to the U.S, and in Seattle, it is a Class B Noxious Weed. It grows thickly and vigorously, and outcompetes other plants from growing. There are no known biological controls of knotweed, although this is an area of active research. I recommend reading this article from Slate, which is a brief fascinating history of knotweed and the biological impacts it is having.

(Side note: I don’t see horsetail on Washington’s Noxious Weed List….)

Knotweed came to my attention via the Community Garden in Seattle. There is a stream near the Community Garden that is lined by knotweed. The city has partnered with a land conservation non-profit company in Washington State, called Forterra, to deal with the knotweed. Forterra has decided to use the synthetic herbicide, Imazapyr, to kill the knotweed. You can bet that the Community Garden group was not too pleased to hear that this chemical would be sprayed near the garden.

There was a flurry emails back and forth between the people at Forterra and the Community Gardeners. Forterra said they would be using a low dose of Imazapyr, that they wouldn’t be spraying when the winds are above 10 mph or if it is raining, and claimed that the herbicide wouldn’t “drift” more than 10 feet away. They also claimed that they wait until after the plants stop flowering so as not to disturb pollinators…..

….There are definitely still flowers and pollinators there. This point was brought up in the emails, with an indirect non-response from Forterra. Hmmm… while I trust that Forterra’s mission of conservation is noble, and I also believe that there is a time and a place for certain synthetic herbicides, and that knotweed might necessitate it’s use, you would think they could wait a week or two more for the blooms to die, right?