Hello, I live in West Eugene and we have a problem with Japanese Knotroot on the side of our house. I am wondering if there is a more permanent solution to getting rid of it. I thought of using weedkiller, but I am concerned about the bees and my dogs. Any recommendations?
Lane County Oregon
Knotweed is definitely a hard to manage noxious weed! OSU Extension has a detailed publication called, "Biology and Management of Knotweeds in Oregon: A Guide for Gardeners and Small-Acreage Landowners." It's important to understand the biology of knotweed when you're developing your management plan. Knotweed can readily regrow from even small pieces of stems or roots. Even pulling the roots can encourage more growth of this plant! The knotweed publication goes into much more detail and also lists several herbicides that can be effective. Best application time is the fall (August to October) before the leaves drop. Always read the label for proper application and use instructions.
You can help avoid pesticides affecting pollinating insects by proper timing of your application. It's possible there will still be flowers on the knotweed in the fall so make your herbicide applications when the pollinator are least active - late evening after they have stopped foraging for the day. For much more information on protecting bee and pollinators from pesticide applications check out the OSU Extension publication, "How to Reduce Bee Poisoning for Pesticides". This publication is geared towards commercial growers and beekeepers, but there are some nuggets that are useful to the home gardener.
Hope this helps and thanks for using Ask An Expert!