Knotweed Identification and Control
Knotweed is fast growing and extremely aggressive. It invades river and creek banks, permanently displaces native vegetation, destroys critical fish and wildlife habitat, and reduces recreational opportunities.
After only a few years, it can be virtually impossible to control knotweed if left unchecked, so please ACT NOW by educating yourself.
Download the pdf here to learn how to help save Pacific Northwest rivers from knotweed!
Seasonal Guide to Identifying Japanese Knotweed
Knotweed is a bamboo-like perennial that grows in dense stands 6 to 12 feet tall. It has hollow, reddish cane-like stems with swollen stem nodes that are surrounded by thin papery sheaths. Leathery, spade-shaped leaves are arranged in an alternating pattern along the stems. The former popularity of Japanese knotweed as an ornamental shrub stems from their attractive flower clusters, which emerge during late summer and fall. These showy, plume-like clusters contain hundreds of small, creamy white to greenish white flowers and extend upward from the upper leaf axils. The fruits are 3-sided and papery, each containing one glossy dark seed.
At the end of the growing season, the plants die back to the ground. However, the dead reddish brown canes often persist throughout the winter. These dry canes can be a fire hazard and can also impede access to streams by both people and wildlife. Additionally, due to knotweed's ability to sequester nutrients into its root system at the onset of winter, dead knotweed canes and leaf litter have less value than our native deciduous plants as a food source for soil microbes and aquatic invertebrates, which could have a negative impact on the aquatic food web.
In early summer, the stems emerge from rhizomes deep underground to shoot to over 2.1m (7ft), suppressing all other plant growth in the area. These canes grow quickly and are incredibly strong - even puncturing asphalt, concrete and brick in an effort to reach sunlight. Underneath the ground, the root system is extensive and can persist over many years. If you attempt to dig out the roots of a knotweed patch, you should NEVER put any of the root or stem material in your compost or yard waste. You must carefully gather all root and stem material, bag it, and throw it in the garbage.
In late summer and fall, the knotweed plant prepares for the winter by sending sugars and nutrients to the plants’ rhizomes. As a result, herbicide applications are most effective after flowering, and up until the first killing frost. However, if you are planning to tackle knotweed with herbicides, it’s a good idea to wait until after the plants are done flowering to limit impacts to pollinators. About a month after it has been successfully treated, you will notice knotweed leaves beginning to curl and turn brown (see photo to the left). Review the videos posted below to determine the best treatment strategy for your property.
Instructional Videos - How to Indentify and Treat Invasive Knotweed
A BIG thank you goes to King County, for giving us permission to use and modify these excellent instructional videos! Although these videos were made in Washington state, the information is relevant to the Luckiamute watershed.
Introduction to Knotweed
About the plant, how to identify it, and how it damages river banks.
How to control knotweed without chemicals (for small infestations)
If you have a small knotweed patch, watch the following video to learn about digging out this invasive weed, cutting and starving roots, and cutting, covering and shading roots as control methods. NOTE: This labor-intensive method is suitable for small patches of knotweed only.
How to control knotweed using herbicide spray
Learn how to apply chemical herbicide on knotweed using a backpack sprayer.