Photo courtesy of Janet Carroll

Knotweed is on the Washington State Department of Agriculture Prohibited Plant List. You cannot transport, sell or buy these plants.

Knotweed is one of the world’s most noxious weeds. Once established, it takes on a life of its own, shading out our native plants and destroying habitat.

As I drive up 92nd Street, to my right and to my left I see patches of knotweed; on 44th Avenue and 76th Street, along Goat Trail Road and Japanese Gulch Creek – this menace is there.  

Knotweed was brought in from Asia as a landscape plant.

It takes only a small, one-inch fragment of live knotweed plant to create a new patch.  The plant’s large masses of underground rhizomes move into new areas, producing new plants.

Four types of knotweed choke out or displace native or ornamental plants: Japanese, giant, bohemian and Himalayan.

All four are on the Washington State Department of Agriculture Prohibited Plant List. You cannot transport, sell or buy these plants.

It is also on the Snohomish County Noxious Weed List and must be controlled by the landowner.

 I have seen at least three of the four knotweed species in Mukilteo; one clump is in a nicely landscaped front yard.

Along road right-of-ways, knotweed is spread by roadside mowing that cuts the plant into small pieces. These pieces find some moist soil down the road, where they take root.

Along waterways, pieces are carried from upstream plants, creating patches all the way downstream to the mouth of the waterway.

It can also hide in landscaping-soil material, submerged within the dirt, and placed on someone’s yard.  

Rarely does it reseed itself. And once there is one patch, there will always be more.

This plant is extremely hard to kill. I tried getting rid of the plant by cutting it back and covering it, blocking the sunlight.  It didn’t work.

I tried cutting it back whenever it grew, carefully putting the cut plant into a bag and letting it dry before disposal. Again, it didn’t work.

Digging up the plant could work if it is small, but the entire root must be removed or the plant will come back. That’s a challenge.

Not knowing any other natural way to kill the plant, I tried spraying the leaves of a few Japanese knotweed clumps with glyphosate (Round-Up), an herbicide with somewhat minimal toxicity if there is such a thing.  It didn’t work.

However, after researching methods for killing knotweed, I learned that spraying the plant is actually the best – I just wasn’t doing it right.

According to Sonny Gohrman from the Snohomish County Weed Board, to be effective with the spraying method, a gardener should:

Add a small amount of minimally toxic surfactant to the glyphosate. The surfactant holds the herbicide on the leaves allowing the maximum amount of chemical to be taken into the plant.

On July 4 give or take a few days, cut back the knotweed; let it dry before disposing of it and take care to pick up all the plant waste.

In fall when the plants are full-grown, spray the leaves. Repeat each year until the plants are gone.

Another method is to inject the herbicide into the plant stalk with a specifically designed needle. This approach has been somewhat successful.

  However, it puts a lot more glyphosate directly into the plant from which it can leach into the ground or nearby waterways, and it is costly.

If you live along a body of water, only certain herbicides can be used to get rid of any noxious weed, and you need an applicators license.

To increase your chances of successfully removing knotweed, give Sonny Gohrman a call at 360-862-7523.  He has had lots of experience with this plant.

Hopefully some botanist will find a safe biological control to kill this plant for the long-term.

Janet Carroll is a member of the Mukilteo Wildlife Habitat Project, which has a goal of certifying Mukilteo as a community wildlife habitat.  Help Mukilteo get community certification by certifying your own yard.  For more information on the project, go to http://www.mukilteowildlife.org"www.mukilteowildlife.org.

Do you have questions about how to get rid of noxious weeds?  E-mail Carroll at mukilteowildlife@gmail.com.