It may look innocuous or even pretty but the Japanese Knotweed is invading our landscape. Most people aren’t even aware of how noxious it can be until it’s too late. According to Wikipedia it is listed by the World Conservation Union as one of the world's worst invasive species. It was brought into the USA as an ornamental shrub and for erosion control.
This shrub looks a bit like bamboo and can grow up to 15’ tall but it often appears as a much smaller plant. It has large oval leaves 3-5.5” long and clusters of small cream-colored flowers. It is in bloom now in mid-September and you’ll often see it growing along the side of the road and in fields and the edge of the woods.
If you’ve ever had it in your yard and tried to get rid of it, you will know about its “secret weapon.’ This plant spreads by underground runners. BUT … these aren’t your typical underground runners. The roots grow quickly and they can grow almost 10’ down with a horizontal spread up to 65’. The plant itself can grow up to 8” per day.
It steals water and nutrients from other plants in the area and quickly kills or weakens other plants it encounters. Even though it was introduced to control erosion, it actually causes erosion by crowding out any low-growing plants and leaving bare ground near its base. It can also affect wildlife by eliminating native food plants and changing native habitat.
You can try to dig it out, but leaving even a ¼” piece of root will allow it to reappear. The recommended treatment is to apply a glyphosate weed killer such as Roundup. Even this can take up to 5 years of reapplications to completely eliminate the plants. If you choose to use a different type of herbicide, be sure to verify it will be effective against this particular plant. Some herbicides only put the roots into a dormant state and don’t actually kill them. The best time to spray is when the plant is in bloom and its circulation system is most active.
Do not mow it as some small pieces may actually take root elsewhere. Any parts of the plant that you cut down need to be bagged and put in the garbage. It can also be burned if that is allowed in your area, but be sure that every bit of the plant has been incinerated.
For those who prefer not to use chemicals, it will be a long process to eliminate these shrubs. Cut the plants to the ground and dig out the rhizomes. Be sure to bag all plant matter and dispose of it in your normal trash. Leaving even a ¼” piece of root will allow it to regrow. This plant can grow in literally any conditions and is tolerant to salt.
If you find this in your yard, try to get rid of it before it’s a huge problem and if you see it growing wild, report it to your local Parks Department.