Invasive Alert: Japanese Knotweed
There is a monster lurking along our rivers and streams.
It is creeping along, year after year, destroying property and choking the life out of its surroundings as it goes.
This monster is native to Japan, China and parts of Korea and Taiwan. In Japan, it reportedly grows on volcanic soils, mountainsides and open areas. Survival in its native environment has caused it to be strong, resilient and nearly impossible to destroy.
It is not an ugly monster. In late summer and early fall, you will see its pretty, frilly flowers along the roads and waterways. It is hard to think that these patches of blossoms, covered in bees and other pollinators, could be so dangerous and destructive. However, its beauty belies the damage it does to our native plants and biodiversity.
It is not just here in the U.S., but around the world that Japanese knotweed has become troublesome.
Japanese knotweed (Fallopia japonica) is known globally by many colorful names, including monkey weed, donkey rhubarb, and Hancock’s curse. My father-in-law called it fleece flower.
It was introduced to the U.S. in the late 1800s, as many invasive species were, as an ornamental plant that proved useful for ground cover and erosion control.
The secret to its success is how it spreads.
Like many plants, it spreads through rhizomes underground. However, Japanese knotweed is more aggressive. Its root system reaches farther than many plants. Cutting the plant without destroying the root system causes it to stretch farther and grow thicker. The plant is fast growing and can add up to 4 inches per day and reach a height of 12 feet.
Back in Japan, quick and aggressive growth serves it well, as knotweed is the first species to reemerge on ash-covered hillsides after a volcanic eruption, years before other plants. Surviving there enabled it to thrive nearly anywhere. It is known to grow in nearly any soil and can even “punch” through asphalt and concrete foundations.
That is why I cautioned our friends, Joanna and Mark, last fall. During a rare pandemic visit, we sat outside, masked-up, on a lovely September afternoon. As I looked down from their deck into a valley below, I noticed that the creek had knotweed growing along the bank. I mentioned in passing that it was invasive, and they should keep their eyes on it.
Over the late fall and early spring, that small patch of knotweed swept up their hillside and is now encroaching on their deck and driveway. Together, we discussed some control techniques and they began a counterattack.
It is a battle that many property owners may soon face. In the United Kingdom, where Japanese knotweed was once common in backyard gardens, merely having it on your property can depress its value. U.K. mortgage lenders can deny a loan because of the cost to remove the invasive plant, and the risk it poses to surrounding property. It is a situation that could easily happen here if we don’t do more to eradicate this invader.
So, what can you do if you find Japanese knotweed on your property? The good news is it is not hopeless. Yes, it will be a lot of work, but it is possible to control or eliminate it.
Recommended elimination involves a combination of repeated mechanical cutting, manual removal, and the application of an herbicide. The process involves cutting, clearing, and applying chemicals over the course of a few years to ensure destruction of the plant’s entire rhizome network underground. Even a piece of surviving root will come back to life quicker than a Hollywood zombie.
More information, including a detailed control plan and application calendar, visit the Penn State Extension website, https://extension.psu.edu/japanese-knotweed. Application of chemicals should follow all safety guidelines. Be sure to only use an approved herbicide, especially if the application is near water. For large areas of control, the assistance of a landscape professional may be necessary.
Hopefully with more awareness, a little work, and a lot of luck, we can keep this monster at bay.
If you would like to learn more about invasive species, the extension can help. More information is available here: https://extension.psu.edu/invasive-species.
If you have gardening questions, you can email fayettemg@psu.edu and a master gardener will respond. You can also find us on Facebook at https://extension.psu.edu/programs/master-gardener/counties/fayette where we share gardening news and tips and announce webinars and special events.