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Japanese Knotweed: America’s Most Damaging Weed

Here’s what you need to know about Japanese knotweed from what it is to how to identify it and the damage it causes. We top that off with information on how to protect your home.

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While there are many weeds throughout the U.S., most of which are not only noxious, invasive, and cause damage, it’s the Japanese knotweed that rises to the top of this odious list.

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What Is Japanese Knotweed?

Japanese knotweed grows up to three inches per day in season, reaches up to 10 feet tall, and roots can grow 20 feet deep. The rhizomes can spread up to 70 feet from the nearest stem. 

With this type of growth rate and pattern, it can damage home foundations, driveways, walkways, and gardens, crowding out other plants.

It can re-grow from a tiny bit of the plant, often as small as a half-inch. Plus, its underground network of rhizomes sends out lateral shoots and roots that can lie dormant for years. This makes it extremely difficult to eradicate.

The U.S. Department of Agriculture estimates it has invaded 41 states. Continental states are shown in the map below. It has also invaded Alaska but not yet Hawaii.

How To Identify Japanese Knotweed

The most distinctive aspect of the plant is the hollow bamboo-like stem. It’s green with purple speckles and grows in segments. The leaves are heart-shaped bright green, also with purple speckles. The leaves grow at staggered intervals on the stems and drop off as winter approaches.

From late August through September, creamy greenish-white flowers form in clusters that grow up to four inches long. When you’ve found the plant in flower, you can be assured it has taken firm root and will be quite challenging to remove.

You can find a comprehensive guide to identification, including a video guide, at Knotweed Help

Damage From Japanese Knotweed

The stems and rhizomes of the Japanese knotweed can not only grow high, wide, and far, but they are very strong. In their wide growth pattern, the roots can find drainpipes and any joints or cracks. Just as with tree roots, they can enter the pipes, clogging them and causing cracks and breaks.

They can also find imperfections in concrete and asphalt, with stems growing through any openings and widening the cracks or even breaking up the walkway or driveway. They can do the same thing with stone or brick retaining walls, finding any small openings, growing through them and causing damage.

They can also cause damage to home foundations. Growth along the walls of a home’s foundation can create stress and cause cracks. The stems and rhizomes can also find any cracks in the foundation and enter to do still more damage.

The spread and growth of this invasive weed comes at a huge economic cost. As one example, since 2010, New York City has spent over $1 million on eradication efforts for a 30-acre patch of Japanese knotweed.

That’s an example of the cost of eradicating the weed. There can also be an impact on a home’s value. Lenders are starting to look closely at any infestation before providing a home mortgage. On top of that, there’s also the cost of repairing the damage.

How To Protect Your Home

Eradicating the Japanese knotweed is extremely difficult. There are several steps you can follow that include: cutting the stems, removing the clippings, covering the area with a tarp to eliminate light and water, then placing a plastic barrier around the area to stop root spread. 

Another option is to excavate the entire area, at least to a depth of 20 feet. You can also try a glyphosate-based herbicide, the main ingredient in Roundup. All these approaches take time and considerable effort.

You can also consult an expert in eradicating knotweed who has the expertise and experience to remove the plant without spreading it elsewhere in the process.

If you find Japanese knotweed on your property, contact a professional to inspect your home’s foundation, crawl space, and basement to ensure the weed has not caused damage to your home.