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Japanese Knotweed: Top Weed for Home Damage in South Carolina

Learn how to identify it and how to protect your home and foundation.

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In South Carolina, we have plenty of weeds and invasive plant species. Clemson University has published Exotic Invasive Plant Species of South Carolina, where it lists 54 species. At the top of the list is the Japanese honeysuckle, which is found in every county in our state. Kudzu is ranked high as well, with only seven counties without an infestation. These plants take over wild areas, gardens, and lawns, destroying trees and plants. 

If we dig somewhat deeper, we find the Japanese knotweed. This invasive plant causes the most damage to homes. It wreaks havoc on home foundations, driveways, walkways, and patios. It finds any cracks or weak spots, growing through them, gradually expanding, and causing still more damage.


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What Is Japanese Knotweed?

The Japanese knotweed has a significant growth rate along with an underground network of rhizomes with lateral shoots and roots. During the warm months, the stem can grow up to three inches per day. It can reach 10 feet tall and the roots grow as much as 20 feet deep. And those rhizomes can spread up to 70 feet from the nearest stem. Not only that, but it can regrow from as little as one-half inch of stem, root, or rhizome.

It has the distinction of being listed in the Top 100 World’s Worst Invasive Alien Species. It can grow. It can spread. And that makes it very difficult to eradicate.

How To Identify Japanese Knotweed

The stem is green with purple speckles but is often described as a red color. It resembles bamboo due to its hollow segmented cane.

The leaves are bright green with purple speckles. They are heart-shaped and grow staggered along the stem. 

Creamy white flowers form in clusters up to four inches long from late August through September. You can find a comprehensive guide to identification, including a video, at Knotweed Help

Damage From Japanese Knotweed

The roots and stems are not so powerful that they can overcome all concrete barriers. Instead, they seek out and find weak spots, enter those, start expanding, and over time, cause considerable damage. 

The roots find cracks and joints in drainpipes, clogging and splitting them. They find cracks and openings in home foundations, widening them, allowing in moisture, and bringing all the damage that can cause.

They also grow underneath concrete and asphalt driveways, walkways, and patios where they find any weak spots, growing up through them seeking sunlight. They can also find stone or brick retaining walls, breaking them up. 

The Japanese knotweed’s spread causes a huge amount of economic damage. As just one example, since 2010, New York City has spent more than $1 million on eradication efforts for a 30-acre patch of Japanese knotweed.

If it finds its way into your lawn, it can also impact your home’s resale value. That’s on top of the cost of repair and eradicating the weed.

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 to eliminate light and water, and placing a plastic barrier in the soil 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, considerable effort, and have their own issues regarding residual damage to your lawn and garden.

You can also consult an expert in eradicating knotweed who has the skill and experience to remove the plant without causing further spreading.

We Can Help

We’ve helped homeowners with foundation damage from plants, trees, weather events, and shifting soil from our offices in Columbia, Greer, Charleston, and Greenville.

If you find Japanese knotweed on your property, contact the professionals at Mount Valley Foundation Services for a free inspection to ensure the weed has not caused damage to your home. 

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