If You Spot This Invasive Plant in Your Yard, You May Have a Major Problem

When shopping for a home, you’re probably not thinking about potentially invasive plants on the properties you’re looking at. But it turns out you likely should be. 

Depending on your level of gardening knowledge, you might be familiar with pesky wild violet popping up in your yard or kudzu vines that cling to everything in sight.

However, there’s one invasive species that’s more than just a little annoying. Japanese knotweed can threaten both your home and your yard. It’s so serious that experts recommend home buyers weigh the presence of this plant in real estate decisions. “Japanese knotweed is one of the most invasive, damaging, and difficult-to-remove weeds in the landscape. It’s invasive in 42 states, much of Canada, and Europe,” says Clara Reitz, owner of Bashavia Gardens, a North Carolina nursery specializing in native plants.

“Its success as an invasive plant comes from its ability to propagate itself in many ways — through its 20+ foot long rhizomes, rhizome fragments, and by seed,” Reitz says. New segments can grow up to 10 centimeters daily, quickly overtaking your garden and, eventually, your home. 

If you see it while shopping for a home or notice something that looks like Japanese knotweed has popped up on your lawn, here’s what you need to know about the plant and what you can do about it.

“Japanese knotweed is a highly invasive perennial plant native to East Asia and was introduced to North America in the 19^th century as an ornamental plant,” says Alex Betz, a landscape designer and founder of Plant by Number. With hollow, bamboo-like stems, spade-shaped vibrant green leaves, and small white flowers, it’s easy to see why gardeners initially thought this dense shrub would make an attractive addition to their yards. 

But they didn’t realize how quickly it would take over. In its native Japan, Japanese knotweed faces natural pests, predators, disease, and other plants that prevent it from spreading out of control. Outside of its native region, there’s nothing to stop it.

Invasive plants are non-native plants that, once introduced, thrive at the expense of other native plants. Mary Phillips, head of Native Plant Habitat Strategy/Certifications at the National Wildlife Federation, explains that Japanese knotweed spreads aggressively and quickly. “It forms dense thickets that can crowd out native species, reducing habitat and food sources for wildlife,” she says.

But Japanese knotweed is even more potent than many other invasive plants, and its roots threaten more than your garden. “Japanese knotweed crowds out native plants along waterways and ditches, can grow through cement foundations, and destroy a planned landscape or garden in a few growing seasons,” Reitz says.

“It is a significant environmental and economic threat in regions where it has been introduced,” Betz adds.

This plant looks different throughout the year, with pink asparagus-like stems cropping up in the spring. As it blooms, you’ll see green shoots growing out of the ground, and a bounty of green, shovel-shaped leaves arrives in the summer, according to a UK-based Japanese knotweed removal service. You’ll notice white flowers sprouting from the leaves in September when the plant starts to flower. The leaves will turn yellow and orange in the fall, and the stems will turn brown. While it may look brittle in the winter with no leaves, the shoots will stay upright if not disturbed and come back strong again in the spring. 

Once you’ve observed Japanese knotweed around your home, you should take action quickly. “The denseness of Japanese knotweed is so strong that it can penetrate and damage building foundations, walls, and pavements, especially those with pre-existing cracks or vulnerable areas,” Betz says. He adds that it can cause “significant structural damage and ultimately decreasing property values.”

And you might not immediately see the extent of the damage. According to Betz, if water gets into foundational cracks caused by Japanese knotweed during the growing season, it can freeze in colder weather. When that ice expands in the cracks, it makes the plant’s initial damage even worse. Japanese knotweed roots can also get into the drainage and septic field, quickly growing and spreading into pipes, threatening your home from every angle. 

Invasive species are some of the most tricky to get rid of, and it’s usually not a one-and-done process. Betz recommends going to the professionals if you suspect you have a Japanese knotweed issue. “Specialized treatments by licensed professionals, often involving herbicides, are used to treat the plant, but repeated applications over several years are necessary,” he says.

If you’re a gardening pro considering the DIY route, you should know that digging up Japanese knotweed completely isn’t really feasible, given that its underground rhizomes can extend 20 feet wide and 10 feet deep, per Betz. “To properly excavate it, you need to scoop out all the dirt with the roots and bring in clean fill,” he says. Even if you think you’ve gotten it all, “you may still experience regrowth from deeper roots due to the soft rhizome structure that is easily fractured when disturbed,” he warns.

According to the New Hampshire Department of Agriculture, you can smother Japanese knotweed with “heavy-duty, dark-colored plastic” after cutting the plant out at the base. Be prepared to wait a while for results because it can take about five years for the area to be totally rid of the knotweed and ready for replanting.

The battle isn’t over once you’ve removed it. You need to make sure Japanese knotweed doesn’t begin to rear its ugly head again. “Minimize areas of bare soil in the landscape to prevent Japanese knotweed and other invasive species from taking root,” Phillips says. “[You should] plant dense stands of various native perennials for biodiversity and a healthy, resilient lawn,” she adds.

“Plant [some] native species with a spreading habit to crowd out the knotweed, such as common milkweed, goldenrod, and rudbeckia,” Phillips says. She suggests using the National Wildlife Federation’s Native Plant Finder to find native species appropriate for your region.

Phillips also recommends regularly monitoring your yard and, if you see a plant you don’t recognize, use a plant identifier app to determine whether it’s helpful or harmful to your yard. 

“The root systems of invasive plants are not only incredibly difficult to eliminate, but it is a job for professionals, and it is extremely expensive,” says Patricia Gray Hendricks, a real estate agent with Long & Foster in New Jersey. So, what do you do if you fall in love with a home but realize there’s Japanese knotweed in the yard and are concerned that it might threaten the house? 

She suggests bringing in a home inspector to determine if there is existing damage, then getting a professional to estimate the cost of mitigating the issue and eliminating the Japanese knotweed. “[Homeowners] think that it will be a simple fix, and it’s often not,” Hendricks says.

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