These Are the Safest Ways to Get Rid of Japanese Knotweed in the Garden
Avoid toxic sprays and use our expert-approved tips instead.
If you are one of the unlucky homeowners with unrelenting Japanese knotweed on your property, then we send our sincerest condolences. Introduced from Asia, this rapidly spreading and highly invasive ornamental plant can quickly overtake even the most manicured of lawns—it can quickly grow to 10 feet tall with roots that shoot down twice as deep. Even scarier, it can grow as far as 70 feet from the nearest stem, and it's been known to become so dense that it blocks all light other plants need to grow. To make things worse, Japanese knotweed is incredibly difficult to remove. Due to its rampant growth, its rhizomes can sprout up through asphalt and cement cracks, out of rock walls, through floorboards, and can even cause severe structural problems for your home.
All of this is to say that control and removal of Japanese knotweed is serious business. Yes, the easiest choice might be to grab a bottle of the herbicide known as glyphosate (which is the main ingredient in Roundup), but this chemical has been linked to cancer and is known to have a negative impact on aquatic life and the gut bacteria of important creatures, such as like pollinating bees. Luckily, chemical treatments aren't your only option. In relatively small, newly invaded areas, this frustrating troublemaker can be tackled without the use of any chemicals. Just be forewarned: The process of eradicating it naturally requires hard work, perseverance, an extended period of time (maybe even years), and a multifaceted approach, meaning using not just one but many strategies. If the invaded area is larger (1/4 acre or more), or if you don't have the means to tackle the project yourself, it's best to call a licensed company or a certified agency for possible help.
For those hoping to take on invasive Japanese knotweed on their own, we've rounded up some natural ways to help you win the war against this frustrating plant.
Educate Yourself and Your Neighbors
The first step in dealing with Japanese knotweed is identifying that you have an issue. Japanese knotweed starts as red asparagus-like shoots which become hollow bamboo-like stems. It has green heart shaped leaves and grows white flowers from August to September. "The earlier a new population is detected, the more likely a management effort will be successful—and with less cost in time, effort, and resources," says Tom Lautzenheiser, Central/Western Regional Scientist for Mass Audubon. Another key point to remember is that because Japanese knotweed has an extensive, highly invasive root system, you should reach out to your neighbors if they have it on their property and educate them on its tenacious behaviors.
Cut Down and Remove the Canes
One method is to use sharp pruning shears or loppers to take down the stems as close to the ground as possible, making sure to remove every last cut piece and fragment because as little as half an inch of the root or cut stem can grow into another plant. Begin cutting the stems at least every two to three weeks as soon as the plant appears in the spring (usually April) and continue through August. Sprouting slows around this time which means so can your cutting frequency.
Because Japanese knotweed is considered a controlled waste due to its potential to cause ecological damage, be sure to carefully dispose of the roots. Never put Japanese knotweed in a compost pile where the soil can become contaminated with bits that can continue to sprout and spread. Wondering where to get rid of it then? Before heading to your local landfill site, first check to see if they accept invasive plants and let them know what you plan on bringing. Another option is to pile up and dry out the stems on plastic, before tossing—just make sure they don't re-sprout or get washed or blown away.
Spread a Covering
Once you have cut down the stems, immediately spread a strong, dark material over the area so that the ground underneath will be deprived of necessary sunlight and water in order to starve any new growth. Be sure to weigh down the covering of choice with heavy rocks, bricks, or carpet scraps so it doesn't blow away. Plan on leaving this covering in place for one full growing season. (Japanese knotweed usually starts growing in early spring and continues until autumn before going winter dormant.) Just make sure to watch out for rhizome escapees.
Sink a Barrier
Rhizomes will try to escape from the covered area, so consider sinking a tough plastic barrier down several feet into the ground around the perimeter of your covering. This method is similar to containing running bamboo—with that invasive plant, you typically dig a trench around the area where bamboo is growing, then sink a sturdy liner into the dugout area to stop it from spreading.
Excavation of Japanese knotweed involves thoroughly digging the plant and its roots out from the ground with heavy machinery. Although this is a far quicker method, it's more expensive and often results in a far larger portion of your yard getting destroyed in the process. Isaiah Messerly, team leader for the Great Lakes Invasive Plant Management Team. says, "Excavation is discouraged in large areas as it is difficult to ensure the entire root system has been completely removed. Plus, we don't encourage transporting invasive plant material around due to the risk of it spreading." Lautzenheiser has mixed feeling son the excavating method, explaining, "The first question is how long has the population been established/how large is it? In a new population, the only truly effective non-chemical technique is to dig the plant out completely—but every fragment must go. If you're dealing with a well-established population, however, the most effective, long term and sustainable approach to knotweed management is herbicide, and even that is not a one-and-done solution."