How to Get Rid of Japanese Knotweed

How to Smother Japanese Knotweed With Tarps

Covering Japanese knotweed with tarps suppresses the plant's growth and ultimately kills it. This method is best to do in the spring to catch the plant at the start of its growing season. Be aware that while this is a way to get rid of Japanese knotweed naturally and with relatively low effort, it may take several years.

1. Prepare the Area

   Prepare the area by using pruners to cut the tall stems down to the ground and removing any debris. Bag all the debris to prevent it from taking root.

   Then, cover the entire area with a thick layer of cushiony material, such as mulch, leaves, or grass clippings. The stems have sharp edges that easily puncture plastic sheeting or tarps.

2. Cover the Area

   Cover the plant area completely with thick-grade black sheet plastic or heavy non-canvas tarps. If you must overlap pieces, make sure to overlap them by at least 2 feet. The plastic should extend 5 to 10 feet outside the boundary area of the knotweed growth.

   Use rocks or other heavy materials to weigh down the tarps, so they don't move or blow away. As this covering will need to remain in place for a long time, disguise it with wood chips or mulch.

3. Trample Any New Shoots

   As new shoots emerge over time, they might push up the tarps. To crush them, trample over them walking over the tarps. What growth does occur under the tarps won't amount to much because it lacks sunlight.

4. Remove the Covering and Replant

   After roughly five years, the smothered knotweed and roots should be completely dead. That's when you can safely remove the covering and replant the area with a groundcover, shrubs, or other plants.
   

How to Remove Japanese Knotweed by Cutting

Cutting back Japanese knotweed suppresses it though usually it does not fully eradicate it. Therefore this method must be used in conjunction with other methods to get rid of Japanese knotweed completely.

1. Cut the Plant Down

   Use pruners to cut the plant down to the ground throughout the growing season, so it's not able to photosynthesize efficiently. Because the cuttings quickly sprout new roots and take hold in the soil, make sure you gather them all up and bag them for disposal.
   

2. Monitor and Repeat

   
   Inspect the area weekly, and clip off any new shoots that appear. This is an essential step, as cutting tends to stimulate Japanese knotweed into new growth. So you should not cut Japanese knotweed unless you stay on top of the new shoots, or you might end up with a more serious infestation than you started with.

   After initially cutting the stems, use a lawnmower set at a low height to trim off new growth as it appears. It will likely require weekly cutting over many months to completely eradicate Japanese knotweed.
   

How to Remove Japanese Knotweed by Digging Up the Roots

Another way to get rid of Japanese knotweed naturally is to dig up the ground where the weed shoots come up most vigorously. This is typically used concurrently with other methods.

1. Find the Rhizome Clumps

   Locate and dig up the plant's rhizome clumps (underground stems that send up shoots). In mature plants, these rhizome clumps are often very woody and easily reach widths of a foot or more.

2. Bag Rhizomes for Disposal

   Try to get as much of the rhizomes as possible, and bag them for disposal. Even the tiniest piece left behind is likely to sprout a new plant.

How to Remove Japanese Knotweed Using Herbicide

Some gardeners resort to applying weed killer to kill Japanese knotweed permanently. The best herbicide for Japanese knotweed is one that contains glyphosate. Follow label instructions carefully, including all safety warnings. Late summer is the best time for using a herbicide on Japanese knotweed.

1. Select a Weed Killer

   Select a weed killer appropriate for Japanese knotweed, and read its label instructions and warnings. You'll likely want to wear rubber gloves and protective clothing when using the herbicide to avoid skin contact.

2. Prune the Knotweed

   Cut the Japanese knotweed to roughly 3 to 4 inches above the ground. This will stimulate tender new growth that will readily absorb the herbicide.
   

3. Apply the Weed Killer

   Once new growth has sprouted from the cut stems (within one to two weeks), carefully saturate the growth with herbicide. Glyphosate works when it is absorbed by foliage to inhibit plant enzymes necessary for growth. Foliage that turns yellow and dies indicates that the plant is dying—roots and all.

4. Repeat as Needed

   Watch the area carefully over the next few months, and apply additional weed killer to any new growth that appears. This herbicide method generally will kill Japanese knotweed permanently over time.
   

Other Methods of Japanese Knotweed Removal

Some people have tried killing Japanese knotweed with other methods, including diesel, bleach, lime, and salt. However, these methods are not effective. Diesel only damages the foliage but the plant will regrow just as healthy as ever, and you'll have polluted your yard in the process.

You'll have the same issue when attempting to kill Japanese knotweed with bleach or lime. Moreover, Japanese knotweed has a high tolerance for salt, so that won't be able to get rid of it either.

When to Call a Professional

If your attempts to get rid of Japanese knotweed have been unsuccessful, it's likely time to call in a professional. They often have more substantial tools and products that get the job done. Japanese knotweed removal costs vary widely, depending on how widespread and stubborn your plants are. A lawn service should be able to give you a quote for your specific area. The good news is reputable lawn services often have a money-back guarantee that they can resolve the problem.