Japanese knotweed: how to identify it and get rid of it

Contents

1. What is Japanese knotweed?
2. How do you identify Japanese knotweed?
3. Why is knotweed so hard to kill?
4. Is Japanese knotweed regulated in the US?
5. How do you get rid of Japanese knotweed?
6. Can you get rid of knotweed without herbicides?
7. How do you dispose of knotweed safely?
8. When should you call a professional?
9. Common mistakes with Japanese knotweed
10. FAQ

What is Japanese knotweed?

Japanese knotweed (Reynoutria japonica, also sold in older references as Fallopia japonica or Polygonum cuspidatum, and sometimes called Asian knotweed) is native to East Asia. It was imported as a garden ornamental in the late 1800s and has since escaped across much of the US. It grows almost anywhere: full sun or part shade, poor fill soil or rich streambanks. Two close relatives behave the same way and are treated the same way: giant knotweed and Bohemian knotweed, a hybrid of the two. Even experts sometimes argue over which one they are looking at, and for control purposes it does not matter.

The problem is not just its looks. A knotweed stand shades out nearly every other plant, and it does not stay put: it creeps outward every year and starts new colonies wherever a fragment lands.

How do you identify Japanese knotweed?

Knotweed looks different through the year, so check for the features that match the season. If your plant does not fit, start from our weed identification guide instead.

Plants that look like knotweed

• Bamboo: also has jointed, hollow stems, but they are hard and woody, and the leaves are long, narrow and papery and stay on through winter. Living knotweed stems are soft and look wet when you cut them.
• Giant and Bohemian knotweed: taller, with larger leaves and a heart-shaped base on giant knotweed. Same biology, same plan below.
• Broad-leaved dock: sometimes mistaken for knotweed, but it grows as a low rosette of leaves from a taproot, not as tall cane-like stems.

Why is knotweed so hard to kill?

Almost everything happens underground. Japanese knotweed spreads through rhizomes, creeping underground stems that send up new shoots and roots. They store huge food reserves, break easily when disturbed, and each fragment can grow into a complete new plant. One documented knotweed plant had a connected underground system covering more than 30,000 square feet.

That explains the usual failures. Cutting or spraying the tops early in the season kills shoots, but the rhizomes simply send up more. Digging snaps the rhizomes and leaves pieces behind. Mowing chops stems into bits that root wherever they fall in moist soil. Most new infestations start from fragments carried in fill dirt, on mowers and tools, or by floodwater, rarely from seed.

Is Japanese knotweed regulated in the US?

Japanese knotweed is classified as an invasive species and is listed as a noxious or prohibited weed in a number of states. Depending on where you live, that can mean you may not sell, plant or move it, and in some places landowners are expected to control it. The lists change, so check your state’s noxious weed list (usually published by the state department of agriculture) or ask your county extension office before you start, especially if the patch is near water or crosses a property line.

How do you get rid of Japanese knotweed?

The method that works in yards relies on timing. In late summer and fall, the leaves send sugars down to the rhizomes for winter, and a systemic herbicide travels with them. Sprayed too early, or too soon after cutting, it only burns the tops.

1. Size up the patchNote how big it is, whether it is near a stream, pond or ditch, and whether it reaches a neighbor’s yard. Water and large stands change which products you can use and whether you should hire help.
2. Optional: cut in JuneCutting the canes in early summer makes the regrowth shorter (often 2 to 5 feet), which is easier to spray completely and keeps spray away from water. Then wait at least eight weeks before spraying the regrowth. Bag every cut stem.
3. Spray after flowering, before frostApply a systemic herbicide to all the leaves from late summer until the first killing frost. Glyphosate is the most common choice; use only an aquatic-labeled formulation near water. Imazapyr also controls knotweed, but it stays active in the soil and can injure nearby trees through their roots, so it is rarely a good fit in home landscapes. Products based on triclopyr or 2,4-D brown the leaves but do little to the rhizomes.
4. Treat small patches stem by stemOn a few stems near plants you want to keep, cut each stem and apply herbicide to the freshly cut stem only if the product label allows that method. Stem injection is another targeted option, but it needs special equipment.
5. Repeat the next yearExpect regrowth the following spring, shorter and thinner. Treat it again late in the season. Most stands need at least two seasons of treatment, and older ones three to five.
6. Keep watchingOnce no shoots appear, check the spot and its surroundings every season and treat any new shoot right away.

Read the label: the label lists whether knotweed is a target, the mixing rate, where the product can be used and any restrictions near water. In the US, the label is the law. For the difference between selective and total products, see our guide to weed and grass killers.

Can you get rid of knotweed without herbicides?

Yes, but it takes longer and works best on small, young patches.

• Digging: realistic only for a new, small plant. Dig out every piece of rhizome you can find, and expect to come back for shoots from fragments you missed. On established stands, digging usually spreads the problem.
• Repeated cutting: cutting the stems down whenever they regrow weakens the plant and slows the spread of the rhizomes, but on its own it rarely kills an established stand.
• Smothering: cut the stems to the ground in early summer, then cover the whole patch, plus a wide margin around it, with heavy tarp or thick sheeting weighed down at the edges. Check the edges for escaping shoots. Expect five years or more. Woven landscape fabric alone is not strong enough; shoots can push through it.
• Vinegar, boiling water, salt: they kill the leaves they touch, nothing more, and salt damages the soil. The natural weed killers that work on young annual weeds are no match for knotweed rhizomes.

How do you dispose of knotweed safely?

• Never compost it. Home compost piles do not reliably kill stems and rhizomes.
• Dry, then bag. Let cut stems dry out completely on pavement or a tarp, then put them in heavy contractor bags and tie them shut. Call your transfer station first: not every facility accepts invasive plants.
• Don’t move the soil. Soil from a knotweed patch can carry rhizome pieces. Don’t use it as fill and don’t accept fill from a site with knotweed.
• Clean up. Brush soil and plant bits off shovels, mowers, tires and boots before leaving the area.

When should you call a professional?

Call a licensed pesticide applicator or your local extension office if the stand is large, if it grows along a stream, pond or wetland (only certain products can be used near water, and local rules may apply), if it crosses into a neighbor’s property, or if it grows against a foundation, retaining wall or septic area where digging and spraying are tricky. Professionals can also use stem injection and products that homeowners cannot buy.

Common mistakes with Japanese knotweed

• Spraying in spring and calling it done: the rhizomes survive and regrow.
• Spraying a few weeks after cutting, while the plant is busy regrowing leaves.
• Mowing or string-trimming the patch and leaving the bits on the ground.
• Digging or tilling an established stand, which multiplies the plants.
• Replanting too soon: wait until the knotweed has been suppressed for at least two seasons, or new plants get in the way of follow-up treatments.

Knotweed is one of several weeds that come back from deep underground parts; field bindweed and Canada thistle call for the same late-season logic. Woody invaders are covered in our guide to weed trees.

FAQ

What is Japanese knotweed?

Japanese knotweed (Reynoutria japonica, also listed as Fallopia japonica or Polygonum cuspidatum) is a tall perennial from East Asia that was brought to North America as an ornamental in the late 1800s. It escaped gardens and now forms dense thickets along streams, roadsides, fence lines and yards, crowding out almost everything else.

When does Japanese knotweed flower?

In late summer, usually August into September. Upright, branched clusters of tiny creamy-white to greenish-white flowers grow from the leaf joints along the upper stems. The weeks right after flowering, up to the first killing frost, are the best time for a systemic herbicide treatment.

Can you compost Japanese knotweed?

No. Stem pieces and rhizome fragments can sprout into new plants, and a home compost pile does not reliably kill them. Dry cut stems on a hard surface or tarp, bag them, and put them in the trash, after checking that your local transfer station accepts invasive plants.

How long does it take to get rid of Japanese knotweed?

Plan on several years. A well-timed herbicide program needs at least two seasons of treatment, and established stands often take three to five years before no shoots come back. Smothering alone can take five years or more. After that, check the spot every season and treat any new shoot.

Will vinegar, salt or boiling water kill Japanese knotweed?

No. Vinegar and boiling water only scorch the shoots they touch, and the rhizomes push up new stems within weeks. Salt does not reach the whole rhizome system but does poison the soil for the plants you want to grow there later.

Does Japanese knotweed spread by seed?

Mostly not. It can set small, shiny, triangular seeds, but most new plants come from fragments of stem or rhizome moved by digging, mowing, flooding or fill dirt. That is why disposal and clean tools matter as much as the treatment itself.

Related

• Weed identificationWeed identification made simple: grass, broadleaf or sedge, leaf shape, flowers and growth habit. Use the free weed identifier, then get the fix for each weed.
• BindweedHow to get rid of bindweed for good: tell field and hedge bindweed from morning glory, cut it back on a schedule, time the herbicide and never till it in.
• Weed treesWeed trees like maple, mulberry, elm, black locust and tree of heaven: identify the seedlings, pull them young, and treat stumps so they don’t resprout.
• Weed and grass killerA weed and grass killer kills every plant it touches. Compare glyphosate, glufosinate and contact acids, avoid soil-residual products near trees, and cut drift.
• Landscape fabricLandscape fabric works under gravel and stone, not under bark in planted beds. Woven vs non-woven, how to install it step by step, and better options.

Updated October 3, 2026