How to Identify Knotweed

Common Knotweed

Polygonum arenastrum This species is different than the rest and less problematic than the others. It is an invasive annual, sometimes perennial, species that grows in areas other weeds cannot. It likes compact soils often found along trails, in gardens, turf, and some crop fields -- which is where most problems arise. The plant is short-lived, but can survive the winter if it establishes a taproot. This species of knotweed is in need of further research, but due to the fact that it is not aggressive, and does not substantially alter habitats it is not of pressing concern.

1. 1
   Check the leaves. They are bluish-green, 1/5" to 4/5" long, with short leaf stalks. They are ovate in shape.
2. 2
   Study the stems. Stems are numerous, wiry, slender, and highly branched. They form mats of biomass up to 8" high. They have swollen nodes and membranous sheaths.
3. 3
   Study the roots. Taproot system that can penetrate more than 18".
4. 4
   Check the flowers. The flowers are small and inconspicuous, ranging from white to green with the occasional pink tint. It has fruit that is three-sided, 1/8" long, dark brown, and matte (not shiny).
5. 5
   Check the height. Common knotweed ranges from 4" to 8" in height.
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Japanese Knotweed

Fallopia japonica The easiest way to identify a Japanese knotweed plant is by studying the middle of a branch. This is where the plant has developed the most unique characteristics.

1. 1
   Check the leaves. These should be spade-like, up to 4" long and 2/3 as wide, and thick with a pointed tip. They are hairless on the underside but do have bumps on their veins; these are visible with a hand lens.
2. 2
   Study the stems. These should be cane-like, hollow between each node, green to reddish-brown once matured and die after the growing season.
3. 3
   Study the roots. They should have a rhizome structure, up to 65' long laterally, and are similar throughout the knotweed species.
4. 4
   Check for flowers. These should be from creamy to greenish white, small clusters of iridescent flowers that grow longer than the leaf. They branch from the leaf axil near base of the stem.
5. 5
   Check the height. These can grow anywhere from 4' 10" to 8' 2".
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Giant Knotweed

Fallopia sachalinensis Giant knotweed is the largest of the knotweed species and also interesting in that all such plants are male. Because of its size, this species more resembles bamboo than the others do.

1. 1
   Check the leaves. Of the knotweed species, the leaves of the giant are the largest, growing up to 16" long and 2/3 as wide. They are deep and heart shaped with an abrupt point. On the underside of the leaf are diagnostic, multicellular hairs that are long, thin, wavy, and sparse. These fall off towards the end of September, and you might need a hand lens to see them.
2. 2
   Study the stems. They will be smooth, hollow, light green, and sparingly branched.
3. 3
   Study the roots. These will have a rhizome structure, up to 65' long laterally, and are similar throughout the knotweed species.
4. 4
   Check for flowers. These will be small, creamy-white to greenish-white, and shorter than the leaf. Three-sided shiny black fruit can also be found.
5. 5
   Check the measurements. Giant knotweed can grow anywhere from 9' 9" to 19' 8".
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Bohemian Knotweed (Hybrid)

Fallopia x bohemica Bohemian Knotweed is a hybrid of Japanese and Giant knotweed. Most Bohemian plants are seedless male plants, but recently seed-bearing hybrids have been cited. These seeds are very troublesome due to the prolific nature of knotweed. The Bohemian plants may be hard to differentiate because they are hybrids that might have undergone a series of back-crossing.

1. 1
   Study the leaves. These can be any mixture of those of the Japanese (spade-like) and those of the giant (heart-like). Look for heart-shaped leaves towards the base of the stem and spade-shaped towards the end of the branch. These leaves have triangle shaped hairs found on the underside of the leaf, mid-vein. They have thicker bases and can be seen best with a hand lens.

   • If you find both heart and spade shaped leaves the plant is probably Fallopia x bohemica . All other species will have one or the other, but not both.
2. 2
   Check the stems. These will be hollow between nodes, reddish-brown, cane-like, and branched.
3. 3
   Study the roots. These have a rhizome structure, up to 65' long laterally, and are similar throughout the knotweed species.
4. 4
   Check for flowers. These are creamy to greenish white and grow in small clusters the same length as the leaf.
5. 5
   Check the measurements. Bohemian knotweed can grow from 6' 6" to 16' 5".
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Himalayan Knotweed

Polygonum wallichii Himalayan knotweed is very distinct. The growing patterns, leaf structures, interior stem, and pink flowers set this plant apart. Unlike the other species, the Himalayan knotweed grows close to the ground, aiding in its dense nature. The pink flowers and lance-shaped leaves are a dead giveaway that you have found a Himalayan knotweed bush.

1. 1
   Look at the leaves. These leaves are the most unique of all the knotweed leaves. They are lance shaped, slender, long, leathery, and pointed with heart shaped bases. They grow to 8" long and less than 1/2" wide in an alternate arrangement. At each node are brown sheaths, also exclusive to the Himalayan knotweed.
2. 2
   Check the stems. These are red at the base of each leaf, and green throughout. They zigzag upward and are solid (not hollow).
3. 3
   Study the roots. These have a rhizome structure, grow up to 65' long laterally, and are similar throughout the knotweed species.
4. 4
   Check for flowers. These are different than the other species' flowers and are white to pink in color. They have loose, branched clusters that are fragrant. These flowers are found at the tips of branches. They also have a fruit which is smooth, three-sided, and typically has the old sepal still attached.
5. 5
   Check the measurements. Himalayan knotweed can be from 6' 6" to 9' 10".
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