- An upright, herbaceous perennial with a shrub-like stature. Appears woody. Height ranges from 6 to 15 feet tall.
- Compare to parents (Japanese knotweed and Giant knotweed); characteristics are intermediate
- swollen at joints
- White, five-petaled small flowers that are sometimes tinged with green. The flowers are attractive and grow on branched racemes. Blooms from August to September.
- small, winged fruits, beige to light brown
- contain triangular, shiny, black, smooth, 3-angled seeds
- prefers full sun, tolerates light shade, salt, high temperatures, dry soil
- intolerant of frost
- thrives in wetlands, roadsides, disturbed areas
- winged fruits are wind dispersed
- spreads mostly vegetatively via long rhizomes
Key ID Features:
- triangular pointed leaves
- greenish-white flowers
- winged fruits
- leaves zigzag on stem
- Giant knotweed has leaves greater than 30 cm long, deeply cordate leaf bases, and visible, multicellular hairs on the undersides of the leaves. The hybrid of Japanese and giant knotweed (P. x bohemica) has an intermediate leaf size, cordate to truncate leaf base, and stout, single-celled hairs on the undersides of the leaves, giving it a feel that is rough to the touch. Japanese knotweed has smooth leaf undersides with no hairs. Leaf hairs can be observed with a strong hand lens but on giant knotweed should even be visible to the naked eye.
- Due to its long rhizomes and winged fruits, this plant can travel long distances. This plant creates a monoculture, outcompeting native plants and destroying botanical diversity. Eradication can be quite difficult due to the plant's ability to regenerate from tiny rhizome fragments or even stem nodes. Studies have indicated that this hybrid is more aggressive than either one of its parents.
- It is unclear whether this hybrid is subject to the importation and propagation ban that applies to one of its parents, Japanese knotweed.
Growth Form: Herb
Origin: hybrid origin
Level of Invasiveness for Nantucket: Invasive
Level of Invasiveness for Massachusetts: Not evaluated
Massachusetts Cultivation Restrictions: no