- An upright, herbaceous perennial with a shrub-like stature. Appears woody. Height ranges from 8 to 18 feet tall.
- cordate bases
- tip triangular, fairly blunt
- 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:
- Typically extremely difficult to distinguish between the hybrid and either parent; consult an expert.
- 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.
- On Nantucket, there are three known populations.
Growth Form: Herb
Origin: Japan, China, Korea
Level of Invasiveness for Nantucket: Likely Invasive
Level of Invasiveness for Massachusetts: Does not meet criteria
Massachusetts Cultivation Restrictions: no