Japanese Knotweed

Japanese Knotweed 1Japanese Knotweed (Polygonum cuspidatum)
Noxious clumping perennials with large leaves, hollow stems, and long creeping rhizomes. Plants grow vigorously and create dense colonies that exclude other vegetation. Established colonies are extremely difficult to eradicate. All three were introduced as garden ornamentals, but have widely escaped cultivation and become invasive in moist, disturbed places To 3 m tall.

Colonies are especially problematic in the Eastern U.S. Young shoots, stems, and rhizomes are edible. Plants are used medicinally in China and Japan. Introduced from the mountainous regions of Japan, Taiwan, Korea, and eastern China.

Japanese Knotweed 2Mature Plant
Main stems erect, often arched near top, simple to minimally branched, grooved, thick, hollow, weakly woody, swollen at nodes, usually reddish-brown at maturity. Twigs often zigzag slightly from node to node. Leaves alternate, leathery, on stalks ~ 2-3 cm long. Tips acute to acuminate.

Stems and leaves +/- glabrous except where noted. Ocrea (specialized stipules) fused, membranous, sheathing stem above each node, usually fringed at the top Leaves broadly ovate, ~ 10-15 cm long, sometimes wider than long. Bases truncate to tapered. Ocrea deciduous, short (~ 4-6 mm long), glabrous, sometimes reddish

Roots & Underground Structures
Rhizomes thick, extensive, store large quantities of carbohydrates, and spread aggressively. Fragments can produce new plants.Japanese knotweed Rhizomes often 5-6 m long, but documented to 20 m long. Rhizomes can penetrate 2 inches of asphalt. Rhizomes buried to soil depths of 1 m can regenerate, but fragments regenerate best from just below the soil surface.

Japanese Knotweed 3Flowers
August-October. Panicles branched, open, lax, with numerous flowers. Sepals 5, petal-like, +/- fused at the base. Petals lacking. Plants with unisexual flowers have male flowers with vestigial ovaries and female flowers with infertile stamens (staminodes). Insect-pollinated.

Fruits & Seeds
Outer 3 sepals persistent, enclose and disperse with achene. Achenes +/- ovoid, 3-sided.

Propagation / Phenology
Reproduces vegetatively from rhizomes and by seed. Rhizome fragments disperse with water currents or flooding and with natural or human facilitated soil movement. Fruits (achenes enclosed by sepals) disperse primarily with wind.