Fallopia japonica (Houtt.) Ronse Decr.

Japanese Knotweed


CC = *
CW = 3
MOC = 17

© DETenaglia

Family - Polygonaceae

Habit - Perennial forb, with somewhat woody rootstocks and widely creeping rhizomes, forming dense colonies, usually functionally dioecious (staminate plants occasionally with a few pistillate or perfect flowers).

Stem - Stout, arching or ascending to erect, to 2.5 m, glabrous, sometimes glaucous. Ocreae 4-10 mm long, oblique at the tip, the surface glabrous or minutely hairy, tan.

Fallopia_japonica_stem.jpg Stem, node, and ocrea.

© DETenaglia

Leaves - Simple, petiolate. Blades 4-15 cm long, 2-10 cm wide, ovate, broadly angled or rounded to shallowly cordate at the base, tapered to a sharply pointed tip, the upper surface glabrous, the undersurface roughened or sparsely pubescent along the veins with minute, unicellular, stout, knob-shaped to bluntly pointed hairs, also gland-dotted, sometimes also somewhat glaucous.

Fallopia_japonica_leaf1a.jpg Leaf adaxial.

© SRTurner

Fallopia_japonica_leaf2a.jpg Leaf abaxial.

© SRTurner

Fallopia_japonica_leaf2.jpg Pressed leaves.

© DETenaglia

Inflorescences - Terminal and axillary, panicles or rarely reduced to racemes, the flowers 3-10 per fascicle, mostly staminate or pistillate (rarely perfect), the flower stalks 3-5 mm long, winged to below the midpoint, jointed at or below the midpoint.

Fallopia_japonica_inflorescence.jpg Inflorescence.

© DETenaglia

Fallopia_japonica_inflorescence2.jpg Inflorescence branch.

© SRTurner

Flowers - On stalks 3-5 mm long, jointed at or below the perianth base, winged to below the midpoint. Tepals 5 in 2 whorls, 2-4 mm long, becoming enlarged to 4-10 mm at maturity, those of the inner whorl mostly elliptic, those of the outer whorl obovate, tapered and decurrent along the stalklike base, glabrous, white, greenish white, or pink to reddish pink, the outer 3 tepals with dorsal wings 1.4-2.0 mm wide at fruiting. Stamens (in staminate or perfect flowers) 8, the filaments sometimes hairy toward their bases. Styles 3, sometimes fused toward the base or nearly to the tip, the stigmas divided into numerous, slender fringelike lobes.

Fallopia_japonica_flowers.jpg Flowers.

© DETenaglia

Fallopia_japonica_flowers2.jpg Flowers.

© SRTurner

Fruits - Achenes 2.3-3.5 mm long, enclosed in the persistent perianth, 3-angled, unwinged, elliptic to ovate in outline, short-beaked, dark brown, glabrous, smooth, shiny.

Flowering - July - October.

Habitat - Forest edges, streambanks, saline seeps, ditches, old homesites, open disturbed areas.

Origin - Native to Asia.

Lookalikes - None.

Other info. - This introduced plant has been widely cultivated. It is found in scattered locations throughout Missouri, as well as much of the continental U.S., with the greatest preponderance in the northeastern and northwestern parts of the country. The coarse foliage and sprays of small white flowers are distinctive.

Though often cultivated, the plant is too aggressive for most garden situations, and indeed is considered an invasive exotic in many parts of the country. Variants of this species have been found to be allelopathic, inhibiting nearby germination and growth of other species. Steyermark knew this plant from only five counties, but since his work in the mid 20th century the count has increased to 18, and this number will almost certainly continue to increase.

Photographs taken in Marquette, MI., 9-8-03 (DETenaglia), also in Bonne Terre, St. Francois County, MO, 8-29-2019 (SRTurner).