Identification

illustration

The Knotweeds:

Japanese Knotweed, Fallopia japonica (Houtt.) Ronse Decraene
(= Reynoutria japonica Houtt., Polygonum cuspidatum Siebold and Zucc.),

Giant or Sakhalin Island Knotweed, Fallopia sachalinense (F. W. Schmidt ex Maxim.) Nakai
(= Reynoutria sachalinense (F.W.Schmidt ex Maxim., Polygonum sachalinense F. W. Schmidt.) Nakai.

Bohemian or Hybrid knotweed, Fallopia x bohemicum (Chrtek & Chrtková) J. P. Bailey
(= Reynoutria x bohemicum Chrtek & Chrtková, Polygonum x bohemicum (Chrtek & Chrtková) P.F. Zita & A.L. Jacobson.)

Polygonaceae (the Smartweed Family)

General appearance: Stout, strongly rhizomatous, herbaceous perennials with arching branches and large, broad, alternate leaves. Plants often reach more than 2 m tall.

Leaves: Large, somewhat longer than broad, with a distinct, relatively short petiole. Japanese knotweed has relatively thick leaf blades that are generally less than 18 cm long and often nearly as wide, with a truncate (flat) base and acuminate (concavely tapering) tip. The leaf blades of giant knotweed are thinner and cordate (heart-shaped) at the base, with an acute or blunt tip, and often reach more than 30 cm long. Their hybrid, Bohemian knotweed, is intermediate between these two, with the leaves often approaching one or the other in size and shape.

The hairs on the lower leaf surface (particularly the midvein) of these plants can greatly aid in identification. Japanese knotweed has very short hairs, appearing as low, broad bumps on the lower leaf surface. Giant knotweed has narrow-based, multicellular hairs. The hybrid is intermediate, with broad-based, elongate bumps. While these hairs can sometimes be distinguished by bending the leaf surface over and holding it against the light, they are best viewed with a 10-20x hand lense.

Stems: Hollow, round, glaucus (having a thin waxy coating) and bamboo-like in appearance. Stems typically green with pinkish or reddish streaks and spots and reddish nodes. They are usually simple or little-branched. Japanese knotweed typically reaches up to 2.5 m tall, while giant knotweed can reach over 5 m tall. The hybrid is intermediate between these two in height.

Flowers: An abundance of small (2-3 mm across) creamy-white or greenish-white flowers are produced in loose clusters of 3-6 along branching stalks (a panicle), which grow upward from the axils of the leaves. (Certain Japanese knotweed cultivars have pink flowers.) Japanese knotweed is functionally dioecious - some clones are male and some are effectively female (male-sterile). Giant and Bohemian knotweed are apparently also functionally dioecious. The outer tepals (undifferentiated sepals and petals) are broadly winged at maturity. Knotweeds typically flower from July through August.

Seeds: Three-sided, dark brown and glossy, and about 3 mm long and 2 mm thick. Unfertilized female flowers produce hollow seeds.

Habitat: Along roadsides, railroad tracks, floodplains, woodland edges, shorelines, and other moist to wet, disturbed areas. All three taxa thrive in riparian areas, readily spreading along streams and rivers.

Sources: Voss (1985), Gleason and Cronquist (1991), Beerling et al. (1994), Seiger (1997), and Zika and Jacobson (2003).

Updated March 2006.


Visitor since March 17, 2006