Japanese knotweed

not listed

NNA - Not applicable

SNA - Not applicable

not listed

State Prohibited Noxious Weed, Prohibited – Control

Japanese knotweed is listed as an invasive terrestrial plant by the Minnesota Department of Natural Resources.

In 2004 the World Conservation Union (IUCN) put Japanese knotweed on a list of “100 of the World’s Worst Invasive Alien Species.”

FACU - Facultative upland

FACU - Facultative upland

FACU - Facultative upland

Japanese knotweed is a 3½′ to 10′ tall, erect, perennial forb that rises from a long, horizontal, spreading, underground stem (rhizome). It often forms a dense thicket. It spreads aggressively and can form colonies up to an acre in size.

The stems are annual, erect or arching, stout, hollow, and hairless. They are unbranched or few-branched, round in cross section, somewhat zigzagged, and swollen at the nodes. They are light green, often with reddish spots, or reddish-brown. Above each node the stem is surrounded by a papery, fringed sheath (ocrea). Older stems are finely ridged.

The leaves are alternate, deciduous, stalked, broadly oval, 3″ to 6″ long, and 2″ to 4¾″ wide. The base of the leaf is usually broad and straight across, sometimes slightly heart-shaped. The blade tapers to a point at the tip with concave sides along the tip. The upper surface is hairless. The lower surface is minutely rough-hairy along some of the veins. The hairs are not visible without a hand lens. The margins are untoothed.

The inflorescence is a branched, elongated, erect, 3″ to 6″ long cluster (panicle) of numerous flowers rising from upper leaf axils.

In its native range each plant produces either flowers with both fertile male and fertile female parts or flowers with infertile male and fertile female parts. In North America only the latter have been introduced.

The flowers are white or greenish-white and bell-shaped. There are 5 tepals consisting of 2 small inner petals and 3 larger, outer, keeled, petal-like sepals. The tepals are fused at the base. Male flowers have 6 to 8 reduced, nonfunctional stamens (staminodes). Female flowers have 3 spreading styles.

The fruit is a shiny, dark brown, hairless, three-angled, 1 ⁄16″ to ⅛″ long nut (achene).

3½′ to 10′

White to greenish

Moist to moderate moisture. Woods, fence rows, roadsides. Full to partial sun.

August to September

In North America only plants with fertile female and infertile male plants have been introduced. The plant spreads by an extensive rhizome system. Having no fertile male flowers the plant cannot produce by reseeding. Fragments of stem or rhizome can produce new plants. Dispersal occurs when plant fragments are washed downstream or are inadvertently transported in dirt by humans.

Sources

2, 3, 4, 7, 22, 28, 29, 30.

Native to Japan, Korea, Taiwan, and China. Introduced and naturalized in the United States and Canada. Escaped cultivation.

Plant

Leaves

Stem

Location: Grant (Washington County)

Starting to see it all over the place!

Location: Shelton Washington

pain in the button, taken over acre