Bohemian knotweed

not listed

NNA - Not applicable

not listed

State Prohibited Noxious Weed, Prohibited – Control

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

FACU - Facultative upland

FACU - Facultative upland

FACU - Facultative upland

Bohemian knotweed is an erect, 5′ to 8′ tall, perennial forb. It rises on usually clustered stems from a long, creeping, horizontal, underground stem (rhizome). It is a fertile hybrid between two highly invasive plants, Japanese knotweed (Reynoutria japonica) and giant knotweed (Fallopia sachalinensis), and it shares features of both of those plants. The hybrid was introduced into North American and cultivated as an ornamental. It escaped from cultivation and is now naturalized across northern United States. It is reported to be partially or fully fertile, but it spreads mostly by rhizomes and by the dispersal of plant fragments. It is found on river banks, along roadways, and in other disturbed areas. It often forms large dense colonies.

The bamboo-like stems are erect, stiff, hollow, green, and hairless. There are usually many slender branches. They are not climbing or twining. They are swollen at the nodes and are covered with a whitish waxy bloom (glaucous). Like other knotweeds (Fallopia and Persicaria), there is a sheath (ocrea) that wraps around the stem at each node. The ocrea is papery, membranous, white to tan or greenish-brown, and usually ⅛″ to ¼″ long, sometimes up to ⅜″ long. It may be hairless or covered with short, fine hairs, but does not have longer bent hairs and does not have bristles at the base. It is usually deciduous.

The leaves are alternate, 2″ to 12″ long, and ¾″ to 4″ wide, larger than Japanese knotweed but smaller than giant knotweed. They are on ⅜″ to 1¼″ long leaf stalks. The leaf blade is variable in shape, and may resemble the leaves of either parent. It may be spade-shaped, straight across (truncate) at the base, or slightly heart-shaped, indented (cordate) at the base. Both leaf shapes may appear on the same branch. They are tapered at the tip with concave sides along the tip (acuminate). They do not terminate in a sharp firm point. The upper surface is hairless. The lower surface is glaucous and has minute hairs along the veins. The margins are untoothed and may be hairless of have a short fringe of hairs.

Flowers appear from July to October. The inflorescence is an erect or spreading, 1½″ to 4¾″ long, cluster of flowers at the end of each branch and rising from leaf axils. It may be long, narrow, and unbranched (raceme), or short, broad, branched (panicle), and plume-like, and it may be either shorter or longer than the subtending leaf. The flowers are grouped into elongated bundles (fascicles) of 3 to 15 flowers each. There is an ocrea at the base of each fascicle. The flowers are

Each flower is ⅛″ to ¼″ (4 to 6 mm) long and is constricted and stipe-like at the base. There are 5 petal-like tepals, 8 stamens, and 3 styles. The tepals are egg-shaped to ellipse-shaped, creamy white or greenish-white, and hairless. The outer 3 tepals are long and winged along the midrib, the inner two shorter and unwinged. All of the tepals become larger as the fruit forms. The styles are fused at the base. Each flower appears perfect, with both male and female reproductive parts, but some of the flowers are female, with nonfunctional male parts.

The fruit is a single-chambered seed capsule (achene). The achene is 1 ⁄16″ to ⅛″ (2.6 to 3.2 mm) long, dark brown, shiny, and smooth.

5′ to 8′

Greenish-white to pink

Rivers; roadways and other disturbed areas

July to October

Sources

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

Both parent plants are native to Asia. The hybrid was introduced and cultivated as an ornamental. It escaped cultivation and is now naturalized in North America.

Uncommon

Hybrid
Bohemian knotweed is a fertile hybrid between two highly invasive plants, Japanese knotweed (Reynoutria japonica) and giant knotweed (Reynoutria sachalinensis).

Genus
The parent plants were originally placed in the new genus Reynoutria in 1777. Later, the genus was merged with Fallopia, separated again, and merged again, the last time in 1988.

Species Complex
The Japanese Knotweed Complex includes Japanese knotweed, giant knotweed, and Bohemian knotweed, a hybrid of the former two.

Branch

Plume-like Inflorescence

Shallowly Heart-shaped Leaf