Persicaria virginiana (L.) Gaertn.

Virginia Knotweed


CC = 1
CW = 0
MOC = 66

© DETenaglia

Family - Polygonaceae

Habit - Rhizomatous perennial forb. Stems not rooting at nodes.

Stems - Ascending to erect, to 60 cm below the inflorescence, glabrous or densely appressed-hairy toward the tip toward the tip, not glandular. Ocreae persistent, often tearing with age, 10-20 mm long, loosely cylindric but not noticeably inflated, light brown to brown and papery throughout, more or less truncate but often noticeably oblique at the tip, lacking a flange of tissue at the tip, the margin with bristles 0.5-4.0 mm long, the surface densely appressed-hairy, the hairs sometimes appearing somewhat woolly, not gland-dotted.

Leaves - Alternate, simple, petiolate. Petioles to 1-3 cm on lowermost leaves, shorter and becoming nearly absent upward, appressed-hairy, mostly on the upper surface. Leaf blades 5-18 cm long, 2-10 cm wide, ovate to elliptic or obovate, those of the uppermost leaves sometimes lanceolate, angled or rounded at the base, tapered to a sharply pointed tip, the upper surface glabrous or more commonly with scatted, stiff, bristlelike hairs, the undersurface glabrous or sparsely to moderately pubescent with softer, short, more or less appressed hairs, mostly along the veins, the undersurface often appearing finely bumpy or pebbled but lacking impressed glands, the upper surface sometimes with a reddish or purplish, chevron-to crescent-shaped or triangular area.

Persicaria_virginiana_ocrea.jpg Ocrea.

© DETenaglia


© DETenaglia

Inflorescences - Terminal and axillary from the upper leaves, solitary or few per main stem, 40-130 cm long, 6-15 mm wide, erect or more commonly somewhat curved or arched, open, the nodes well-separated, the fascicles of flowers not overlapping those of the adjacent nodes, the stalk 2-40 cm long, appressed-hairy, sometimes glabrous or nearly so toward the tip, not glandular. Ocreolae not overlapping, the margins with bristles 1-3 mm long, the surface appressed-hairy, not gland-dotted.

Persicaria_virginiana_inflorescence2.jpg Inflorescence.

Flowers are well separated with the central axis clearly visible.

© SRTurner

Flowers - Perfect, solitary or 2 or 3 per fascicle. Perianth white, greenish white, or rarely pink, bell-shaped at flowering but becoming more or less urn-shaped soon after flowering, not gland-dotted, the tepals 4, 2.5-3.5 mm long, fused toward the base, the nerves inconspicuous, irregularly few-branched, open-branched, not forming a network, not anchor-shaped. Stamens 4, somewhat exserted, the anthers yellow or pink. Filaments whitish, glabrous, 2 mm long. Anthers pale yellow to whitish, 1 mm long. Styles 2-branched from the base, exserted, persistent in fruit to form a beak.

Persicaria_virginiana_flower.jpg Flowers.

© DETenaglia

Fruits - Achenes 3.5-4.0 mm long, 2.0-2.8 mm wide, not or more commonly slightly exserted, bluntly 2-angled, tapered to a prominent, hooked beak, the faces convex, lacking a central hump, the surface smooth or finely wrinkled, brown to black, shiny or dull.

Persicaria_virginiana_fruits.jpg Immature fruits.

© SRTurner

Flowering - June - October.

Habitat - Rich, moist woods.

Origin - Native to the U.S.

Lookalikes - None when flowering.

Other info. - This species can be found throughout Missouri and the eastern half of the U.S. It is easy to identify in the field from its hairy ocrea, big, alternate leaves, and long inflorescences with well-spaced flowers. The plant is very common in moist, shaded areas. Tension in springlike tissues at the joint of the fruit stalk can shoot mature fruits up to 4 m.

Synonyms are Polygonum virginianum L. and Tovara virginiana (L.) Raf.

Photographs taken at the Kansas City Zoo, 7-30-99, and in the Ozark Scenic Riverways, Shannon County, MO., 7-29-04 (DETenaglia); also at Cuivre River State Park, Lincoln County, MO, 8-25-2008, and Don Robinson State Park, Jefferson County, MO, 9-16-2021 (SRTurner).