Polygonum douglasii (Douglas' Knotweed)

Plant Info
Also known as:
Genus:Polygonum
Family:Polygonaceae (Buckwheat)
Life cycle:annual
Origin:native
Habitat:sun; dry sandy or rocky soil; rock outcrops, savanna, along railroads, disturbed soils
Bloom season:July - August
Plant height:6 to 30 inches
Wetland Indicator Status:GP: FACU MW: FACU NCNE: FACU
MN county distribution (click map to enlarge):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

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Detailed Information

Flower: Flower shape: 5-petals

[photo of flowers] Clusters of 2 to 4 flowers arising from widely-spaced leaf axils on branching stems. Flowers are about 1/8 inch long, with 5 tepals (petals and similar sepals) that are all about the same size, white to pale rose-purple often with a green or darker purplish midvein, and blunt at the tip. In the center are 8 stamens surrounding a cone-shaped ovary. Flower stalks are slender, shorter than the flower, ascending to spreading in flower, becoming nodding in fruit. All flower parts are hairless.

Leaves and stems: Leaf attachment: alternate Leaf type: simple

[photo of leaves] Leaves are alternate, mostly ascending, ½ to 2 inches long, up to 1/3 inch wide, linear to narrowly lance-oblong, pointed at the tip, and have a prominent midvein. Edges are toothless and may be minutely hairy (magnification required) and/or rolled under (revolute); surfaces are hairless. Leaves near the base typically fall off early, and those in the flower clusters are reduced to bracts.

[photo of ocrea] At the base of the short leaf stalk is a brown, membranous sheath (ocrea) that extends up around a branch stem, tearing away with age, its upper edge typically shredding. Stems are erect, weakly angled, usually many branched from the lower stem, smooth or slightly rough.

Fruit: Fruit type: seed without plume

[photo of fruit] The tepals persist in fruit, tightly wrapping the single seed and drooping as it matures.

[photo of seed] Seeds are black, 3-sided, somewhat diamond-shaped, smooth to minutely textured, and shiny or dull.

Notes:

Douglas' Knotweed is primarily a western species, more common in the western third of North America, with disjunct populations around the Great Lakes and in New England. It somewhat resembles the related Slender Knotweed (Polygonum tenue), which has leaves that are shorter and pleated forming a “W” in cross-section, fruits that are mostly erect to ascending and not drooping, and currently found only in the southern half of the state.

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More photos

Photos courtesy Peter M. Dziuk taken in St. Louis County.

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