Polygonum tenue (Slender Knotweed)

Plant Info
Also known as: Pleat-leaf Knotweed
Genus:Polygonum
Family:Polygonaceae (Buckwheat)
Life cycle:annual
Origin:native
Habitat:sun; dry, sandy soil; bluffs, savanna, hill prairies
Bloom season:July - September
Plant height:4 to 20 inches
Wetland Indicator Status:none
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 Cluster type: raceme

[photo of flowers] 1 or 2 flowers (rarely 3) 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 pinkish with a green midvein, and blunt to pointed at the tip. In the center are 8 stamens surrounding a cone-shaped ovary. Flower stalks are minute, erect to ascending both in flower and in fruit. All flower parts are hairless.

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

[photo of leaves] Leaves are alternate, erect to ascending, ½ to 1½ inches long, up to 1/3 inch wide, narrowly lance-linear, pointed at the tip, with a prominent midvein and a pleat on each side, forming a “W” in cross-section. Edges are toothless and may be minutely hairy (magnification required); surfaces are hairless. At the base of the short leaf stalk is a membranous sheath (ocrea) that extends up around a branch stem, tearing away with age, its upper edge typically disintegrating into a few brown fibers. Stems are erect, angled, smooth or slightly rough, green to brown, usually branched from the lower stem, the branches mostly erect.

Fruit: Fruit type: seed without plume

[photo of fruit] The tepals persist in fruit, tightly wrapping the single seed and remaining erect to ascending at maturity.

[photo of seed] Seeds are black, 3-sided, oval-elliptic, mostly smooth and shiny.

Notes:

Slender Knotweed reaches the northern edge of its range in Minnesota, and is often overlooked due to its slender, spindly appearance and inconspicuous flowers and fruits. It is similar to the related Douglas' Knotweed (Polygonum douglasii), which has leaves that are not pleated, its flowers and fruits are more spreading to drooping, and is currently found only in our northeast counties.

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More photos

Photos courtesy Peter M. Dziuk taken in County.

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