Polygonum achoreum (Leathery Knotweed)

Plant Info
Also known as: Blake's Knotweed, Striate Knotweed, Blue Knotweed
Genus:Polygonum
Family:Polygonaceae (Buckwheat)
Life cycle:annual
Origin:native
Status:
  • Weedy
Habitat:sun; moist to dry disturbed soil; roadsides, fields, shores, waste places, gravel pits, shores
Bloom season:July - September
Plant height:4 to 28 inches
Wetland Indicator Status:GP: FACU MW: FAC NCNE: FACU
MN county distribution (click map to enlarge):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

Pick an image for a larger view. See the glossary for icon descriptions.

Detailed Information

Flower: Flower shape: 5-petals Flower shape: indistinct Cluster type: whorled

[photo of flower] 1 to 3 short-stalked flowers (rarely 4 or 5) in the leaf axils of branching stems. Flowers are tiny, less than 1/8 inch long, with 5 erect to spreading tepals (petals and similar sepals) that are yellowish-green and whitish or yellowish around the edges, rarely pink. The 3 outer tepals are boat-shaped, the 2 inner tepals are narrower, flatter and rounded at the tip. In the center are 5 to 8 yellow stamens surrounding a rounded ovary. Flower stalks are less than 2 mm long, mostly hidden within associated the leaf sheath.

[close-up of tepals and perianth] The tepals are at the tip of a bowl-shaped tube, the entire structure known as the perianth. At maturity, the perianth is 2.5 to 4 mm long, the tube more or less as long as the tepals, 40% to 60% the length of the perianth.

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

[photo of leaves] Leaves are alternate, toothless, hairless, to 1-3/8 inches long, up to about 2/3 inch wide, oval to egg-shaped, rounded at the tip, on a short stalk. Color is blue-green to bright green; leaves are often covered in a whitish, powdery mildew.

[photo of ocrea] At the base of the leaf stalk is a thin, membranous sheath (ocrea), silvery with brown veins, commonly soon shredding into brown fibers. Stems are green, finely ribbed, hairless, moderately to many-branched, initially erect, becoming sprawling with upturned branches.

Fruit: Fruit type: seed without plume

[photo of fruit] The perianth persists in fruit, wrapping the single seed and becoming pear-shaped at maturity. Seeds are shorter than the perianth, dull yellow-green to yellow-brown, minutely bumpy on the surface, 3-sided, egg-shaped to nearly triangular, tapering to a blunt tip.

Notes:

Leathery Knotweed is a common native with a weedy disposition, typically found in disturbed soils such as roadsides, pavement cracks, weedy shorelines and the margins of agricultural fields. It is one of several Polygonum species with branching stems and 1 to several tiny flowers in the leaf axils of the branches. They can be difficult to distinguish; things to look for are plant color (blue-green or yellow-green), leaf shape, whether leaves on flowering branches are much smaller than those on the main stems, length of the perianth tube relative to the tepals, tepal shape and color, seeds dull or shiny. Identification is best earlier in the season when both flowers and fruit are present, as late season plants may not exhibit the same traits consistently, particularly the fruit.

Leathery Knotweed may be the most recognizable of this group with its rounded leaves that are more or less the same size on branches as the main stems; tepals yellowish-green usually with whitish or yellowish edging; outer tepals boat-shaped, inner tepals narrower and rounded at the tip; perianth tube about as long as the tepals (or longer, best seen at maturity); and dull yellowish to brownish seeds shorter than the perianth. Overall color is typically blue-green and foliage is often covered in a whitish, powdery mildew. The leaf sheath (ocrea) tends to shred into brown fibers early.

It is most likely to be confused with Erect Knotweed (Polygonum erectum), which tends to have longer and proportionately narrower leaves that are more yellow-green, and the perianth tube is distinctly shorter than the tepals (20% to 38% the length of the perianth).

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More photos

Photos by Peter M. Dziuk taken at various locations around Minnesota.

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