Ontario Weeds: Prostrate knotweed
Excerpt from Publication 505, Ontario Weeds,
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Table of Contents
- Other Names
- General Description
- Stems and Roots
- Flowers and Fruit
- Similar Species
- Related Links
Name: Prostrate knotweed, Polygonum
Other Names: renouée des oiseaux, Doorweed, Knot-grass, Mat-grass, Road-spread, traînasse, renouée aviculaire
Family: Buckwheat or Smartweed Family (Polygonaceae)
General Description: Annual, reproducing only by seed.
Prostrate knotweed (A - plant on a dry lawn; B - portion of stems showing ocrea).
Prostrate knotweed. A. Plant. B. Branch tip with flowers in several leaf axils. C. 1 older leaf with 1 "seed" in its axil, the "seed" still enclosed in its sepals.
Stems & Roots: Stems prostrate
or semi-erect from a thin, tough, wiry, deeply penetrating taproot;
stems slender, tough and wiry; growth habit variable: in open sunny
situations stems fully prostrate, to 1m (40in.) or longer, or main
stems prostrate with short, upturned branches, but in partly shaded
situations, such as a grain crop, stems are nearly erect to 30 or
40cm (12-16in.) high; stem nodes (joints) distinct, thickened, and
surrounded by a thin, papery ocrea (membranous sheath) that usually
has a torn or jagged margin.
Leaves: Leaves alternate (1 per node), up to 5cm (2in.) long but usually much shorter, their width about 1/3-1/5 of their length, usually broadest near or past the middle, and narrower towards both ends.
Flowers & Fruit: Flowers small and inconspicuous, 2mm (1/12in.) or less long, without petals but with 5 tiny greenish, pinkish or purplish sepals, produced in axils of leaves and partially enclosed in the ocrea; mature "seed" more or less enclosed by the drying sepals, slightly rough, dull brown, triangular in cross-section and about 2mm (1/12in.) long. Flowers from June to September.
Habitat: Prostrate knotweed occurs throughout Ontario in areas of moderately heavy foot- or wheel-traffic where the soils may be low in fertility and so heavily compacted that other plants are unable to survive. It is one of the most common weeds along roadsides, edges of or cracks in sidewalks and pavement, and heavy-traffic areas in lawns. It also occurs in gardens and cultivated fields where it tends to have a more erect habit of growth.
Similar Species: It is distinguished by its thin, wiry stems with small leaves, the ragged ocrea surrounding the stem above every leaf, and its tiny greenish to pinkish or purplish flowers in axils of leaves. Prostrate knotweed, an introduction from Eurasia, is similar to several, closely related, native species of knotweed. One of the more common of these is Striate knotweed, Polygonum achoreum Blake,[POLAH, renouée coriace, Erect knotweed] This plant sometimes occurs in similar situations, especially roadsides, and is distinguished from Prostrate knotweed by its coarser, more erect stems, broader and more rounded leaves, and by its triangular seeds being smooth and olive-coloured. All knotweeds have wiry stems with swollen nodes, membranous ocrea, and tiny flowers arising inside the ocrea in leaf axils.
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