Ministry of Agriculture, Food and Rural Affairs

Ontario Weeds: Lady's thumb

Table of Contents

1. Name
2. Other Names
3. Family
4. General Description
   • Photos and Pictures
5. Stems and Roots
6. Leaves
7. Flowers and Fruit
8. Caution
9. Habitat
10. Similar Species
11. Related Links

Name: Lady's-thumb, Polygonum persicaria L

Other Names: renouée persicaire, Red shank, Smartweed, persicaire pied rouge, persicaire

Family: Buckwheat or Smartweed Family (Polygonaceae)

General Description: Annual, reproducing only by seed.

Photos and Pictures

Stems and Roots: Stems erect from a taproot, 20-100cm (8-40in.) high, green or reddish, smooth except for slightly swollen at the distinct nodes; each node with a hairy ocrea (cylindrical membranous sheath surrounding the stem).

Leaves: Leaves alternate (1 per node), narrowly elliptic, 2-15cm (4/5-6in.) long, greenish above and slightly paler below, usually with a reddish to brownish or purplish blotch near the middle; undersurface of leaf often slightly rough with tiny bumps, but never glandular or hairy, ocrea arising with the leafstalk at each node, membranous and somewhat papery, its surface covered with short, upward slanting hair and its upper margin ciliate with a fringe of short, erect hair about 1-2mm (1/25-1/12in.) long.

Flowers & Fruit: Flowers small, densely crowded into narrow cylindrical spikes (1-4.5cm, 2/5-2in.) long at ends of stems and branches; each flower with 5 pinkish sepals 2-4mm (1/12-1/6in.) long, sometimes nearly white; fruits ("seeds") more or less enclosed by the sepals when mature, shiny, smooth, black, broadly ovate in outline, about 2mm (1/12in.) long; of 2 kinds, either rounded-triangular or flattened or somewhat lens-shaped in cross-section; the seed often slightly thickened near the middle. Seedling with cotyledons (seed leaves) about 8-12mm (1/3-½in.) long by 2-3mm (1/12-1/8in.) wide, tapered towards both ends, reddish on the undersurface; stem below the cotyledons often reddish to brownish-green; cotyledons soon withering on developing stems. Flowers from June to September.

Habitat: Lady's-thumb is an introduced weed which occurs in cultivated land on nearly all soil textures throughout Ontario as well as along roadsides and waste places.

Similar Species: It is distinguished from other members of the Smartweed Family by the following combination of characteristics: undersurface of leaves without matted white hair or yellowish glands, ocrea with hair on the surface and a fringe of longer hair on the margin, and the stem lacking glands on the upper portion near the spikes of flowers. The reddish or purplish blotch usually present on the upper surface of the leaves cannot be relied upon as a distinguishing feature of Lady's-thumb. Occasionally it is absent from this species and frequently can be found on leaves of other species as well.

Caution: The seed is a frequent contaminant in small grains. 

Related Links

... on general Weed topics
... on weed identification, order OMAFRA Publication 505: Ontario Weeds
... on weed control, order OMAFRA Publication 75: Guide To Weed Control

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