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Ministry of Agriculture, Food and Rural Affairs


Scientific Name: 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.

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.


  • Cotyledons about 8- 12 mm (1/3- ½ in.) long by 2- 3 mm (1/12- 1/8 in.) wide, tapered towards both ends, reddish on the undersurface
  • Stem below the cotyledons often reddish to brownish-green
  • Cotyledons soon withering on developing stems


  • Erect from a taproot
  • 20- 100 cm (8- 40 in.) 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)


  • Alternate (1 per node)
  • Narrowly elliptic
  • 2- 15 cm (4/5- 6 in.) 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
    • Surface covered with short, upward slanting hair
    • Upper margin ciliate with a fringe of short, erect hair about 1- 2mm (1/25-1/12in.) long


  • Small
  • Densely crowded into narrow cylindrical spikes (1- 4.5 cm, 2/5- 2 in.) long at ends of stems and branches
  • Each flower with 5 pinkish sepals 2- 4 mm (1/12- 1/6 in.) long, sometimes nearly white
  • Seeds:
    • More or less enclosed by the sepals when mature
    • Shiny
    • Smooth
    • Black
    • Broadly ovate in outline
    • About 2 mm (1/12 in.) long
    • Either rounded-triangular or flattened or somewhat lens-shaped in cross-section
    • Often slightly thickened near the middle
  • Flowers from June to September

Often Confused With
Other members of the Smartweed family (Lady’s-thumb is distinguished by (1) undersurface of leaves without matted white hair or yellowish glands (2) ocrea with hair on the surface and a fringe on longer hair on the margin (3) stem lacking glands on the upper portion near the spikes of flowers)

The seed is a frequent contaminant in small grains. 

Lady's-thumb in flowerLady's-thumb leaf Lady's-Thumb ocrea Lady's-Thumb Lady's-Thumb seedling
Click to enlarge.