Lady’s-Thumb
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.
Seedlings
- 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
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)
Leaves
- 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
Flowers
- 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)
Caution:
The seed is a frequent contaminant in small grains.