Lady's thumb, Polygonum persicaria

Life Cycle



Reproducing only by seed.


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


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

Flowers and Fruit

Flowers small, densely crowded into narrow cylindrical spikes 1 - 4.5 cm long at ends of stems and branches; each flower with 5 pinkish sepals 2 - 4 mm long, sometimes nearly white; fruits (“seeds”) more or less enclosed by the sepals when mature, shiny, smooth, black, broadly ovate in outline, about 2 mm 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 - 12 mm long by 2 - 3 mm wide, tapered towards both ends, reddish on the undersurface; stem below the cotyledons often reddish to brownish-green; cotyledons soon withering (f) on developing stems. Flowers from June to September.

Roots and Underground Structures

Fibrous root system from a shallow taproot.


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. The seed is a frequent contaminant in small grains.


Corn yield loss (%)*: 3 % at 1 plant/m2 13 % at 5 plant/m2 Soybean yield loss (%)*: 4 % at 1 plant/m2 15 % at 5 plant/m2 *assumes that the weed has emerged with the crop and has been left uncontrolled all season.

Distinguishing Features

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 (b, B) with hair (d) on the surface and a fringe of longer hair (e) on the margin, and the stem lacking glands on the upper portion near the spikes of flowers. The reddish or purplish blotch (c) usually present on the upper surface of the leaves cannot be relied upon as a distinguishing feature of Lady’s-thumb. Occasionally the blotch is absent from this species and frequently can be found on leaves of other species as well.


Lady's thumb is not known to be toxic.

Human Health Issues

Lady's thumb is not a known allergen.

Forage Quality

No information exists at this time.

Species Benefits

The leaves are edible and some members of the smartweed family possess a "peppery" flavour. From: Peterson, LA, 1977, A Field Guide to Edible Wild Plants of Eastern and Central North America, Houghton Mifflin Company, New York.

Often Mistaken For

Green smartweed (POLSC) and Pennsylvania smartweed (POLPY)

Power Ranking Corn

Power Ranking

↑ 33


Power Ranking Soybeans

Power Ranking

↑ 24


Biological Control

Currently none available for this weed. For the latest research on biological weed control:

Biopesticide Control

Currently none available for this weed in corn and soybean.

Herbicide Resistance

No documented cases of herbicide resistance in Canada. For more information on weed resistance:

Figure #1.

Lady's Thumb. A. Plant. B. Portion of stem with leafstalk and ocrea. Pale smartweed. C. Portion of stem with lower surface of leaf blade, leafstalk and ocrea. Pennsylvania smartweed. D. Portion of lower stem with leafstalk and ocrea. E. Portion of upper stem with leaf stalk and ocrea showing stalked glands.

Figure #2.

Lady's thumb at the 3-leaf stage.

Figure #3.

Lady's thumb leaf.

Figure #4.

Lady's thumb ocrea.

Figure #5.

Flowering head of lady's thumb.

Figure #6.

Mature lady's thumb plant.