Virginia Tech Weed Identification Guide

Ladysthumb: Polygonum persicaria

ladysthumb4-28.jpg (173046 bytes) Weed Description:  A summer annual weed of horticultural, agronomic, and nursery crops that may reach 3 1/2 feet in height.  Ladysthumb is distributed throughout the United States.

Seedlings: Cotyledons are elliptic to lanceolate in outline, whit hairs along the margins.  First true leaves are alternate, lanceolate in outline, and hairy on the upper surfaces.

Leaves:  Arranged alternately along the stem, lanceolate to elliptic in outline, approximately 2 to 6 inches long and 1 1/4 inches wide.  Older leaves are usually only slightly hairy.  Leaves taper to short petioles, which have an ocrea that encircles the stem.  Leaves often, but not always, have a purple spot in the middle of the leaf which resembles the mark of a lady's thumb, thus the name of this weed.

Roots:   Fibrous roots with a shallow taproot.

ladysthumb4-10b.jpg (171031 bytes)
ladysthumb4-10.jpg (57953 bytes) Stems: Branched, often reddish in color and swollen at the nodes.  A thin membranous sheath called an ocrea encircles the stem at the base of each leaf petiole.  The ocrea's of ladysthumb have stiff hairs arising from the top of the ocrea, which are approximately 2 mm long.

Flowers:  Flowers are clustered in terminal spikes at the ends of stems.  Individual flowers are small and usually pink in color but can occasionally be white.

Fruit:  A black achene.

Identifying Characteristics: The elliptic to lanceolate leaves with a purple spotted 'lady's thumb' print in the middle and distinctive ocrea with stiff hairs are all characteristics that help to distinguish ladysthumb from other similar weeds.  Pennsylvania Smartweed (Polygonum pensylvanicum) is very similar in appearance and growth habit, but does not have hairs on the ocrea like that of ladysthumb.  Tufted Knotweed (Polygonum caespitosum var. longisetum) is also similar in appearance and growth habit, but has hairs on the ocrea that are much longer (5 to 10 mm) than those of ladysthumb.