Virginia Tech Weed Identification Guide

Pennsylvania Smartweed: Polygonum pensylvanicum

Weed Description:  A summer annual weed of horticultural, agronomic, and nursery crops that may reach 3 1/2 feet in height.  Pennsylvania smartweed 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.
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 Pennsylvania smartweed do not have any hairs arising from the the ocrea, as is the case with many other similar weeds.

Fruit:  A black achene.

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.

Identifying Characteristics: The elliptic to lanceolate leaves with a purple spotted 'lady's thumb' print in the middle and distinctive ocrea are all characteristics that help to distinguish Pennsylvania smartweed from other similar weeds.  Ladysthumb (Polygonum persicaria) is very similar in appearance and growth habit, but has stiff hairs on the ocrea that are approximately 2 mm long unlike that of Pennsylvania smartweed.  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.