Virginia Tech Weed Identification Guide

Tufted Knotweed: Polygonum caespitosum var.longisetum

smartweed9-18.jpg (164463 bytes) Weed Description:  A summer annual weed of horticultural, agronomic, and nursery crops that may reach 3 1/2 feet in height.

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.

Roots:  Fibrous roots with a shallow taproot.

Fruit:  A black achene.

smartweed9-18c.jpg (98680 bytes)
smartweed8-7.jpg (56801 bytes) Leaves:  Arranged alternately along the stem, lanceolate to elliptic in outline, approximately 3/4 to 3 inches long and 1/2 to 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.
Flowers:  Flowers are clustered in terminal spikes at the ends of stems.  Individual flowers are small and are dark pink to red in color. smartweed9-18b.jpg (93884 bytes)
tuftedknot8-10.jpg (58923 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 tufted knotweed have stiff hairs arising from the top of the ocrea, which are from 5 to 10 mm long.

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 tufted knotweed 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 tufted knotweed.  Ladysthumb (Polygonum persicaria) is also similar in appearance and growth habit, but has hairs on the ocrea that are much shorter (2 mm) than those of ladysthumb, and also has generally smaller leaves and dark pink to red flowers.