Pennsylvania smartweed

August 28, 2015

Polygonum pensylvanicum L. 

Life cycle

Erect summer annual. 

Pennsylvania smartweed seedling
Pennsylvania smartweed seedling.

Leaves

Cotyledons are narrow oval to lance-shaped with rounded tips. Leaves are alternate, lance-shaped with pointed tips and smooth margins, usually hairless and occasionally with a purple watermark.

Pennsylvania smartweed leaf
Pennsylvania smartweed leaf.

Stems

Branched, erect up to 4 feet tall and jointed with swollen nodes. A smooth, membranous sheath (ocrea) surrounds the stem at the base of each petiole.

Pennsylvania smartweed & ladysthumb ocreas
Pennsylvania smartweed ocrea (left). Ladysthumb ocrea (right).

Flowers and fruit

Small, pink to white flowers form in dense, spike-like clusters at the tips of stems. The seed is enclosed in a single-seeded, flat, glossy black, round to oval fruit with a pointed tip.

Pennsylvania smartweed flower
Pennsylvania smartweed immature flower cluster.

Reproduction

Seeds.

Similar weeds

Ladysthumb (P. persicaria L.) Differs by having a fringe of bristly hairs at the top of the ocrea. Usually has a purple watermark on leaf.

Pale smartweed (P. lapathifolium L.) Differs by having young leaf undersides with whitish hair, older leaf undersides with yellow glands and a nodding inflorescence.

Swamp smartweed (P. amphibium var. emersum Michx.) Differs by having perennial, creeping, woody rhizomes and usually hairy foliage; found in wetter environment.

swamp smartweed foliage
Hairy foliage of swamp smartweed.

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