Pennsylvania smartweed

Polygonum pensylvanicum L.

Life cycle

Erect summer annual.


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 seedling
Pennsylvania smartweed leaf (left), and seedling (right).


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 ocrea
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.



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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The MSU IPM Program maintains this site as an access point to pest management information at MSU. The IPM Program is administered within the Department of Entomology, fueled by research from the AgBioResearch, delivered to citizens through MSU Extension, and proud to be a part of Project GREEEN.