Pennsylvania smartweed

Smartweed (Polygonum pensylvanicum). Photo: Betty Marose

Updated: March 3, 2023

Other common names are Pennsylvania knotweed, Pennsylvania smartweed, pinweed, purple-head

Life cycle: summer annual

Growth habit: prostrate; grows from 1 to 4 ft. tall; leaves alternate, lanceolate, smooth margins, sometimes with a purplish thumbprint marking the 2 to 3 in. leaf; conspicuous ocrea surrounding point where petiole meets stem

Reproduction: seeds from pink flowers in the spike-like clusters; can become almost invasive if allowed to go to seed

Conditions that favor growth: common weed of nursery crops and landscapes; rarely a problem in turfgrass; tolerates a range of soil types and conditions

Cultural control: Remove young plants before flowers form

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