Pennsylvania smartweed (Persicaria pensylvanica). Wirestock / iStock / Getty Images Plus
Getting to know Pennsylvania smartweed
Pennsylvania smartweed (Persicaria pensylvanica, formerly Polygonum pensylvanicum) is a wiry, multi-branched plant that has small lance-shaped green leaves that come to a point. The leaves are arranged alternately up the red-tinted stems, have smooth edges, and may have a black or purple mark in the center.
The most defining trait is the plant’s flowers, which usually begin appearing in July and continue into early fall. The flowers are inch-long spikes of light pink that form at the top of the wiry flower stems.
Pennsylvania smartweed plants typically grow about 18 inches tall, although left untouched in rich, moist soil, heights can reach three or even four feet.
Smartweed seeds sprout into young plants in late spring to early summer. The summer flowers ultimately produce small black seeds that drop to overwinter in the soil and produce a new crop the following spring.Each season’s plants die in fall.
Birds also help distribute Pennsylvania smartweed. At least 50 species of birds from geese to sparrows (plus several mammals) feed on smartweed seeds, but since they don’t digest all of the seeds, their droppings spread seeds to new locations.
Each smartweed plant is capable of producing 20,000 or more seeds per year. About half of that seed is still viable in the soil four years later.
Seeds germinate best in damp soil, so smartweed is often less of a problem in dry climates or drought conditions.