Pennsylvania Smartweed

Polygonum pensylvanicum

Pennsylvania smartweed
Pennsylvania. Photo courtesy of F.D. Richards. Licensed under CC BY-SA 2.0.
Pennsylvania smartweed is a summer annual weed that is often appreciated for its attractive pink flower cluster. It is native to North America and prefers to grow in moist areas, where it has been known for growing to about 3 feet tall.
Seedlings generally germinate in the spring with its cotyledons being either lance-shaped or elliptic and hair on the margins. The first true leaves are lance shaped and alternately arranged. Their topside is usually hairy.
Pennsylvania smartweed seedling
Pennsylvania smartweed seedling. Photo courtesy of John D. Byrd. Licensed under CC BY 3.0.
Older leaves on the Pennsylvania smartweed usually are only slightly hairy and continue the trend of being lance-shaped or elliptic. They are anywhere from 2 to 6 inches long and often have a purple spot in their center. The stems are branched, having a reddish color and swollen nodes. Each leaf petiole is surrounded by a hairless ocrea where it meets the stem. Pennsylvania smartweed has a taproot surrounded by a fibrous root system. The entire plant is generally no more than 3 feet high, but some are known to become as tall as 4 feet.
Mature Pennsylvania smartweed
Mature Pennsylvania smartweed as it begins to flower. Photo courtesy of F.D. Richards. {{CCBY-SA2.0}}
Flowers of the Pennsylvania smartweed form in dense, spikelike clusters at the tips of each stem during the late summer and early fall. The flowers produce seeds that are inside of a small, shiny black fruit with a sharp tip.
Pennsylvania smartweed flowers
Pennsylvania smartweed flowers. Photo courtesy of Dendroica cerulea. {{CCBY-NC-SA2.0)
Pennsylvania smartweed are able to tolerate well dry spells where many other, more desireable, plants may struggle. It competes with other surrounding plants for nutrients, space, and moisture.
Simple tillage and establishment of healthy crops will be enough to crowd out the Pennsylvania smartweed.
Pennsylvania smartweed can be effectively controlled by 2,4-D, acifluorfen, and carfentrazone when it's used alone or in combination with propanil or quinclorac. Please contact your local weed specialist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.