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Pennsylvania Smartweed (Polygonum pensylvanicum L.)

Crop Impacts: Hay fields and cultivated lands

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About Pennsylvania Smartweed:

Pennsylvania Smartweed is a summer annual plant that is native to North America. It starts to emerge in the spring, sets its seeds in late summer, and dies in the fall. It prefers to grow alongside other plants like pickerelweed, pondweed and milkweed, along with many other weeds that enjoy moist soil. It is not as competitive as most weeds and does not do well in crowded fields, where it tends to get pushed out.

Family: Buckwheat or Smartweed Family (Polygonaceae)

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Pennsylvania Smartweed Scouting and Prevention:

When looking at the Pennsylvania Smartweed leaves, you will notice they are long, narrow and alternate from one another. Like many other plants in the smartweed family, Pennsylvania Smartweed grows many small, pink flowers on .5 to 2 inch long spiked clusters that rest on hairy stalks and bloom from May until October. Pennsylvania Smartweed also produces small, two-sided achene’s that have one seed in them. One plant can have up to 800 achenes. At full maturity this plant can stand from 1 to 4 feet tall with a reddish purple, smooth stem and a rhizomes system/taproot that can grow up to 8 inches underground.

Common locations

  • - Hay fields
  • - Cultivated lands


The best way to prevent this weed from growing on your land is to plant competitive and dense varieties of crops and grasses (if worried about invasion on your lawn). Tilling your fields at night, which is also known as dark tillage, has been seen to reduce the weeds emergent by 30 to 50%. If you are using farming equipment in contaminated areas, make sure you give your machines a good cleaning to ensure that none of the rhizomes wrap around and/or hang off of the tillage equipment.

Pennsylvania Smartweed Control:

Cultural Control

Pennsylvania Smartweed can be controlled by hand removal or frequent mowing. This weed thrives off of moist soil. By restricting water and improving drainage, the weed will struggle to survive. It also cannot live through any hoeing before it grows larger than ¼ inches tall. Rotating your crops with varieties of small grain plants will reduce the growth of Pennsylvania Smartweed. The final cultural control method is planting your crops at a specific time. It is important that you till during the spring to trigger the germination of Pennsylvania Smartweed. This means that you should be tilling your fields in early May and have your fields planted by late May.

Chemical Control

If you decide to use herbicides, it is very important that you apply it while the weed is actively growing and the flowers are still in the seeding part of their growth cycle. The most effective way of chemically controlling Pennsylvania Smartweed is by applying herbicides that have dicamba in them, such as atrazine, Banvel and Clarity.

Latin / Alternative Pennsylvania Smartweed names:

  • - Polygonum pensylvanicum L.
  • - Lady Thumb
  • - renou?e de Pennsylvanie
  • - Pinkweed
  • - Persicaire glanduleuse

Additional Pennsylvania Smartweed Resources