Field Scouting Guide: Pigweed

Field Scouting Guide: Pigweed

Herbicide-resistant smooth pigweed (redroot pigweed). Photo by Mark VanGessel

This month’s guide focuses on Amaranthus hybridus/Amaranthus retroflexus (smooth pigweed/redroot pigweed). Each month, we bring you a different crop protection issue, ranging from weeds and diseases to insects and wildlife.

We’ve reached out to pathologists to learn how to spot and treat this weed. This month, our contributor is Mark VanGessel from the University of Delaware Research and Education Center.



  • Scientific name: Amaranthus hybridus/Amaranthus retroflexus
  • Common name: Smooth pigweed/redroot pigweed
  • Geographical Range: Both are found throughout the U.S.
  • Crops affected: All vegetable crops are affected.


You can find both pigweed types throughout the U.S. They can cause significant loss in vegetable crops. Both are summer annual weeds that emerge in early spring, so they can be a particular problem for all spring-planted vegetables.


Like waterhemp and Palmer amaranth, Powell pigweed, prostrate pigweed, and others can be confused with eastern black nightshade or common lambsquarters.

Lynn Sosnoskie with the University of California, Riverside, has an excellent, detailed guide on how to identify this weed. She walks you through each variety’s characteristics, from the roots to the leaves and seeds.


Many herbicides will effectively control this weed, and there are too many to list. Remember that many herbicides are prone to developing resistance. Triazine and ALS (acetolactate synthase) herbicides are some that are notorious for developing resistance in this area.

For integrated pest management, I recommend small-seeded species that germinate from only an inch or so deep in the soil. Cover crops are effective. Cultivation and rotary hoe, or harrows, are effective on this species. Pigweed produces a tremendous number of seeds, so it can quickly infest fields if growers aren’t careful.