Noxious Pigweed Management Guidelines for Field Crop Growers

For palmer amaranth and waterhemp.
Noxious Pigweed Management Guidelines for Field Crop Growers - Articles

Updated: March 14, 2016

Noxious Pigweed Management Guidelines for Field Crop Growers

Joseph M. DiTomaso, University of California - Davis, Bugwood.org

General Guidelines

It is important to learn about these two invasive pigweeds (Palmer amaranth and Waterhemp).

  • Know where they are prevalent across the country.
  • Know where they have been found in Pennsylvania.
  • Know the weed biology (aggressive germination, competitiveness, seed production, and herbicide resistance).
  • Know how they spread (equipment, feed, grain, hay, manure, mulch and seed).
  • Know how to reduce their impact including preventing their movement and spread.

Develop strategies to address Palmer amaranth and Waterhemp in Commerce.

  • Identify and address pathways for the movement and spread of these weeds.
  • Identify and address effective, consistent and complementary containment measures.
  • Collaborate with academia, government and industry to take effective preventative actions.

Best Management Practices

If you don't currently have the problem

  • Educate yourself about these two new noxious pigweeds.
  • Scout fields starting in mid-April through June looking for pigweed seedlings.
  • Diversify in-season herbicide sites of action.
  • Closely monitor fields before and after herbicide application. o If some weeds were not adequately controlled, quickly assess potential reasons for nonperformance.
  • If you recently purchase used equipment, use custom machinery operators, use noncertified crop seed, buy imported feed or hay, import manure or compost or import other materials that could harbor weed seeds; ensure they are not contaminated before used on your farm.
  • Monitor field edges and ditches and fencerows for pigweed plants and aggressively control to prevent seed production and spread.

If you have either of these pigweeds

1. Verify identification and report to your local Penn State Extension Office and to your Professional Crop Advisor.

2. Use an integrated management approach and aggressively manage the weeds to prevent seed production and spread. These practices include:

  1. Rotate away from soybeans or crops where management is more difficult. A diverse crop rotation that includes winter cereals such as wheat and perennial forages including alfalfa will help reduce the potential for seed production and spread.

    i. Use cover crops to help diversify the crop rotation

    ii. Consider using high biomass cover crops prior to soybean (cereal rye) to help suppress emerging pigweeds

  2. Start clean so you do not plant into existing emerged weeds.
  3. Manage the field with no-till if possible leaving any potential seeds near the soil surface. Leaving the seeds near the soil surface will increase biocontrol and weather related mortality

    i. There may be value in initially using deep tillage to bury seeds, but then manage infested field with no-till.

  4. Where herbicides can be employed, soil residual herbicides and effective postemergence products should be used.

    i. Small plants that are less than 4 inches tall are easier to control.

  5. Diversify in-season herbicide sites of action
  6. In addition to the available cultural weed control tactics, organic growers will need to rely more on mechanical weed control with additional cultivator passes followed by hand removal.
  7. Closely monitor fields before and after herbicide application

    i. If some weeds were not adequately controlled, quickly assess potential reasons for nonperformance.

  8. For plants not yet flowering:

    i. With smaller infestations, physically remove plants - do not rely just on herbicides. Pull by hand or use a hoe. Remove plants from field so they do not re-root. Bury or burn removed plants along field edge.

  9. For flowering plants:

    i. If no viable seed is yet present (darkened seeds that shatter), hand rogue smaller infestations and remove plants from within field. Bury or burn removed plants along field edge.

    ii. If discovered after mature seed has formed, physically cut or remove plants prior to harvest and either leave in the field or bury or burn removed plants along field edge.

  10. Harvest corn or soybeans as silage. This will remove most of the pigweed plants with the forage and potentially subject immature and mature seed to ensiling which should destroy much of the seed.
  11. Do not combine harvest mature pigweed plants if at all possible as this will quickly promote spread.

    i. If combine harvest cannot be avoided, harvest infested fields last to avoid moving seeds away from the infested fields.

  12. Mark or flag areas where pigweed plants produced seed. The areas should be intensively scouted the following season and an aggressive management plan implemented
  13. Clean tillage and harvest equipment before leaving infested fields. This is particularly important when harvesting crops with a platform header.
  14. If you purchase used equipment, use custom machinery operators, use noncertified crop seed, buy imported feed or hay, import manure or compost or import other things that could harbor weed seeds; See
  15. Control weeds in non-crop areas. Monitor field edges and ditches and fencerows for pigweed plants and aggressively control to prevent seed production and spread.