University of Illinois
Zero Tolerance For Pigweed Weed Species

[July 27, 2019]    When it comes to keeping pigweed in check, scientists across the Corn Belt, including Dr. Aaron Hager, Associate Professor, Department of Crop Sciences, University of Illinois, are recommending "zero" tolerance for palmer amaranth escapes.

This is due to the fact that one female plant will produce 460,000 seeds. To put this into prospective consider the fact that even if 95% of the seeds produced are controlled, there is still the potential for 23,000 plants to survive.

Two particularly troublesome pigweeds are Tall Waterhemp and Palmer Amaranth, which have and are continuing to develop resistance to herbicides. In Illinois, Tall Waterhemp is resistant to 6 different classes of herbicides and Palmer Amaranth is resistant to 3. If that is not bad enough, members of the pigweed or amaranth family can cross pollinate between species which aids in the rapid spread of resistance as well.

A weed free field is the key starting point to managing the situation. Tillage or an herbicide burndown plus a residual herbicide are essential when Palmer weed is present. This must be followed by vigilant scouting and herbicide applications combining post-emergence and residual products when new seedlings are identified.

If Palmer is identified after it is too big for herbicide control, hand removal is the best option. Any field in which Palmer weed reaches maturity should be harvested last and left untilled. Deeply burying the seed may lead to a decade of Palmer problems as future tillage brings old seeds up to the surface. However, leaving any seed on the surface will allow for natural forces to reduce the viability of seed and help reduce germination the following spring, and burndown herbicides can be applied to reduce the population potential. Research shows that viable seeds left near the surface will "burn out" in about 4 years if no future seeds are added to the seed bank.

[Doug Gucker, Local Food Systems and Small Farms Educator, University of Illinois Extension]