By Greg LaBarge, CCA, Ohio State University Extension agronomist

There are a few questions you can work through to understand why weeds that should have been controlled with your herbicide package are still growing. The list of questions is from the Take Action Pesticide Resistance Management website (

First, rule out factors that affect herbicide performance.

  • Misapplication

• Wrong rate, nozzle, or volume

• Antagonism with tank-mix products

• pH of spray solution

• Boom height

• Calibration

  • Unfavorable weather conditions.
  • Weed factors

• Size

• Stressed plants

• Weed flushes that emerge after herbicide application.

  • Soil conditions that lower residual herbicide performance.

Resistance is likely if you answer yes to one or more of the questions.

  • Are other weeds listed on the product label satisfactorily controlled? Chances are, only one weed species will show herbicide resistance in any given field situation. However, if several normally susceptible weed species are present, go back to the list of factors affecting herbicide performance.
  • Is there a spreading patch of non-controlled plants of a weed species?
  • Are there surviving plants mixed with dead plants of the same species?

What should you do in 2022 if you suspect resistance?

  • Re-treat with a different mode of action. Consult the 2022 Weed Control Guide for Ohio, Indiana, and Illinois to identify a herbicide site of action not used in previous applications.
  • Contact your local adviser, extension educator, and state weed specialist for guidance.
  • If weeds are too large, the second application may not be effective. Consider appropriate non-chemical weed control methods to prevent the weeds from going to seed.
  • Several labs test seeds for herbicide resistance. The results help develop an alternative herbicide system for the 2023 crop.

What common Ohio weeds have resistance?

Mark Loux’s weed science group at OSU has done a great job tracking herbicide resistance in Ohio Weed populations. If you attended a pesticide license recertification program over the winter, you likely saw Table 1. The table list some of our herbicide-resistance weeds and what sites of action have shown resistance. 


Know your pigweeds

Weed identification is important to put the correct control program in place. Several pigweed species can be found in Ohio fields. It is essential to identify them correctly since a couple are tougher weeds to control. First, Palmer amaranth and, more recently, waterhemp have gotten a fair amount of airtime in weed management talks. Competitiveness, later emergence, high seed production, and herbicide resistance are a few factors that make these weeds to focus on keeping out of your fields.

The first thing to look for on the plants is hairs. Plants with hair on stem or leaves are smooth/red pigweed. No hair on stem leaves could be waterhemp, Palmer amaranth, or spiny amaranth. 

To identify waterhemp, Palmer amaranth, or spiny amaranth, focus on the leaf and petiole length. The leaf is the blade portion. The petiole is the stocky stem that attaches the leaf to the plant’s main stem. 

  • Waterhemp has a long narrow leaf with a short petiole. 
  • Palmer amaranth has a petiole as long as or longer than the leaf. 
  • Spiny amaranth has quarter to half-inch long spines where the petiole attaches to the stem.

Pictures are always better than words for plant ID, and our weed group has a nice color factsheet you can find here

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