University of Delaware Weed Science:

Red clover and rye blow in the wind at a cover crop experiment the University of Delaware research station in Georgetown, DE. Photo: Annie Klodd

Weed scientists at the University of Delaware, led by Dr. Mark VanGessel, conduct a number of field studies on integrated weed management practices. Among these are several experiments focused on cover crops. These studies help us understand how cover crops can best be used to help manage weeds as part of an integrated weed management program. One study, which is a collaboration with Penn State, examines how several cover crop mixes interact with different herbicide programs to manage troublesome weeds like marestail (horseweed) and pigweed. Another ongoing study at UD looks at how cover crop management affects its ability to suppress weeds. These management factors include soil fertility, planting timing, and termination timing. They also conduct herbicide trials for field crops as well as specialty crops, and evaluate a range of cash crops as cover crops such as canola and oats.

Mark VanGessel pulls back a large stand of rye cover crop in late May, revealing largely weed-free soil underneath. This rye is part of a study on weed management by cover crops. Photo: Annie Klodd

Herbicide Resistant Weeds in Delaware

Updated: September 22, 2016. Mark VanGessel, University of Delaware; – Delaware

Equipment is a primary vector for the introduction and spread of resistant weeds into and within Delaware.

Reading this list: Each herbicide resistant weed in the state is listed. Underneath each species is a list of the unique resistant biotypes of that weed in the state, according to the herbicide group(s) they are resistant to.

First priority: Horseweed. Highly prevalent. Blew in from Ohio Valley thru TN & KY, to DE within 1 year. Also a problem in veggie crops.

  • Group 9 (glyphosate)
  • Group 2
  • Group 2 (ALS) + Group 9 (glyphosate)
  • Group 22 (Paraquat)

Second priority: Palmer Amaranth. This pigweed species has become very widespread throughout the state’s agricultural areas.

  • Group 2 (ALS)
  • Group 9 (glyphosate)
  • Group 2 + Group 9

Third priority: Smooth/redroot pigweed

  • Group 5 (atrazine)
  • Group 2 (ALS)

Other herbicide resistant weeds in Delaware:


  • Group 5

Common chickweed – problem in small grains

  • Group 2 (ALS)

Italian ryegrass

  • Group 2 (ALS)

Common ragweed

  • Group 2 (ALS) + Group 14 (PPO)