Annual bluegrass

Are you training your weed management staff properly to avoid herbicide resistance and employing best management practices? We spoke with Jeffrey Derr, professor of weed science at Virginia Tech, about ways to identify and prevent herbicide resistance throughout the nursery.

Nursery Management: What are the telltale signs of herbicide resistance?

Jeffrey Derr: The most obvious sign is when you see a given weed species increasing in density, especially when you used to get really good control of that weed. If weeds are escaping more often, you likely have an issue.

If you tried increasing the rate and still don’t see control, that’s another red flag.

When a weed develops resistance, it often tolerates a particular herbicide at an increased rate. You may have heard the term ‘superweed,’ but that’s a real misnomer. Those weeds aren’t going to be any bigger or grow faster. In most cases, they’re resistant to a certain herbicide.

Herbicide rotation is critical for a successful weed management program. If you use the same herbicide year after year, you’re definitely increasing the likelihood for herbicide resistance.

When a grower contacts us and suspects herbicide resistance, we collect specimens of that weed and bring it back to the greenhouse for testing. We do herbicide applications of varying rates to try and prove resistance in greenhouse trials. In our research, we look at elevated application rates. Some resistant species will tolerate four to eight times the labeled rates.

NM: Besides lack of rotation, what are some other reasons herbicide resistance may occur?

JD: There may be some genetic diversity in the population of that weed that is displaying herbicide resistance. With horseweed for example, there will be thousands of seeds in a field and one out of a 100 may have a mutation that was resistant. When it survives, you may get a large population of resistant weeds.

Annual bluegrass has a lot of variability in the species, which may increase its potential for being resistant to herbicides.

Couple that with repeated use of that same herbicide over time and the ‘survivor’ produces seed, and now you have a larger population of resistant plants. It becomes the dominant biotype.

NM: What are some other reasons an herbicide may not be working properly besides resistance?

JD: There are many reasons herbicides are not working properly that don’t point to resistance issues. They may have been mixed or applied the at wrong rate.

The weather conditions may have played a factor. Did it rain right after you applied a post-emergence herbicide? Was it really cold when you treated? Was it windy?

Even distribution is critical, so calibration of equipment is important.

Perhaps the products weren’t stored properly. Proper storage includes keeping the chemicals dry and preventing liquids from freezing.

Horseweed, pre-bolt stage
Annual bluegrass photo courtesy of Michigan State University; horseweed photo by Gary Stone, U of Nebraska, Lincoln

Try to mix up only what you can use in a day because mixes can break down.

Water quality can be an issue, including pH and sediments in the water.

Make sure surfactants were used properly.

NM: What are examples of weeds that have become problematic in terms of management?

JD: Glyphosate-resistant weeds, including horseweed, have become a problem. A lot of that resistance is coming from glyphosate used in no-till crop production. Those seeds can blow in from a soybean field, for example, into a nursery setting. Those seeds can travel a long distance. That’s why it’s important to know what your neighbors are growing and what they’re using for treatment.

There are a few key weed species that are more likely to develop resistance based on their physiology, such as horseweed, annual bluegrass and palmer amaranth. One of the earliest cases of an herbicide-resistant weed in the nursery industry was common groundsel.

NM: What is your checklist for preventing herbicide resistance?

JD: First and foremost, rotate with different modes of action. When tank mixing, use herbicides from different modes of action. Use both pre- and post-emergence herbicides. Integrate non-chemical control methods when you can. Do what you can to prevent weeds from setting seed to begin with by hand weeding, for example.

Watch for weeds in your rooted cuttings, media components and other products you’re bringing into the nursery.

Clean your equipment, including tractors and mowers that are used between different fields. That equipment could be housing weed seeds, including resistant weed seeds.

For more information on herbicide resistance, visit and search for “Herbicide resistance in the nursery crop production and landscape maintenance industries” by Jeffrey Derr, Joe Neal and Prasanta Bhowmik.

Jeffrey Derr is a professor of weed science at Virginia Tech and the director of the Hampton Roads Agricultural Research and Extension Center in Virginia Beach.