Horseweed can germinate in late fall, form an overwintering rosette, then bolt the following spring and summer.
All photos provided by authors.

Horseweed (Conyza canadensis) occurs throughout the continental United States and most of Canada. It has long been one of the most problematic weeds across all crops in agriculture, but its economic impact became greater when it developed resistance to glyphosate-containing herbicides (e.g., Roundup).

Horseweed is a winter annual. It often germinates in late summer or early fall and forms a rosette to overwinter. The overwintering rosette allows the plant to bolt quickly in the spring and grow without competition from other summer annual weeds. Horseweed bolts in early spring, growing up to five feet tall before releasing seed. It can produce up to 200,000 seed per plant, which can be disseminated via wind for several hundred miles. Seeds germinate almost immediately after being released from the plant (no dormancy), and can germinate year-round, as long as soil temperatures are greater than 55°F.

When discussing herbicide-resistant weeds, it is important to remember that only some populations of the species are herbicide resistant, not the entire species. Glyphosate-resistant horseweed can tolerate normal rates of glyphosate. Treated weeds may be chlorotic, stunted, and have excessive rosetting of the foliage. However, these plants can still produce viable seed and perpetuate the population.

How do weeds become herbicide-resistant?

Weeds can become resistant to herbicides by one or a combination of mechanisms. Some weeds have altered leaf surfaces that limit the amount of herbicide absorbed into foliage or stems. Some weeds have adapted to reduce the amount of herbicide translocated to target sites within the plant. For example, glyphosate is most effective when it kills plant roots, thus reduced translocation from foliage to the root system will reduce control. Some weeds can actively detoxify or metabolize the herbicide once it is absorbed. Finally, some plants can sequester the herbicide in specialized locations within the plant to prevent its herbicidal effects. Glyphosate resistance in horseweed is believed to be caused by limited translocation of the herbicide once it is absorbed by the plant.

Individual plants do not respond to herbicides by becoming stronger or resistant. Remember that herbicide resistance occurs to a population, not individual plants. Consider, for example, a 1-acre field with a million weed seedlings. An herbicide is applied, killing 99.99 percent of the weeds. Fewer than 100 weeds survive because they randomly expressed a gene for a slightly thicker cuticle, which prevented herbicide absorption. Those few weeds grow poorly, but still disseminate 10,000 seed each (a very modest number for most weeds). Next year, just 25 percent of those seeds germinate and express the gene for a thicker cuticle. These weeds survive the next herbicide application, so now there are over 200,000 weeds that continue to grow (albeit poorly), and produce another 10,000 seed each. If only 25 percent of those plants have the gene for a thicker cuticle and are able to produce another generation of seed, then there will be a population of 500 million weeds that survive the third herbicide spray. In three years, a small and unnoticeable scattering of stunted and chlorotic weeds that were able to resist the normal herbicide application will turn into an entire population of herbicide-resistant weeds, resulting in complete herbicide failure. It was not a single plant that evolved herbicide resistance, but a random gene mutation or expression that allowed a few weeds to survive the standard herbicide rate, reproduce, and eventually develop into a large uncontrolled population.

Where did glyphosate-resistant horseweed come from?

Glyphosate-resistant populations of horseweed most likely developed in no-till crops that utilized “Roundup-Ready” technology. These crops rely primarily on Roundup (glyphosate) for controlling weeds while the crop establishes. After annual repeated applications of the same herbicide, a few horseweed plants randomly expressed some genetic property that allowed them to resist normal rates.

Nursery growers in the Midwest have recently reported difficulty in controlling horseweed in field-nursery crops. Field-grown nursery crops share some similarities to no-till agronomic crops. Most importantly, soils in field nurseries are not tilled after planting, at least within the planting row. While many nurseries use preemergent herbicides in the spring and fall, glyphosate is routinely used for spot-spraying and maintaining weed control throughout the growing season. Some nurseries use glyphosate exclusively to periodically remove weed infestations and forego using any preemergent herbicides. Either way, this reduction in tillage and reliance on glyphosate for weed control has led to expanding populations of glyphosate-resistant horseweed in nursery fields.

Lack of tillage within field nursery rows allows horseweed to establish.

Controlling glyphosate-resistant horseweed

Farmers deal with glyphosate-resistant horseweed in agronomic crops (corn, soybeans, etc.) by using a combination of 2,4-D and other phenoxy-herbicides, as well as a variety of ALS-inhibiting herbicides. However, these solutions are not viable for nursery crops due to the likelihood of plant injury, as well as the limited labeled applications for using these products in nursery production.

Considering the prolific seed production and fertility of this weed, its ability to germinate throughout the growing season, and its tolerance to glyphosate, the only practical method for controlling horseweed is through an effective preemergent herbicide program. Unfortunately, very few of the preemergent herbicides registered for use in nursery crops have been evaluated for horseweed control, so there is limited knowledge regarding which herbicides provide the most effective control. Therefore, experiments were conducted to develop a comprehensive herbicide management plan for controlling horseweed year-round in field nurseries.

An experiment was conducted to determine which herbicides provide the most effective preemergent horseweed control.

Experimental procedure

Experiments were conducted in a horseweed-infested field in Wooster, Ohio. The field was first plowed and disked. Herbicides were spray-applied during the first week of October while the field was still weed-free. All herbicides were applied with a spray volume of 40 gal/acre, at the rates listed in Table 1. Each herbicide-treated plot was 3 feet wide and 10 feet long. Even though the field had historically high horseweed pressure, about 2,000 horseweed seed were also broadcasted to each plot. There were five plots per herbicide treatment. The following spring, the weeds present in each plot were identified and counted to assess control of horseweed and other weed species. The experiment was repeated in an adjacent field the following spring (April).

In addition to horseweed, the most prevalent weed species observed in the plots included annual bluegrass (Poa annua), lambsquarter (Chenopodium album), and Pennsylvania smartweed (Polygonum pensylvanicum). Following fall application, Marengo and SureGuard provided the most effective horseweed control, each with less than one horseweed per square foot compared to more than 13 horseweed per square foot in non-treated control plots (see Fig. 1). In addition, they provided nearly 100 percent control of all weed types. Gallery and Goal were also very effective at reducing horseweed numbers. Princep and Tower were not effective and resulted in similar horseweed numbers to non-treated controls. While Gallery provided effective control of horseweed and other broadleaf weeds, it provided no control of grass weeds. Pendulum provided effective grass control, but moderate to poor control of horseweed and other broadleaf weeds.

Results were different for the spring application of preemergent herbicides. Marengo and SureGuard still provided effective control, although the spring application of Marengo was not quite as effective as the fall application (see Fig. 2). Princep and Tower provided excellent control after spring application, in contrast to poor control after fall application. Finally, Gallery and Goal provided relatively poor control following spring application, despite providing excellent control in fall application.

Herbicides and rates applied to a field soil to determine efficacy on preemergence horseweed control.

Year-round horseweed control

The easiest way to control horseweed is through cultivation. Recall that horseweed becomes problematic in no-till agronomic crops and in nursery fields that lack cultivation (within tree rows). Disking or light cultivation should be used wherever it is practical and efficient. When cultivation is not possible, apply preemergent herbicides to weed-free soil to form a chemical barrier and prevent horseweed establishment. SureGuard and Marengo can provide effective control when applied either spring or fall. Furthermore, both products provide broad-spectrum weed control that will also minimize other grass and broadleaf weeds. SureGuard and Marengo should be used cautiously and according to the label to avoid crop injury.

In sensitive crops where injury is a concern, or in crops where SureGuard and Marengo are not labeled, other herbicide tank mixes should provide effective control. Fall applications of either Gallery or Goal tank mixed with Pendulum will provide effective broad-spectrum weed control and should provide excellent horseweed control. Spring applications of Princep and Tower would likewise provide an effective tank mix for both broad-spectrum weed control and excellent horseweed control. Always follow label directions with any herbicide application.

Silvia Valles Ramirez is a visiting scientist at the Ohio State University. Dr. James Altland is a Research Horticulturist with the USDA-ARS in Wooster, Ohio; Mention of any commercial product in this article is for educational purposes only, and should not be considered an endorsement by the Ohio State University or USDA-ARS.