Horseweed (Conyza canadensis or Erigeron canadensis), also called marestail, is a highly prolific and difficult-to-manage nuisance weed. A single mature plant can generate between 200,000 and 400,000 tiny seeds, which are easily carried long distances by the wind. This massive seed output contributes to its pervasive nature, especially in disturbed or no-till areas, making control a significant challenge.
Understanding the Horseweed Life Cycle
Horseweed is categorized as either a winter annual or a summer annual, meaning it can germinate in both fall and spring. The life cycle begins with the development of a basal rosette, a small, low-growing cluster of leaves that is the plant’s most vulnerable stage. Fall-emerged horseweed overwinters in this rosette stage until spring.
In mid-spring, the plant begins to “bolt,” rapidly growing an erect, hairy central stem that can reach heights of up to six feet. Once the plant transitions into this tall, bolting stage, control becomes significantly more difficult. The mature plant flowers in mid-to-late summer, producing wind-dispersed seeds from August to October. Control measures are most effective during the rosette stage.
Non-Chemical Removal and Suppression Techniques
For small infestations, manual removal is a practical solution, provided it is done thoroughly. Hand-pulling is most effective when plants are in the rosette stage and the soil is moist. It is important to remove the entire shallow taproot and fibrous root system, as the plant can regrow if the root crown is left behind.
Intensive tilling or cultivation can be an effective mechanical control, particularly in spring when overwintered rosettes are easily uprooted and killed. However, repeated tillage can bring buried seeds to the surface, potentially triggering new germination events. Tilling should also be avoided if mature plants are present, as this action can inadvertently spread seeds throughout the area.
Soil solarization utilizes the sun’s heat to kill weeds and seeds in the topsoil layer. This process involves covering moist, bare soil with clear plastic sheeting during the hottest months for several weeks. The trapped solar energy raises the soil temperature high enough to kill most horseweed seeds and seedlings. A thick layer of organic mulch, such as wood chips or straw applied 3 to 4 inches deep, can also suppress germination by blocking light and creating a physical barrier.
Chemical Control Strategies and Timing
Managing horseweed often requires a chemical approach due to its extensive seedbank and widespread resistance to common herbicides. Horseweed is known for its resistance to Group 9 herbicides, such as glyphosate. Effective control relies on targeting the plant when it is small and actively growing, typically in the fall or very early spring rosette stage.
Successful chemical programs integrate both pre-emergent and post-emergent herbicides with different modes of action. Pre-emergent products must be applied before seeds sprout in late fall or early spring to prevent germination. Post-emergent treatments are best applied when the rosette diameter is less than two inches, as the waxy stem of a bolting plant makes chemical control significantly less effective.
For post-emergent control of glyphosate-resistant populations, herbicides containing the following active ingredients are commonly used:
- 2,4-D
- Dicamba
- Glufosinate
- Paraquat
These are often applied with a residual herbicide, such as metribuzin or saflufenacil, to control later-emerging seedlings. Rotating herbicide classes is a recommended practice to prevent the development of further resistance. Always follow label instructions precisely regarding application rates and safety precautions.
Preventing Future Infestations
Long-term management focuses on minimizing bare ground and reducing the weed seed bank. After initial control, establishing a dense, competitive ground cover or crop is necessary to prevent new seedlings from taking hold. Since horseweed is not shade-tolerant, competition from desirable plants is highly effective.
In agricultural settings, utilizing cover crops, such as cereal rye or forage radish, is a proven method for suppressing horseweed density. These crops grow quickly and create a dense canopy that shades the soil surface. Regular monitoring and spot-treating new rosettes in both fall and spring will prevent plants from reaching the seed-producing stage. Finally, thoroughly cleaning all equipment after working in infested areas will prevent the accidental spread of seeds to new locations.