Horseweed (Conyza canadensis), also known as marestail, is a widespread and highly challenging annual or biennial weed that has become a major problem in agricultural and disturbed landscapes. This prolific plant quickly colonizes bare ground and outcompetes desirable vegetation for resources like water and light. Its capacity to produce a vast number of easily dispersed seeds, combined with its ability to grow under varying conditions, makes it difficult to manage once established. Controlling horseweed effectively requires a precise strategy that targets the weed at its most vulnerable stages.
Understanding Horseweed Growth Stages
Successful horseweed management relies on understanding its two primary growth phases: the rosette and the bolted stage. Horseweed typically behaves as a winter annual, with seeds germinating in the fall and forming a small, ground-hugging cluster of leaves called a rosette. These rosettes overwinter, becoming well-established before spring arrives.
As temperatures rise, the plant rapidly transitions into the bolted stage, where a single, erect central stem can quickly grow to heights of one to five feet. Once the plant bolts, its tissues become woody and less susceptible to control methods. A single mature horseweed plant can produce up to 200,000 tiny seeds, which are easily carried long distances by the wind due to their parachute-like pappus.
Non-Chemical Eradication Methods
Physical removal offers an effective control strategy for horseweed, provided it is timed correctly before the plant bolts. Manual hand-pulling is most successful when the horseweed is still in the small, pliable rosette stage, usually in the fall or early spring. The plant has a relatively shallow root system, making it easy to remove completely from the soil at this size.
Tillage is an excellent mechanical method for controlling horseweed populations, especially since the seeds require light to germinate. Lightly tilling the top quarter-inch of soil can eliminate a high percentage of emerged seedlings and bury surface seeds too deep to sprout. This method is particularly effective for controlling fall-emerged rosettes in the spring before planting.
Cultural practices can also suppress horseweed emergence by limiting the light and bare soil it needs to thrive. Applying a heavy layer of mulch creates a physical barrier that inhibits seed germination. Planting a dense cover crop, such as cereal rye, in the fall can outcompete and suppress horseweed rosettes by shading the ground.
Chemical Control Strategies
Chemical control is divided into pre-emergent and post-emergent strategies, both relying on precise timing for maximum effectiveness. Pre-emergent herbicides are applied to the soil before the horseweed seeds germinate, providing a protective barrier against the emergence of new seedlings. These applications are best timed for late summer to early fall to target the primary germination window of winter annual horseweed, or in early spring for summer annual emergence.
Post-emergent herbicides are applied directly to the emerged plant, but success is highly dependent on the plant’s size and growth stage. Effective control is achieved when the horseweed is still in the small rosette stage, preferably less than four inches in diameter. Once the plant bolts and begins to develop a thick, waxy stem, the efficacy of most chemicals drops significantly, often resulting in poor control.
Effective active ingredients for post-emergent control include growth regulators like 2,4-D and dicamba, or contact herbicides such as glufosinate or paraquat. These chemicals are commonly used as “burndown” treatments in the fall or spring to clear fields of overwintered rosettes before planting. However, the choice of herbicide must account for the widespread issue of chemical resistance in horseweed populations, which can render common herbicides like glyphosate ineffective.
Managing Herbicide Resistance
Horseweed has developed resistance to several herbicide groups, most notably glyphosate, which has severely complicated management efforts. Relying on a single herbicide or chemical class will often result in control failure and the selection of more resilient weed populations. To combat this, advanced management relies on using multiple effective modes of action (MOA) simultaneously.
The most successful chemical strategy for dealing with resistant horseweed is tank mixing, which involves combining two or more herbicides with different MOAs in a single application. For example, combining a growth regulator (like 2,4-D) with a contact herbicide (like paraquat) provides a multi-pronged attack that is significantly more difficult for the weed to overcome.
Integrated weed management (IWM) provides the most sustainable defense against herbicide resistance by combining chemical and non-chemical methods. This approach emphasizes using effective burndown treatments with multiple MOAs, followed by residual pre-emergent herbicides to prevent new emergence. Incorporating cultural tactics like crop rotation and tillage diversifies the control pressure, reducing the exclusive reliance on chemicals and slowing the evolution of further herbicide resistance.