Horseweed, also known as mare’s tail, is a widespread annual plant that originated in North and Central America. Scientifically classified as Erigeron canadensis, it has since naturalized across much of the globe. It is characterized by a rapid growth habit and a strong ability to colonize disturbed soils, making its biology a priority in agricultural fields and landscapes.
Identification and Life Cycle
Horseweed begins its life cycle as a small, low-lying cluster of hairy leaves called a basal rosette. This rosette acts as a winter annual, allowing the plant to overwinter. The leaves in this stage are typically spoon-shaped, sometimes with slightly toothed margins.
From this rosette, the plant rapidly develops a single, mostly unbranched stem that can reach heights up to seven feet tall. The upright stem is densely covered with slender, unstalked leaves that are linear to lance-shaped. Flowering occurs from mid-summer into the fall, presenting as numerous small, inconspicuous flower heads clustered near the top of the plant.
The flower heads are daisy-like, featuring a center of yellow disc florets surrounded by white or purplish ray florets. Each mature plant can produce over 200,000 seeds. These minute seeds are tipped with a dirty white pappus—a tuft of bristles that acts like a parachute—enabling them to be carried long distances by the wind.
Agricultural and Ecological Impact
Horseweed’s rapid growth and massive seed production make it highly competitive, especially in agricultural settings. Its dense, upright growth habit allows it to shade out and compete with major crops like soybeans and corn for light, water, and nutrients. Uncontrolled horseweed populations can lead to significant yield reductions, with losses in soybean production potentially reaching over 80%.
The plant’s impact is particularly pronounced in no-tillage farming systems, which have become a common practice for soil conservation. No-till practices rely heavily on chemical weed control, which has inadvertently fueled the rise of herbicide-resistant horseweed. Horseweed was the first weed species in the world documented to develop resistance to glyphosate, the active ingredient in many broad-spectrum herbicides, with the first case reported in 2001.
Resistance is not limited to glyphosate (Group 9); populations have also developed resistance to other herbicide classes, including ALS-inhibitors (Group 2) and paraquat (Group 22). Multi-herbicide resistance complicates management significantly, forcing farmers to employ combinations of herbicides with different modes of action. Furthermore, horseweed exhibits allelopathy, releasing toxic chemicals into the soil that inhibit the germination and slow the growth of neighboring plant species.
Strategies for Management
Effective management of horseweed focuses on targeting the plant when it is most vulnerable, which is during the small rosette stage. Control efforts should ideally be applied in the late fall or early spring before the plant begins to elongate its central stem, a process known as bolting, which makes it much harder to eliminate.
For smaller areas or gardens, mechanical control methods are highly effective, such as hand-pulling or shallow tilling of the soil. While tillage works, it is often avoided in large-scale agriculture due to the desire to maintain soil structure in no-till systems. Cultural control methods, such as the use of cover crops like cereal rye, can suppress horseweed by creating a dense canopy that limits light and physically interferes with seedling emergence.
Chemical control requires a strategy that relies on herbicide rotation and tank-mixing different modes of action to circumvent resistance. Since many populations are resistant to glyphosate, alternative herbicides are necessary, such as growth regulators like 2,4-D or dicamba, or PPO-inhibitors. The application of residual herbicides, which remain active in the soil for a period, is also a useful tactic to prevent new seedlings from emerging throughout the growing season.