Horseweed (Marestail) (Conyza canadensis (L.) Cronq.)
Life cycle:
Winter/summer annual. Horseweed emerges in fall or early spring as a rosette. Fall-emerged horseweed becomes dormant over the winter, plants start to bolt in April/May, begin to flower in July, set and disperse seed from August to October and die. Fifty-nine to 91% of fall-emerging plants survive the winter.
Emergence:
Horseweed is primarily a fall emerger. In one study, 5-32% of total emergence happened in the spring.
Emergence is greatest at soil depths of less than 1/2", but it can emerge from soil depths of up to 1".
Seed:
Production Average: An average of 200,000 seeds are produced per horseweed plant.
Dispersal Mechanisms: Seeds can be dispersed long distances by wind. There have been reports of 12,500 and 125 seeds per yd^2 at 20 and 400 feet, respectively, away from the seed source.
Longevity: Seed longevity of horseweed has not been established. However, there is one report of finding viable seeds in the seedbank of a 20-year-old abandoned pasture.
Dormancy: Horseweed seeds are not very dormant. Up to 86% of the seeds can germinate immediately upon seed shed. As burial depth increases, seed dormancy increases.
Competitiveness:
Little information is available on the competitiveness of horseweed. An estimated 83% of soybean yield is lost from 105 plants per 10ft^2. Severe infestations have reduced sugar beet yield by 64%.
Preferred Soil / Field Conditions:
Horseweed prefers coarse, well-drained, and fertile loam soils. It can occur on organic soils and tolerates drought.
Management:
Biological
No information is available at this time.
Mechanical
Tillage: Horseweed is controlled by tillage, but is primarily a weed in no-till systems.
Cultural
Crop rotation: Small grains and crops planted in narrow rows and high populations in the rotation suppress horseweed because horseweed is not very shade-tolerant.
Planting date: Tilling in the spring and planting later (mid-May) will reduce horseweed infestations.
Chemical
Application timing and effectiveness: Horseweed is effectively controlled in the rosette stage. It is difficult to control once plants begin to bolt. There are known resistant populations to ALS-inhibitors and glyphosate in the United States.
For more specific information regarding control with herbicides go to the Horseweed section of the 2007 Weed Control Guide in the Publications section.