Marestail

Horseweed

Conyza canadensis

Marestail plant
Marestail plant. Photo courtesy of F.D. Richards Licensed under CC BY-SA 2.0.
Marestail is an annual broadleaf plant, growing to a height of between 2 and 6 feet. It is considered to be a noxious weed, with some infestations reducing soybean yields by up to 83 percent.
Marestail can follow either a summer or winter annual life cycle. Some emerge in the spring, while others emerge in the fall and overwinter with a basal rosette. Marestail cotyledons are oval to egg-shaped and range in hair density from hairless to dense coverage with short, stiff hairs. Leaves are arranged alternate, with the first leaf being shaped like a football and having a hairy stalk. Subsequent leaves are similarly shaped with the edges being slightly toothed.
Marestail seedlings
Marestail seedlings. Photo courtesy of the United Soybean Board. Licensed under CC BY 2.0.
As the marestail bolts, the leaves grow in a spiral up the stem, continuing with the alternate leaf pattern. They are simple and linear in design with the edges being entirely or slightly toothed. Leaves become progressively larger towards the base of the stem, with remains unbranched unless damaged. The marestail plant is easily misidentified as a whitlowgrass or fleabane species. Whitlowgrass has shorter and narrower leaves with a an entirely to slightly smooth leaf edge. Fleabane species tend to have purple spots at the tip of each leaf's tooth, whereas marestail does not. Marestail also has dark green leaves, while fleabane's are gray-green.
Marestail stand
Marestail stand. Photo courtesy of Delaware Cooperative Extension. Licensed under CC BY 2.0.
Flowers of the marestail plant bloom white from June through September. The flowerhead has an urn-shaped green base with a small daisy shaped flower coming out of the top. This will then mature to a dandelion-like puff which will become the seed head. The seed bristles are dirty-white and attached to narrow, elliptical and slightly hairy structures that allow the seed ot be dispersed by the wind. Each plant can produce up to 200,000 seeds.
Marestail plant displaying flowers at all stages of development
Marestail plant displaying flowers at all stages of development. Photo courtesy of Bruce Ackley. Licensed under CC BY 3.0.
Marestail is highly adaptable and tolerates drought conditions well, continuously growing in environments that are stressful for desireable crops. It is especially prevalent in no-till farmland and has been known to reduce yields by up to 83 percent. Along with competing for nutrients, space, and light, marestail can be a host for various insect pests and possible plant viruses.
Increasing tillage intensity in fields infested by marestail has seen a reduction of 50 percent or more as compared to a no-till field. Crop rotation has minimal effects on reducing marestail.
The primary method of controlling marestail in no-till fields is to implement a program prior to planting. It is important that 2,4-D ester be included in any herbicide treatments, and that the treatments be applied before the weeds reach 4 to 6 inches. Note that fall applications will control currently emerged marestail but not spring-emerging plants. Applications made in the spring should include a residual herbicide in order to control subsequent emergence. Treatments are overall most effective in the rosette stage (less than 2 inches tall) There have been confirmed populations of marestail that are resistant to ALS and glyphosate. Please contact your local weed specialist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.