No. 6 Article 6/April 29, 2005

Horseweed (Marestail) Identification and Biology

Horseweed is an annual weed species that historically has been found in waste areas, fallow fields, and fencerows. However, with increases in reduced tillage acres, horseweed has become a major problem in no-till fields and more recently a problem in some tilled fields.

One reason for its occurrence in these situations is its life cycle. Horseweed is an annual, but may follow either a winter or summer annual life cycle. A majority of plants throughout Illinois emerge in the fall, with much less emergence occurring in the spring. Horseweed does not mature until late summer, unlike most winter annual weed species that complete their life cycle early in the spring. This delay in maturity allows horseweed to compete with soybean during the growing season and interfere with harvest.

Cotyledons of horseweed are oval and 2 to 3 mm long. Following emergence, horseweed forms a basal rosette in the fall or early spring. Rosette leaves are hairy, elliptic to lanceolate, with entire or slightly toothed margins. Shortly into the spring, the plant bolts and produces numerous hairy leaves that alternate around the elongated stem. Once the stem has elongated (1.5 to 6 feet in height), the basal leaves deteriorate. The stem leaves are linear and gradually become smaller up the stem. Stems of horseweed plants tend to be unbranched unless damaged by herbicides, mowing, animals, or insects.

Numerous tiny white ray flowers are present from July to October; these flowers produce thousands of tiny seeds (up to 200,000) that are easily dispersed by wind because of the attached pappus (tiny hairlike bristles). This dispersal mechanism allows horseweed seeds to travel great distances and establish themselves in several fields. Approximately 80% of horseweed seed can germinate almost as soon as it falls off the mature plant.

For more information on horseweed identification and biology, we suggest reading the Biology and Management of Horseweed publication available online at (Adobe PDF), or you may request a free copy by contacting one of us.--Dawn Nordby and Aaron Hager

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