Horseweed
Conyza canadensis
Aster family (Asteraceae)

Description: This native annual plant is 1-7' tall and unbranched, except for the flowering stems near the apex. The stout central stem is ridged and covered with long white hairs. The leaves alternate all around this stem (appearing almost whorled) and differ little in length, except beneath the inflorescence, creating a columnar effect. They are about 3-4" long and " across, narrowly lanceolate or oblanceolate, with a few teeth toward the outer tips, and fine white hairs along their margins. The smaller leaves near the inflorescence are more linear and less likely to have any teeth.

When mature, several flowering stems appear at the apex, which branch frequently and spread upward and outward, terminating in a multitude of tiny composite flowers. These composite flowers are individually less than 1/8" across, but subtended by smooth green bracts that are somewhat longer. In each composite flower, there are numerous yellow disk florets in the center, which are surrounded by tiny white ray florets that remain erect, rather than spreading outward. There is no noticeable floral scent. The blooming period can occur any time from mid-summer to the fall, lasting about 2-3 weeks for individual plants. The small slender achenes are light brown and have tufts of white or light brown hair. Seed distribution is by wind. The root system consists of a branching taproot.

Cultivation: The preference is full sun, moist to dry conditions, and rich fertile soil. However, this plant flourishes in other kinds of soil, including those containing considerable amounts of gravel and clay. This weedy plant is easy to grow, and sometimes forms large colonies in favorable disturbed sites. Drought resistance is very good.

Range & Habitat: Horseweed is a very common plant that occurs in every county of Illinois (see Distribution Map). In black soil prairies, it may appear in dry disturbed areas, particularly near the margins of more developed areas, such as fields. It can also be found in dry disturbed areas of gravel prairies, clay prairies, hill prairies, clay banks along rivers, and meadows near woodlands. In more developed areas, Horseweed often appears in abandoned fields, pastures, fence rows, vacant lots, garbage dumps, areas along railroads and roadsides, or in gardens and lawns. This plant is more common in disturbed areas. It has become an adventive weed in Europe.

Faunal Associations: Wasps and flies are the most common visitors of the flowers, where they seek nectar. Among the wasp visitors, this includes Sphecid wasps, Vespid wasps, and Ichneumonid wasps. Among the flies, this includes Syrphid flies, Thick-headed flies, Tachinid flies, Flesh flies, Blow flies, and Muscid flies. A few small bees may also visit the flowers. Fertile seed can be produced without cross-pollination. Some insects have been observed feeding on this plant, including the caterpillars of Agrapha oxygramma (Sharp-Stigma Loop Moth), and Nysius niger (Black Seed Bug). Mammalian herbivores usually leave this plant alone because the foliage is bitter. Rabbits may eat the tops off of young plants occasionally. The ecological value of this plant to wildlife is low, except to some flower-visiting insects.

Photographic Location: The photographs were taken at the webmaster's apartment complex in Urbana, Illinois.

Comments: Initially, Horseweed may appear to be a tall, columnar goldenrod with hairy stems, and leaves that tend to angle upward from their bases. However, it dull inflorescence sets it apart. Upon close inspection, the tiny flowers have some resemblance to the more attractive flowers of the Erigeron spp. (Fleabanes), to which genus this plant was assigned in the past.

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