Crooked Run Valley
Canadian horseweed (Conyza canadensis)
TAXONOMY: The currently accepted scientific name of Canadian
horseweed is Conyza canadensis (L.) Cronquist.
NATIVE STATUS: Native, United States and Canada.
GENERAL BOTANICAL CHARACTERISTICS:
Habit: Canadian horseweed are erect with one to several stems typically reaching 30 to 150 cm (1 to 5 ft) tall while mature plants grow erect, 6-1/2 to 10 feet (2–3 m) tall. Usually a single main stem branches out from the upper half of the plant. Stems are typically unbranched at the base unless damage has occurred to the apical growing points. An abundance of dark green leaves are alternate to one another along the stem. Cutting the main stem of horseweed can lead to the growth of several basal branches.
Leaves: The leaves are linear to oblanceolate, 2 to 8 cm (0.8 to 3.1 in) long and 2 to 8 mm (0.08 to 0.31 in) wide. The leaf margins are ciliate-serrate.
Flowers: The inflorescence is a loose panicle. The numerous flower heads are very small, 2 to 4 mm (0.08 to 0.16 in) tall and 3 to 7 mm (0.12 to 0.28 in) wide. Many flower heads grow at the ends of branched stems. Initially the flower head has an urn-shaped green base with a very small daiseylike flower (head) at the top. The rays are white or purplish and very small, only reaching 0.5 to 1.0 mm (0.02 to 0.04 in) in length.
Eventually the flower heads mature and expand into seed heads, which look somewhat similar to dandelion puffs.
Fruit/Seed: The fruit is an achene that is small, roughly 1/16 of an inch (1.5 mm) long, narrow, elliptical or oblong, slightly hairy, and attached to soft, dirty-white pappus bristles. There are approximately 700,000 seeds per pound.
Roots: The root system consists of a short taproot with secondary fibrous roots.
REGENERATION PROCESS: Canadian horseweed propogates itself by reseeding.
HABITAT TYPES: Canadian horseweed is common in grasslands and in moist disturbed sites including riparian and wetland areas. It has be-come a common pest in agricultural locations throughout its range. It also occurs in ditch and canal backs, urban sites, landscaped areas, orchards, vineyards, roadsides and potentially any disturbed, unmanaged places.
SITE CHARACTERISTICS: Canadian horseweed prefers full sun, mesic to dry conditions, and fertile loamy soil. However, it also adapts to a varieyt of soils, including those that contain considerable amounts
of clay or gravel.
SEASONAL DEVELOPMENT: Flowers bloom from June through September.
GENERAL DISTRIBUTION: Canadian horseweed is found through-out the United States and Canada.
SKY MEADOWS DISTRIBUTION: To be determined.
IMPORTANCE AND USES: The flowerheads of Horseweed (Conyza canadensis) attract small Halictid bees, Sphecid wasps, Vespid wasps, Perilampid wasps, Syrphid flies, Tachinid flies, flesh flies (Sarcophaga spp.), Muscid flies, plant bugs (Miridae), and other insects. Bee visitors suck nectar or collect pollen, fly visitors suck nectar or feed on pollen, while the remaining floral visitors feed on nectar. A variety of insects feed on the leaves, bore through the stems, or consume other parts of Horseweed. These species include the stem-boring larvae of a tumbling flower beetle (Mordellistena pustulata), larvae of Calycomyza humeralis (Aster Leafminer) and other leaf-miner flies, larvae of Asteromyia modesta (Horseweed Blister Midge) and Neolasioptera erigerontis (Horseweed Stem Gall Midge), Lygus lineolaris (Tarnished Plant Bug) and Taylorilygus apicalis (Broken-backed Bug), Uroleucon erigeronense and other aphids, Melanoplus differentialis (Differential Grasshopper) and Oecanthus quadripunctatus (Four-spotted Tree Cricket), and the larvae of such moths as Cucullia alfarata (Halloween Paint), Cucullia speyeri (Speyer's Paint), Schinia arcigera (Arcigera Flower Moth), and Schinia lynx (Lynx Flower Moth)
Mammalian herbivores usually leave Horseweed alone because the foliage is resinous and bitter. However, deer and rabbits sometimes browse on young plants, while to a minor extent muskrats eat the stems of plants that grow near bodies of water. Horseweed contains a substance (herpene) that can irritate the noses of horses that feed on it. The foliage of this plant can also irritate the skin of some people.