Canadian horseweed
(Erigeron canadensis L.)

Overview

Appearance

Erigeron canadensis is a winter or summer annual herbaceous plant in the sunflower family. The plants are erect reaching 0.5-7 ft (30-150 cm) tall. Stout angular-terete stems are usually unbranched at the base, however damage to the apical growing points can cause branching. It has a short taproot with secondary fibrous roots.

Foliage

The alternate leaves are elliptic, narrowly lanceolate, or narrowly oblanceolate, 0.8-3.1 in (2-8 cm) long and 0.08-0.31 in (2-8 mm) wide, with ciliate-serrate leaf margins. The leaves are sessile or almost sessile and are spaced closly together along the stem, appearing almost whorled.

Flowers

The inflorescence is a loose panicle. The numerous flower heads are very small, 0.08-0.16 in (2-4 mm) tall and 0.12-0.28 in (3-7 mm) wide. The rays are white or purplish and very small, only reaching 0.02-0.04 in (0.5-1.0 mm) in length.

Fruit

It is a prolific seed producer with approximately 700,000 seeds per pound. The fruit is an achene with a white to grayish white bristly pappus.

Ecological Threat

E. canadensis, Canadian horseweed is common in grasslands and in moist disturbed sites including riparian and wetland areas. It has become a common pest in agricultural locations throughout its range. This plant has become an invasive weed in Eurasia.

Selected Images

Maps

Invasive Listing Sources

• Indiana Noxious Weeds
• Jil M. Swearingen, Survey of invasive plants occurring on National Park Service lands, 2000-2007
• Ohio Noxious Weeds
• WeedUS - Database of Plants Invading Natural Areas in the United States

References