Horseweed; also known as Canadian Horseweed, Mare's Tail, or Butterweed
Kingdom: Plantae; Division: Angiosperms; Class: Eudicots; Order: Asterales; Family: Asteraceae; Genus: Erigeron; Species: Erigeron canadensis · Asteraceae (Daisy/Sunflower/Aster family) · Simple; deciduous annual herb
Species
Erigeron canadensis (High confidence)
Leaf Type
Simple; deciduous annual herb
Family
Asteraceae (Daisy/Sunflower/Aster family)
Shape
Linear to lanceolate; lower leaves can be slightly more spatulate, while upper leaves are narrow and needle-like
Size
Approximately 2-4 inches long and 0.2-0.4 inches wide; middle and upper leaves are considerably smaller and narrower than the basal leaves
Margin
Variable; lower leaves are often coarsely toothed/serrate, while upper leaves are typically entire (smooth)
Arrangement
Alternate; the leaves are crowded along the central stem, often appearing spiral
Rarity
Very Common; frequently considered a pest or agricultural weed
Color & Pattern
Uniform dull green to olive green; lacks variegation; the main stem is often a lighter green and can be quite bristly
Venation Pattern
Pinnate with a prominent light-colored midrib; secondary veins are often obscure due to the narrow leaf width
Texture & Surface
Rough and hairy (hispid/pubescent); the presence of stiff, bristly hairs on both surfaces and along the margins is a key trait
Description
An erect annual herb reaching up to 5-7 feet in height. It features a single bristly stem that branches extensively at the top into a large panicle of tiny, numerous flower heads. The flowers are small (only 3-5mm wide) with white ray florets and yellow centers, eventually turning into fuzzy dandelion-like seeds
Key Features
Single upright bristly stem; narrow, hairy, alternate leaves; massive terminal panicle of tiny white/yellow composite flowers; wind-dispersed seeds with white pappus (fuzz)
Habitat
Disturbed sites, roadsides, agricultural fields, sidewalk cracks, and waste areas; prefers full sun and well-drained soil
Geographic Range
Native to North America; now a cosmopolitan weed found throughout Europe, Asia, and Australia, often considered invasive outside its native range
Condition Notes
The specimen appears healthy but is transitioning into its reproductive stage; many flower heads have gone to seed (pappus visible), typical of late summer or early autumn. Health grade: Good
Interesting Facts
Horseweed was the first weed to develop resistance to the herbicide glyphosate (Roundup) in North America, making it a significant challenge for modern farmers
Medicinal & Uses
Historically used by Native Americans as a diuretic and to treat dysentery. It contains essential oils used in the perfume industry. It has no significant timber or modern culinary value
Ecological Role
Provides nectar for small bees, flies, and wasps; serves as a host plant for certain lynx spiders and specialized moths. It is a pioneer species that quickly colonizes bare soil
Similar Species
Erigeron bonariensis (Flax-leaf fleabane), which is generally more grey-green and more heavily branched from the base; and various Goldenrods (Solidago spp.), which have much larger, brighter yellow flowers
Foraging Tips
Look in disturbed urban environments or the edges of farm fields. Late summer is the best time for identification when the characteristic mass of tiny cream-colored flower heads appears
Toxicity Warning
Mildly Toxic; the plant contains terpenes that can cause skin irritation (dermatitis) in sensitive individuals or livestock if consumed in large quantities. Generally safe to handle with care