- Name also: Canadian Horseweed, Canada Fleabane
- Family: Daisy Family – Compositae, subfamily Asteroideae
(formerly Aster Family – Asteraceae)
- Growing form: Annual herb.
- Height: 5–100 cm (2–40 in.). Stem branchless–branching, white-haired, green.
- Flower: Single flower-like approx. 3–4 mm (0.12–0.16 in.) capitula surrounded by involucral bracts. Capitula’s ray-florets (25–45) yellowish, thread-like/tongue-like; disc florets tubular, small. Stamens 5. Pistil of 2 fused carpels. Involucre cylindrical, involucral bracts narrow, with membranous margins, sparsely haired–glabrous. Capitula usually 50–300 in a long, quite narrow racemose cluster.
- Leaves: Alternate, stalkless–short-stalked, dense, withering early. Blade (narrowly) lanceolate, margin sparsely toothed–entire, often with spreading hairs especially from margins.
- Fruit: Achene, tip with dirty white unbranched hairs.
- Habitat: Railway and roadsides, loading areas, banks, sand pits, wasteland, gardens, yards, ballast soil deposits.
- Flowering time: July–September(–October).
- Harmfulness: Potentially or locally harmful alien species.
Canadian fleabane is, as its name suggests, from North America, but it has spread with people to inhabited areas almost all across the temperate zone in the northern hemisphere. It was first observed in Finland at the end of the 19th century but it has been slow to colonise the country. The species has spread to Finland primarily from Russia on the railways and it is still moving. The downy-haired cypselas have made steady progress on their journey by riding air currents created by trains or by attaching to sticky surfaces. It has accessed rail-side warehouse and industrial areas with car and rail traffic and also grows on roadsides, snow dumps and waste ground. Many stands are quite new and strange to most Finns. Global warming has apparently helped the species to spread and will do so in the future.
Fleabane doesn’t demand much of its habitat: it likes gravel and sand but its size varies with the nutrition in the soil from a few-centimetres-high dwarf in poor places to even a metre (3.3 feet) in more fruitful places. A large plant has many capitula and produces a lot of seed. All in all the species is noticeably flexible, and it can germinate in spring or any stage of the summer: this is the key to its success. As an annual which cannot handle competition it certainly favours open habitats, but these are abundant around urban environments.
Canadian fleabane is regarded in Finland as quite a harmless weed (classified as a potentially or locally harmful alien species). In its homeland, North America, it is quite a valued medicinal plant, especially for congealing blood. The plant has also been used to treat rheumatic complaints and gout. The medicine is made from the cut and dried flowering tops of the plants.