Information on habit, biology, persistence & spread for Canadian fleabane.

Other names

Bitterweed, Horseweed

Latin names

Conyza canadensis (L.) Cronquist (Erigeron Canadensis L)

Weed Type

Annual Broad-leaved Weeds


Canadian fleabane is an introduced, annual to biennial weed naturalised on waste ground, waysides and walls. Despite the common name it is said to be native to Asia and was introduced into Europe in the 17th century, probably from North America. The fluffy seeds are reputed to have been brought into the UK almost 300 years ago as the stuffing in bird skins mounted by taxidermists. It was found on walls in London in the early 1800's.


Canadian fleabane flowers from June to September/October. The flowers are primarily self-pollinated. Insects visit the flowers, however, and a low level of outcrossing is known to occur. There may be 45-70 seeds per flower head and the average number of seeds per plant ranges from 25,000 to 60,000. A large plant may produce 200,000 seeds. There is no dormancy but seeds require light for germination.

The seeds require light for germination. Fewer seeds germinate under a canopy of leaves than in diffuse light. The estimated base temperature for germination ranges from 12.5 to 13.0°C.

Canadian fleabane is intermediate between a summer and winter annual. Spring emerging seedlings appear in April-May but a low level of emergence continues until mid-August before peak emergence begins in late August to early September. More than 59% of seedlings that emerge in the autumn survive the winter. Canadian fleabane overwinters as a rosette of leaves from which the flowering stem develops in spring. Plants with rosettes in excess of 5 cm diameter survive the winter better. Spring emerging plants do not form a rosette before flowering. Seedlings emerge mainly from the upper 10 mm of soil.

Persistence and Spread

Viable seeds have been recorded in the seedbank of a 20-year old pasture despite its absence from the vegetation. Under dry storage, seed longevity is 2-3 years.

The seeds are light and have a pappus of hairs that facilitates wind dispersal for distances of up to 122 m. Seeds have also been recovered from irrigation water. Seeds submerged in water for 9 months gave 1-2% germination.


Canadian fleabane can colonise cultivated land but it will only become established if there is no further tillage. Shallow cultivations prior to drilling are sufficient to prevent survival or establishment. In the USA, Canadian fleabane numbers increased with the change to no-tillage growing systems. It had previously occurred along roadsides and field edges but then rapidly colonised the uncultivated fields.

Crop residues that cover the soil surface prevent or delay seedling emergence. Residues of rye (Secale cereale in the soil reduce seed germination.

Phytoparasitic bacteria have been considered as potential biological weed control candidates for Canadian fleabane. Spray applications of Pseudomonas syringae with surfactant added have caused severe disease symptoms and death of the plant.

Updated October 2007.

Fully referenced review