The Biology of Canadian Weeds. 147.Onopordum acanthiumL.

Abstract

Cavers, P. B., Qaderi, M. M., Threadgill, P. F. and Steel, M. G. 2011. The Biology of Canadian Weeds. 147. Onopordum acanthium L. Can. J. Plant Sci. 91: 739–758. In Canada, Scotch thistle, Onopordum acanthium L. (Asteraceae) is primarily a weed of well-drained natural areas and ruderal habitats such as abandoned gravel pits. It is classed as a noxious weed in Ontario and the North Okanagan region of British Columbia but is common only in southern Ontario. Native to southern Europe and southwestern Asia, it has been introduced to almost 50 countries in all continents except Antarctica. In parts of the United States it is a strongly competitive species that infests small grain fields and competes with desirable forage species in rangelands. It does have desirable attributes and has many uses including being consumed as a vegetable, used as a source of honey, yielding a burnable oil, incorporated in medications for many human ailments, providing grazing for goats, sheep and cattle and being grown as a garden ornamental. Despite its limited range in Canada, several distinct biotypes, differing in morphology, phenology and important ecological attributes, such as dormancy and longevity of cypselas (fruits) in the soil, have been described from southern Ontario. In Australia, a fertile hybrid of O. acanthium and O. illyricum is a major weed and there are concerns that these two species could be progenitors of a similar hybrid in North America.