Biological Flora of the British Isles: <i>Cirsium arvense</i> (L.) Scop.

Summary 1. This account presents information on all aspects of the biology of Cirsium arvense that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles : distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, history, conservation and management. 2. Cirsium arvense , creeping thistle (Californian thistle, Canada thistle), one of the world’s most troublesome and persistent weeds, is native to Europe and the east northern hemisphere but introduced to North America and the southern hemisphere. Latitudinal distribution north or south is limited by low winter and summer maximum temperatures and by a long day requirement for flowering. 3. Cirsium arvense is believed to have originated in the temperate Middle East and its spread has closely followed human migration and agricultural activity. Colonization of new sites is by seed which establishes best in bare or disturbed ground, mirroring its prehistoric ecology as an opportunist pioneer of bare ground and organic residues. It is now a widespread and scheduled agricultural weed in both arable crops and pastures and also a constituent in over 70 British (National Vegetation Classification) plant communities, occurring mainly on waste neglected land, roadsides, hedgerows and disturbed areas. 4. Its presence in crops leads to yield losses and in pastures seriously interferes with utilization due to the deterrent effect of the leaf spines on grazing animals. This has led to a long history of investigation into control measures: mechanical, chemical, biological and integrated, which are summarized. Combination treatments and integrated control have achieved some success but effective control requires follow‐up procedures over a number of seasons. Climate change studies suggest C. arvense could grow better and be more difficult to control in future. 5. Success and persistence derives from an extensive, far‐creeping and deep rooting system which ensures survival and rapid vegetative spread under a wide range of soil and management conditions, and a means of escape from sub‐aerial control treatments. New adventitious buds capable of shoot development can arise at any point along the horizontal roots, even when these are cut into pieces or damaged. Root buds remain dormant until released from dormancy through damage or decay of the aerial shoots. Carbohydrate root reserves, stored in swollen cortical tissue, fall to a minimum just before flowering and are then replenished for perennation during the subsequent winter. Strategies for control aim to treat the plant when root carbohydrate reserves are at a minimum, to exhaust these reserves and to prevent replenishment for further perennation. 6. Balanced against its difficulty as a weed, C. arvense has significant conservation value as a host to numerous insects, many attracted by copious and accessible nectar and strong flower fragrance. It is however a strong competitor to low‐growing plants in natural communities. 7. Cirsium arvense is dioecious and for flowering has a 14–16 h day length requirement. Seed set is successful if male and female plants are no more than 50–90 m apart to allow insect pollination. In spite of the conspicuous wind‐borne pappus, this rarely carries a seed which normally falls near the parent plant. The flower heads and other plant parts are regularly attacked by numerous insects and less frequently by diseases. 8. Germination of seed is mainly during the high temperatures of early summer in the year following dispersal and establishment is most successful in open areas. Development of the branching root system and vegetative spread follow rapidly. 9. A combination of dioecy and vegetative reproduction has resulted in the maintenance of genotypic and genetic diversities within populations allowing efficient colonization and persistence, contributing greatly to success in the species.

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