Abstract:
Achenes of Cirsium vulgare germinated at constant temperatures of 7-32�C. Fresh achenes had a higher optimum temperature of germination (23.5�C) than older achenes, and also germinated more slowly and had a lower maximum level of germination. At depths of 2 cm achenes either germinated or were destroyed. At greater depths the achenes showed an exponential decay rate in viability with time. Germination was synchronous with rainfall pattern over the summer, although subsequent germination may occur if achenes are brought to the surface. Young seedlings were not damaged by frosts of -2�C. After germination a major root system developed rapidly, whilst a rosette more slowly formed above ground. Rosettes increased in diameter until the winter when growth ceased, although horizontal growth was reinitiated if the rosette became damaged. In late winter or early spring vertical growth is initiated, leading to formation of a bushy plant with subsequent flowering and production of achenes. C. vulgare flowers throughout spring and summer with maximum number of plants flowering in late spring or early summer. Each capitulum holds c 200 achenes and a large plant may produce > 50 000 fertile achenes. Afte the capitulum has matured the achenes are wind dispersed. However, despite the presence of a pappus the majority of achenes fall within a circle of radius 1.5 times the height of the parent plant. In conjunction with the production of non-viable achenes through self-fertilisation, this is expected to result in marginal spread of this species from existing infestations.
