Germination and Phenology of Seven Introduced Thistle Species in Southern Australia

Abstract

Seeds of seven introduced thistle species, common and widespread in southern Australia, were germi- nated over a range of different day!night temperatures and water potentials. The thistles were Carduus nutans L. (nodding), C. pycnocephalus L. and C. tenuiflorus Curtis (slender), Carthamus lanatus L. (saffron), Cirsium vulgare (Savi) Ten. (spear), Onopordum aff. illyricum (illyrian) and Silybum marianum J. Gaertn. (variegated).

Fresh seed of all thistle species germinated over a wide temperature range (15/5° to 40/30°C). Differences between taxa were expressed mainly at low temperatures (15/5° and 15/0°C), at which C. pycnocephalus germinated well and Onopordum only minimally. Germination of C. lanatus seeds was the most sensitive to moisture stress and that of C. nutans and C. vulgare the least. Radicle elongation of germinating seedlings differed at different water potentials, with C. tenuiflorus being the most sensitive to low water potential and that of C. lanatus the least, even at a water potential of - 1.5 MPa.

The seven thistle species were separated into three groups based on the patterns of their phenological responses over two seasons when grown in a common environment. Carduus pycnocephalus and C. tenuiflorus behaved as short-season annuals, C. vulgare and Onopordum showed a strongly biennial response, and the other species were annual except for C. nutans, which behaved as an annual or a biennial depending on time of establishment.

We conclude that the differences in germination responses to temperature and water potential between the different introduced thistles are minor. In combination with the major differences in phe- nology revealed in this study some of these differences may help to explain the predominance of the seven thistles in different regions of southern Australia. Other factors will largely determine their weediness in any one region.