Sequential Flowering of Thistles (Cynareae, Asteraceae) in Southern Australia

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Abstract
Flowering patterns of four and seven species of thistles were examined in Wagga Wagga, N.S.W. and Canberra, A.C.T. respectively, during the 1982-83 and 1983-84 flowering seasons. Flowering patterns at each location were sequential, consistent and differed significantly among species. The progression of anthesis among the species was as follows: Carduus pycnocephalus, Silybum marianum, Onopordum acanthium, O. illyricum, Centaurea calcitrapa, Carthamus lanatus and Cirsium vulgare. Early-flowering species typically had brief flowering periods whereas later species flowered for con- siderably longer periods. Seed (achene) dormancy patterns and optimum temperatures for germination of the first and last species of the sequences were analysed. Seeds of C. pycnocephalus exhibited pro- nounced innate dormancy for at least 2 months after seed maturation and germinated poorly at high temperatures. In contrast, some seeds of C. vulgare were able to germinate soon after seed maturation and over a wide range of temperatures. Early-flowering species probably benefit from pronounced dormancy by avoiding precocious seed germination during wet springs, while high temperatures prob- ably restrict germination after rare rainfall during the long and dry summers of Mediterranean-like southern Australia. Such dormancy is unnecessary for late-flowering species in this environment as their seeds are produced coincidentally with the onset of more reliable autumn rains.