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

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.