The ecology and physiology of two species of Carduus as weeds of pastures in Victoria

Slender thistles (Carduus pycnocephalus and C. tenuiflorus) were introduced to Australia about the 1880s. They are now important weeds of pastures in much of southern Australia and are difficult to control with present methods. This study was undertaken to investigate several aspects of the ecology and physiology of the plants with the belief that a knowledge of some of these aspects, particularly of seed germination and seedling establishment, might disclose some "weakness" in the plants' growth which could be exploited to improve control measures. Because of confusion over differences between the two species which occur in Australia the initial step was to evaluate the morphological features which have been used to distinguish between the two species. Although they are very similar morphologically, cytological evidence confirmed that the two species were quite distinct and, in fact, had quite different evolutionary origins. Germination of seeds of slender thistles is controlled by three separate forms of dormancy; these are known as innate, induced and enforced dormancy. Dormancy ensures that the plants will survive in a Mediterranean-type climate and also colonize areas with quite different climates and, most importantly, survive natural catastrophes such as drought, fire, and flood. The germination of slender thistles in the field is confined to a very short period (about 6 weeks) after the autumn break in any year. This is a "weakness" in the plants' survival mechanism because they are vulnerable in that year, at least, to any treatment which can kill seedlings. The herbicide, diquat, was found to kill young seedlings of slender thistles and not affect seedlings of desirable pasture plants associated with the thistles in southern Australia. This treatment is economical and leads not only to a reduction in thistles but an increase of about 30% in pasture production. Several other aspects of the plants' growth were investigated. Slender thistles have early growth characters which give them advantages over more desirable components of pastures. They are more competitive than subterranean clover and ryegrass over a wide range of levels of nutrients, and the traditional approach to pasture improvement in southern Australia of applying superphosphate and sowing subterranean clover will encourage, not suppress, slender thistles. Since grazing animals generally do not eat slender thistles the presence of thistles in pastures at densities commonly occurring in Victoria considerably reduces pasture production