Some aspects of the biology, ecology and control of slender thistle, Carduus pycnocephalus L. and C. tenuiflorus Curt : (Compositae) in Tasmania
Creators: name: Bendall, Glen Max
Publisher: name: University Of Tasmania
Year: 2023

description: The germination, dormancy and nutritional requirements of slender thistle, and the effects of grazing management on the ecology of slender thistles in improved pasture were studied. Heterocarpy was found to be well developed in slender thistles. Approximately 85% of the seed produced contained a soluble germination inhibitor which required leaching from the seed to facilitate germination. The remainder of the seed did not possess an inhibitor and would germinate without leaching. Seedlings arising from seeds Without an inhibitor had greater root growth at low soil moisture than seedlings arising from seeds containing an inhibitor, thus conveying to the former seedlings a greater tolerance of moisture stress. The survival value of heterocarpy to slender thistles is discussed. Following leaching, seed germinated over a range of well defined temperatures (10¬¨‚àûC-30¬¨‚àûC) and this was related to the germination pattern of slender thistles in the field, Effect of burial of seed at varying depths on germination, dormancy, and longevity was observed. Seed buried at a depth of 1.3cm gave the highest percentage emergence. Twenty to 25% of seed buried at 5cm and 10cm remained dormant and formed the source for germination in following seasons, Five percent of seed buried at 10cm remained dormant for more than two years, Slender thistles responded more to applications of nitrogen‚Äöthan to applications of phosphorus or potassium when grown man infertile soil, Nitrogen greatly stimulated vegetative growth and also directly increased the reproductive capacity of the plants by enhancing branching and hence the number of flower heads produced. High pH (pH6.5) also favoured thistle growth. The effects of grazing management on slender thistle populations and botanical composition of improved pasture were investigated in two field trials. Deferred autumn grazing in winter and spring significantly reduced thistle population densities. Thistle control was obtained by two different ecological mechanisms: 1. Deferred autumn grazing caused pasture/thistle competition for light which resulted in etiolation of the thistles to the extent that they were readily eaten during subsequent grazing. The increase in acceptability of the thistles to the sheep appeared to be mainly due to morphological changes. An increase in nitrate (N) and total reducing sugars with etiolation may have also favoured acceptability. 2. Deferred autumn grazing apparently reduced the availability of moisture to thistle seeds and germination was partially inhibited. Grazing management, especially spring grazing, favourably altered pasture botanical composition by reducing the proportion of weed grasses. These changes are discussed in relation to thistle control It is suggested that deferred autumn grazing may be an alternative to herbicides for slender thistle control in pastures and that control would be expected to be mainly by the first mechanism. It is also suggested that deferred autumn grazing and herbicides may be combined to give effective control, as etiolated thistles were more easily killed with MCPA than normal rosette thistles.