Thistles

"Thistle" covers a range of related but distinct weeds from the genera Cirsium, Carduus, Onopordum and Sonchus, all sharing spiny leaves and, in most cases, purple or yellow flower heads. Common types found across Australia include spear thistle, saffron thistle, variegated thistle, Scotch thistle, slender thistle and sowthistle. Despite differences in size and detail, they are grouped together because they cause similar problems and are managed in similar ways.

The plant itself is usually a rosette-forming biennial or annual, though some, like Californian thistle, are deep-rooted perennials. Leaves are typically deeply lobed or serrated with sharp spines along the margins, ranging from grey-green to dark green, and often have a waxy or slightly hairy surface. Flower heads appear from late spring through summer, sitting atop tall, sometimes branching stems that can reach 30cm to over 1.5m depending on species. Flowers are usually purple, pink or yellow, surrounded by spiny bracts, and mature into a fluffy white seed head that disperses on the wind, much like a dandelion. Roots vary from a simple taproot in annual types to extensive lateral root systems in perennial thistles like Californian thistle, which allows them to regrow from root fragments.

Thistles are widespread throughout Australia in pastures, roadsides, disturbed ground, lawns and garden beds. Most germinate in autumn or spring, grow through winter as a rosette, then bolt to flower in spring and summer. They thrive in compacted, overgrazed or nutrient-poor soil and readily colonise bare patches in lawns or garden beds where turf or ground cover is thin.

Management relies on preventing seed set, since a single plant can produce thousands of wind-borne seeds. Physical removal by digging out the taproot before flowering works well for scattered plants, especially in garden beds. In lawns, maintaining dense, healthy turf through correct mowing height, regular feeding and overseeding of bare patches reduces the open ground thistles need to establish. Slashing pastures before flowering helps prevent seed spread but won't kill established perennial roots. For larger infestations, selective broadleaf herbicides registered for thistle control are commonly used, with best results achieved when plants are young and actively growing (seedling to rosette stage).