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Star thistle is declared noxious in the Southern Tablelands and South East Region only in Bombala LCA, in class 4, although it can be quite common elsewhere on the tablelands.

Description

Most thistles are erect single-stemmed or branching biennial herbs ranging from 30cm to 2m high depending on species and growing conditions. They are characterised by having long spines on the leaf margins and stems. The plant begins life as a rosette, from which an elongated flowering stem arises. Flower heads consist of numerous small flowers clustered into cylindrical or hemispherical heads at the branch tips, and surrounded by spiny bracts. Flower colour is pink to purple for most of the species found in the region, except for a few with yellow flowers.

Star thistle is distinctive in often being quite short (about 40cm), though it may reach 1m high, and densely branched from the base. The rosette leaves are narrow, grey-green, deeply lobed right to the central vein (midrib) and not spiny. The purple flower heads are enclosed in bracts which have extremely long bone coloured spines. For additional thistle photos, check the entries for Scotch thistle, nodding thistle and soldier thistle.

preferred habitat and impacts

Thistles are invasive weeds of pasture, reducing carrying capacity. The broad flat rosette habit in the early stages of growth smothers surrounding grass plants, and the density of stands which can occur after disturbance such as over-grazing or cultivation can choke out all other vegetation. Unpalatable to stock because of the spines, they are favoured by heavy grazing. The spiny nature of thistle plants restricts stock and human movement in infested pasture.

Thistles are a more troublesome weed in the drier tablelands and slopes of southern NSW than on the coast, and more species are present in these areas. Black or spear thistle is the most common thistle found on both the coast and the tablelands, but other thistle species are quite uncommon on the coast.Thistles are also environmental weeds, invading grasslands and grassy woodlands.

dispersal

Seed of star thistle lacks the tuft of fine hairs which aids wind dispersal in some thistle species, so is mostly moved around by water, in soil and on animals, vehicles and machinery. The seed tends to stay in the spiny heads and the long spines help these to adhere to animals, bags, machinery and so on.

look-alikes

All thistles are broadly similar in appearance and star thistle is distinguished by its compact habit, spineless leaves and the long bone coloured spines on the bracts. Its rosette leaves are similar to other thistles in the genus Centaurea, of which the commonest locally is St Barnaby's thistle (Centaurea solstitialis). However, it has yellow flowers and less ferocious spines.

Some plants have a resemblance to thistles although they are not closely related. The weed Mexican poppy (Argemone ochroleuca) has thistle-like spiny foliage which is silvery grey in colour. It is in the poppy family, and has a cream coloured poppy flower and a prickly seed capsule which splits at the top to release numerous small black seeds. It grows to about 1m high. Wild teazel (Dipsacus fullonum) has more elongated cylindrical heads of mauve flowers with long spiny bracts at the base, and short prickles on stems and the vein on the leaf underside, but not on leaf margins. It is also a weed.

There is a native plant with thistle-like foliage, the blue devil (Eryngium rostratum). It is a plant of native grasslands and grassy woodlands on the tablelands and slopes, and is very unlikely to be found on the coast. It has blue flowers in branched heads, and is not in the daisy family. Its leaves are narrow, deeply lobed with very narrow lobes and not spiny, so young plants could be mistaken for star thistle.

control

Small infestations of thistles can be chipped out, but may regrow if the cut is not made deeply enough. Hold the top of the plant down to the ground with one foot to get the spiny leaves away from your hands while chipping, or catch them while still in the rosette stage, when they are very much easier to cut. Spot spraying or boom spraying can be used for larger infestations. Slashing can be effective on star thistle but viable seed may form on the cut plants if slashing is done after heads have been fertilised.

Goats and donkeys may help reduce seed-set by eating the flowers, although the very long spines of star thistle might prove a better deterrent to browsing than the shorter spines of most other species.