Thistles: Black or spear thistle (Cirsium vulgare)

Slender thistle (Carduus pycnocephalus)

Nodding thistle (Carduus nutans)

Saffron thistle (Carthamus lanatus)

Scotch thistle (Onopordum acanthium)

Variegated thistle (Silybum marianum)

Family: Asteraceae (daisies)

Status: Nodding thistle is declared noxious in category W2 in Shoalhaven, Eurobodalla and Bega Valley Local Government Areas (must be continually suppressed and destroyed), and Scotch thistle only in Eurobodalla and Bega Valley LGA’s, in category W2. No other thistles are listed as noxious on the south coast.

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 on the south coast, except for saffron thistle, which has yellow flowers.

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, it is 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 NSW. Black or spear thistle is the most common thistle found on the coast. It is often mistakenly referred to as Scotch thistle. Slender thistle is less common, but widespread. The remaining four species are not well established on the coast, although they may become locally common in some areas as a result of feeding contaminated hay. Scotch, variegated, saffron and nodding thistle should all be destroyed wherever they are detected on the coast, to prevent them becoming well established here.

Thistles generally prefer more fertile soils, and hence seldom invade forests on the coast, except occasionally along road edges.

They are sometimes a serious weed of remnant grassy native vegetation in farming areas.

Dispersal:
Seed is wind-blown, and moved around in soil and on vehicles and machinery. Contaminated hay and agricultural seed are also a source of infestations. The degree of wind movement of thistle seed varies with the species. Although all have a parachute of bristles to help keep the seed aloft, in some species such as nodding thistle this breaks off readily and does not assist much with dispersal. In other cases such as black thistle, seed can drift for long distances on the wind.

Look-alikes:
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.

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.

Control:
Thistles can be chipped out with a mattock (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). Spot spraying or boom spraying can be used for larger infestations.

Goats and donkeys can help reduce seed-set by eating the flowers.