Family Asteraceae > Subfamily Carduoideae> Tribe Cardueae > Subtribe Carduinae.
Commonly known as just Thistles or Plume thistles most are native to Eurasia and North Africa.
Mabberley recognises around 250 species but others up to 300.
A few species are cultivated but most are considered weeds.
Cirsium differ from other thistles such as Cynaria, Centaurea, Silybum and Onopordum in having feathered hairs on their seeds.

Some of the annual or perennial herbs have no stem with others up to 2 (5) m high.
Many species have spiny wings on the stems.
Stems may have hairs that are sometimes matted and cobweb-like.

Some plants have only basal leaves while others also have alternate leaves along a stem.
Leaves may have a petiole, sometimes with wings from the tapering blade base.
Leaves can be undivided or part or all of the edge can be toothed or shallow to deeply lobed.
There are usually spines around 2 mm long on the edge.
The green to yellowish-green leaves usually have no hairs on the upper surface but there may be a few.
The lower surface may be the same but is often greyish-white with dense matted hairs.

The terminal globular, ovoid or bell-shaped flower heads, with or without a peduncle can be solitary or clustered.
At the base of the head is the involucre with up to 10 or more whorls of involucral bracts or phyllaries.
The phyllaries in each whorl can all be similar or differ in size and/or shape.
Many are ovate, lanceolate or narrowly triangular with a 1 to 5 mm long spine while others are reduced to just a long sharp spine.
Bracts may have wings on each side that often have a fringed edge and some have a wider appendage at the tip.
Bracts may be smooth or have bristles or hairs that can be matted and cobweb-like.
Commonly the outer, middle and inner bracts have different combinations of the above features.

The flowers or florets are on a flat or convex receptacle.
The florets are all similar with a pappus of bristles and a corolla with a tubular base and 5 similar lobes.
Lobes can be around the same length as the tube or half to twice as long.
Petals are red, purple, yellow or rarely white.

Florets are mostly bisexual but when unisexual the males and females are on different plants.
The 5 stamens have short soft hairs on the filaments and the anther sacs have a small appendage at both ends.
The inferior ovary has 2 styles that are fused except at the tip.

The laterally compressed seeds are cypselae (from an inferior ovary) but commonly called achenes (from a superior ovary).
They are smooth or ribbed and have 3 or 4 rows or feathery bristles (the pappus) that are joined at the base so all fall off together.

Australia has 4 (5) naturalised species with Cirsium vulgare and Cirsium arvense being the most common.