Carduus

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Carduus: plumeless thistles

Carduus is a group of flowering plants in the daisy family. It is one of the two true thistles, along with Cirsium. Plants in this group are commonly called plumeless thistles.

Where they grow
Carduus plants come from temperate Eurasia and North Africa. Many species have been introduced to other parts of the world.

What they look like
- They are usually annuals or biennials, sometimes perennials.
- Most plants grow 2 meters tall, but they can reach up to 4 meters.
- Stems are erect, winged, spiny, and often have woolly hairs.
- Leaves can be hairy or smooth, may be whole or lobed, and have spine-toothed edges.
- Flowers are in heads that can be solitary or in groups of up to 20. The heads are round to cylinder-shaped with spine-tipped bracts. Flowers themselves are tubular and can be white, pink, or purple.
- The fruit is a small dry seed (cypsela) with a tuft of bristles or scales that helps it drift in the wind.

Why some Carduus plants are a problem
Many Carduus species are invasive outside their native range, including in Australia and the United States. Notable weedy species include C. acanthoides, C. nutans, C. pycnocephalus, and C. tenuiflorus, which grow easily in disturbed habitats like overgrazed pastures.

A note on C. nutans
C. nutans can release chemicals that inhibit the growth of other plants (allelopathy).

Controlling them
Biological control methods have been used to manage weedy Carduus plants. These include insects and a fly:
- Thistle head weevil (Rhinocyllus conicus)
- Thistle crown weevil (Trichosirocalus horridus)
- Thistle crown fly (Cheilosia corydon)
- Musk thistle rust (Puccinia carduorum) may also be used.

Number of species
The Carduus genus contains about 82 accepted species, along with several natural hybrids.