Thistles (Various Genera: Primarily Cirsium, Carduus, and Onopordum)
Common Names
- Bull Thistle
- Canada Thistle
- Musk Thistle
- Scotch Thistle
- Creeping Thistle
Description
Thistles are a group of spiny, herbaceous plants in the Asteraceae family, characterized by their prickly leaves and stems. They are typically biennial or perennial, though some species can be annual. Thistles are known for their aggressive growth habits, sharp spines, and distinctive purple to pink flowers. Many species are considered invasive weeds in agricultural and natural settings.
Identification
Stems & Leaves
- Stiff, erect stems that may be branched or unbranched.
- Leaves are deeply lobed with sharp spines along the margins.
- Surfaces often covered with fine hairs or a woolly texture.
Flowers
- Typically bright purple, pink, or occasionally white.
- Flower heads are made up of numerous tubular florets, giving them a brush-like appearance.
- Flowers are surrounded by spiny bracts.
- Blooming period varies by species but generally occurs from late spring to summer.
Fruits & Seeds
- Produce small, dry seeds (achenes) with a feathery pappus, aiding in wind dispersal.
- Seeds can remain viable in soil for many years.
Spread
Thistles primarily spread through seed dispersal by wind, water, animals, and human activity. Some species, such as Canada Thistle (Cirsium arvense), also spread through creeping root systems, making them particularly difficult to control.
Similar Species
- Milk Thistle (Silybum marianum) – Larger, with distinctive white-mottled leaves.
- Teasel (Dipsacus spp.) – Resembles thistles but has cone-shaped flower heads and lacks true spiny leaves.
- Burdock (Arctium spp.) – Features burr-like seed heads instead of feathery seed dispersal.