The Southeastern Grasslands Institute presents Genus of the Month: Cirsium, the plume thistles
The genus Cirsium is a group of important native grassland plants in the larger plant group Asteraceae. The the Flora of the Southeastern United States, there are 20 species of Cirsium you may come across in our region. Of these, 17 are native to the United States, and 3 are considered invasive. Because of the feathery structures on their seeds/fruit, the plants in this genus are commonly called plume thistles.
Benefits of Cirsium
Pollinators - thistles in the Cirsium genus produce nectar that attract and support many pollinator species, including bees, butterflies moths, wasps, and hummingbirds.
Wildlife Food Source - the Cirsium thistles produce seeds that provide food for birds, and the seeds have a fluffy material (called pappus) that birds use to build nests.
Identifying Characteristics of Cirsium
Please find a key to the genus Cirsium linked here. Below are some defining genus characteristics.
Composite Flowers - flowers are made of many smaller flowers called florets. The head of the flower is made up of all the individual florets and the involucre, which is the ring of bracts (a type of modified leaf) at the base of the flowering head. The bracts are important in identifying individual species of Cirsium, which can be distinguished by differences of presence or absence of spines on the phyllaries (bract structures) on the flower head.
Selected Native Cirsium Species
Cirsium altissimum (tall thistle)
Habitat: pastures, woodlands, thickets
Images by aggie_wildlifer (CC0)
Cirsium discolor (field thistle)
Habitat: pastures, woodlands, thickets
Images by casey_harless (CC0)
Cirsium muticum (swamp thistle)
Invasive Cirsium Species in the SGI Focal Area
Cirsium vulgare (bull thistle)
Native to Europe, endemic and origins in Europe
Identifiable by stems with spiny wings and leaves that have bases that extend downward and wrap around the stem (decurrent leaves)
Images by andreakautz(CC0) and philmaynard (CC0)
Cirsium arvense (Canada thistle)
Native to Euraisa, endemic and origins in Europe
Identifiable by creeping rhizomes which form colonies and tend to be in large dense stands, no spines on the phyllaries (modified leaf structures on base of flower head), and is the only perennial Cirsium species in the U.S. (all others are biennial).
Blog by SGI Staff; based on work by Alaina Krakowiack, SGI Central Appalachian Grasslands Coordinator/Ecologist; blog prepared by Dr. Eleanor Lopez, SGI Communications Specialist.