Freely branching stems that grow one to four feet tall.
Stems have leaf-line spines.
Leaves in a basal rosette the first part of the life cycle.
Stem leaves are alternate and sparsely hairy underneath.
Purple to pink flowers erect on the stem, solitary or in clusters of two to five.
Fruit is an achene capped with cream-colored bristles.
Habitat and Ecology
Spiny plumeless thistle (Carduus acanthoides) is found in the Northeast, the Midwest, and many western states, except Oregon, Nevada, Utah, and Arizona. It is native to Europe and was first discovered in the United States in Camden, New Jersey, in 1878. Spiny plumeless thistle does not compete well with established vegetation, but disturbance creates prime sites for colonization. The thistle is typically found in open sites, roadsides, pastures, annual grasslands, and waste areas. It easily hybridizes with other Carduus thistles.
Spiny plumeless thistle is an herb with leaf-like spines that extend up to the flower heads. Most of the plants are biennials, germinating in winter and spring and then existing as a basal rosette until flowering in the spring and summer of the following year. Mature plants are one to four feet tall. The freely branching stems arise from a single, thick taproot that can grow to a foot long. Spiny plumeless thistle reproduces by seeds.
Leaves The leaves of the basal rosette are 4-8 inches long and have spiny teeth. The stem leaves are alternate and attach directly to the stem. They are sparsely hairy underneath. The spines on the leaf margin can be up to 0.2 inches long.
Flowers and Fruits The flowers are purple or pink and are up to one inch in diameter. Under the flowers are spiny bracts. The flowerheads are erect on the stems, and the end of the stem may have only one flower or flowers in clusters of two to five. Spiny plumeless thistle flowers from June through October. The fruit is an achene (a dry, one-seeded fruit with a thin wall that does not open at maturity; for example a sunflower “seed”.) The achenes are 2-3 mm long, and they are light brown with faint lengthwise stripes. The achene is capped by a ring of cream-colored bristles that are 4-5 inches long.
Car’duus is the classical Latin name for thistle. Acantho’ides means appearing like a spike or spine.
Spiny plumeless thistle is similar to musk thistle (Carduus nutans), but its flowers are a third the size and its stems have many leaf-like spines that extend up to the bases of the flower heads. It can also be confused with Canada thistle (Cirsium arvense), bull thistle (Cirsium vulgare), and Scotch thistle (Onopordum acanthium). Canada thistle has small flower heads, plumes on the achene bristles, and smooth stems. Bull thistle has plumes on the achene bristles and stiff bristly hairs on the upper leaf surfaces. Scotch thistle is not densely covered with bristles and has deeply- pitted, honeycomb-like receptacles (the thickened part of the stem below the flower).
Possible control methods are explained at these websites: