Slenderflower Thistle
Carduus tenuiflorus
Washington State Classification
Class A — Eradication Required Statewide
Slenderflower thistle invades range lands and pastures, outcompeting desirable forage plants and native plant species by forming dense stands.
Quick Reference
Identification
Growth Habit
Stems grow up to around 6.5 feet and are unbranched to branched and plants are covered in loose, woolly hairs. It germinates in the fall, then overwinters as a rosette (a radiating cluster of leaves) and produces flowering stalks in late spring.
Leaves
Leaves at stem base are generally 3.9 to 9.8 inches long. They have 6 to 10 spine-tipped lobes and tapered leaf bases forming winged leaf stalks. Leaves are often covered with loose woolly hairs. Stem leaves are shorter, less lobed and stalkless.
Flowers
Flowerheads are clustered in groups of 5 to 20+ at branch tips. Flowers pinkish, 0.4 to 0.6 inches long. Outer bracts at the base of flower head with 1 to 2 mm long spine tips and inner bracts without spines. Bract margins slightly membranous.
Fruit & Seeds
Slenderflower thistle has a seed (achene) within a dry covering, with bristles on the top end. It is 0.16 to 0.20 inches (4 to 5 mm.) long.
Impact
Slenderflower thistle invades range lands and pastures, outcompeting desirable forage plants and native plant species by forming dense stands.
Ecology & Spread
Habitat
Slenderflower thistle grows in dry open areas such as pastures, ranges, right-of-ways and areas of disturbance. Please click here to see a county level distribution map of slenderflower thistle in Washington.
Spread Mechanisms
Reproduction
Slenderflower thistle reproduces by seed.
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Control Methods
Mechanical
- Tilling or digging out slenderflower thistle are effective methods of mechanical control.