slenderflower thistle Carduustenuiflorus W. Curtis
Appearance
Carduus tenuiflorus is an herbaceous winter annual or biennial that invades disturbed areas. The most serious infestations occur in the Pacific coastal states, but Carduus tenuiflorus also occurs sporadically in the eastern United States. Plants grow up to 6 ft. (1.8 m) tall with single or multiple spiny stems with wings arising from the base of the spines.
Foliage
Stem wings can be up to 0.4 in. (10 mm) wide. Leaves are deeply lobed and also very spiny.
Flowers
The small flower heads (less than 1 in. [2.5 cm] long) are slender, purple, and lack stalks. Flowering occurs from May to July.
Fruit
Fruit has a dry outer coating and is 0.16-0.20 in. (4-5 mm) long.
Ecological Threat
Carduus tenuiflorus invades pastures, old fields, vacant lots, and other disturbed areas. Its presence reduces the forage productivity of pastures. It is native to Europe and was recognized as a serious invader in the western United States as early as 1956.