The photo above shows the flower head of tumble knapweed as seen at about 3000' on the south side of Mt. Adams...................August 25, 2005. Note the long spine at the tip of each light involucral bract, and the numerous comb-like lateral spines.
Tumble knapweed is an erect, widely branched annual or biennial from 10-60 cm tall. The pinnatifid leaves are small. The leaves are reduced in size upwards on the stem, becoming entire in the inflorescence.
The numerous flower heads are narrow, with the involucre 8-10 mm tall, and the middle and outer bracts coarsely comb-like or spiny. The spines range from 1.5-4 mm long. Individual flower heads are few flowered, with the flowers white to occasionally purple.
Tumble knapweed may be found in disturbed places such as fields, waste places, and roadsides.
Native to the Mediterranean region, tumble knapweed is found over much of the United States.