This native Eurasian weed is a diffusely branched annual or short-lived perennial. It grows 1 to 2 feet (30-60 cm) tall and has stems that are rough to the touch
Leaves are finely divided except for the reduced leaves of the inflorescence which are mostly entire. Most are covered with fine hairs.
The plant produces many narrow flower heads. The flowers are white to rose or sometimes purplish. Flowering occurs from July to September. The margins of the bracts on the base of the flowers (involucre) are divided like the teeth of a comb. Each bract is tipped with a yellow, slender spine.
The seeds or achenes are brown or grayish and lack a tuft of hairs (pappus).
Diffuse knapweed infests roadsides, waste areas, dry rangelands and disturbed sites. It is a highly competitive plant which threatens to exclude many desirable species from pastures and wildlands. The seed is commonly spread by vehicles or by the sale and movement of contaminated sand and gravel.
Seedlings of diffuse knapweed have finely divided leaves covered with short hair.
Bracts under the flowers have yellow spines with teeth along their margins.