Diffuse knapweed has an urn-shaped flowerhead that can be white, pink, or lavender. Unlike other knapweeds, diffuse knapweed has terminal spines on the floral bracts.
Diffuse knapweed plants start with just a basal rosette of leaves. These leaves grow throughout the first year and if they reach a critical growth stage, the plant will bolt the second year sometime around May. This plant flowers between July and September. The fruits can begin developing as early as August. After the plant has died (often after fruiting), the stems often break off and behave in a tumbleweed fashion, spreading seeds as they go.