Weed
lesser knapweed
AcheneAchene:
A dry, hard, one-chambered, one-seeded indehiscent fruit, as in buckwheat (Fagopyrum) or sunflower (Helianthus). The fruit wall is not united with the seed coat.
Oblong
Yellow to light brown
Longitudinal ridges; surface is covered with a fine pubescence.
Lateral scar; collar is thin and finely serrate.
AchenesAchene:
A dry, hard, one-chambered, one-seeded indehiscent fruit, as in buckwheat (Fagopyrum) or sunflower (Helianthus). The fruit wall is not united with the seed coat. usually do not retain a pappusPappus:
Modified calyx in Asteraceae, composed of hairs, bristles, awns, or scales. The pappus-like structure attached to milkweed seeds is termed a coma. 
. Surface rough with longitudinal ridges and soft pubescence. Size measurements refer to mature achenesAchene:
A dry, hard, one-chambered, one-seeded indehiscent fruit, as in buckwheat (Fagopyrum) or sunflower (Helianthus). The fruit wall is not united with the seed coat.; immature achenesAchene:
A dry, hard, one-chambered, one-seeded indehiscent fruit, as in buckwheat (Fagopyrum) or sunflower (Helianthus). The fruit wall is not united with the seed coat. also shown here.
State Noxious Weed: Arizona, Colorado, Washington, Wyoming
Not included
Asteraceae
Centaurea nigra L.
Centaurea nigra L. var. radiata DC.
