There are many species of knapweed that have been introduced to North America, four of which are among the worst noxious weeds in the western US. Spotted knapweed is the only one with black tipped involucral bracts (the leaf-like structures that surround the cup upon which the flowers are attached), creating a spotted appearance.The flowers are generally purple in color. The garden plant Bachelor's buttons (Centaurea cyanea) is similar and also can occur in disturbed habitats. However, it has smooth involucral bracts and entire leaves, but is not a major weed.
Common camas is a stout, robust, 12-28 inches (30-70 cm) tall plant with a dense inflorescence. Camases are liliaceous, perennial herbs that grow from an edible bulb. The leaves are long and narrow, grass-like, and emerge from the base. The flowers are light to deep blue; more than 3 flowers in an inflorescence may be open at one time. Flowers have 6 tepals, 6 stamens, and 3 stigmas. The inflorescence is a spike-like cluster borne on a leafless stem that is held above the leaves. Common camas is distinguished from great camas (Camassia quamash ssp.quamash) by the following: the flowers are slightly irregular, with the lowest tepal curving outward away from the stem; the anthers are bright yellow; the plant is relatively short and stout, with shorter flower stalks and smaller bulbs; and there is no waxy powder on the leaves. Common camas blooms from April through June. The fruits are barrel-shaped to three-angled capsules,splitting into three parts to release many black, angled seeds.