The common dandelion (Taraxacum officinale) is a perennial plant--most would say a weed--with long, deeply toothed green leaves growing in a low rosette around tall, hollow milky stems which bear a shaggy, cheery yellow flower head. Dandelion flowers must be popular in the plant kingdom, as the flowers of numerous other plant species strive their best to look just like dandelions, even turning into a white puff of drifting seed heads at maturity.
Coltsfoot (Tussilago farfara) bears its flowers on stems ranging 3 to 18 inches tall and blooms at about the same time as dandelions. The coltsfoot flower is about the same size and happy shade of golden yellow as the dandelion; however, on closer inspection, the coltsfoot flower has central disc flowers surrounded by the outer ray flowers most typical of the composite flower varieties, while dandelions lack the inner disc flowers and consist solely of a ring of outer ray flowers. The other distinct difference between coltsfoot and dandelions is the leaves--coltsfoot blooms before the leaves emerge from the ground, and when the leaves arrive, they are large and flat, with a shape resembling a horse's hoof, according to the Purdue University Horticulture Department description. Coltsfoot also loves damp soil and thrives in ditches and along brooks, while dandelions tend to prefer good drainage.
The smooth hawksbeard (Crepis capillaris) grows throughout most of the southern and western United States. The Virginia Tech Weed Identification Guide maintains that smooth hawksbeard primarily lives in pastures and hayfields, as well as along roadsides. Its sunshine-yellow flowers sport only ray flowers, like the configuration of the common dandelion. However, smooth hawksbeard tends to grow in a multi-stemmed clump. Its flowers are distinctly dandelion-like, and its leaves are thin and deeply pointed, but its growing habit differs considerably from that of the dandelion. The leaves clasp the stems, which are more upright than the dandelion rosette form, and the flowers bloom in an open spray on multiple thin, dark, branching stems.
The annual sowthistle (Sonchus oleraceus) is a widespread annual plant which grows throughout the western United States, but is particularly abundant in California's Central Valley and coastal areas, where it thrives year-round, according to the U.C. Davis Integrated Pest Management Program Weed Gallery. The plant bears spiny, pointed leaves, somewhat dandelion-like in shape, but considerably larger, with the plant form reaching about 4 feet tall. The plant also bears flat, clasping leaf rings around stem branching points. Its flowers are golden dandelion-like composite flowers with no visible disc flowers, but they grow in clusters on the tip of the central, main stem of the plant and flower a bit later in the summer than do dandelions.