There's a joke in wildflower classes that the first yellow-flowered daisy species students learn is ADYC, short for another D*** Yellow Composite. This has some truth to it, because superficially many species look similar in flower. The goal of wildflower classes is to look closer, to see the details that make one species distinct from all others, even DYCs.
Today I present a primer on four introduced species, Crepis capillaris, Hypochaeris radicata, Leontodon saxatilis and Taraxacum officinale.
All are tap-rooted biennial to perennial species in the Asteraceae, or composites, which have compound flower heads of dozens to hundreds of tiny yellow flowers. All are common on open ground, including pastures, lawns, roadsides, and in the dunes, back from the open west edge. So how to tell them apart? Look closely at the backs of each flower, and seed head, at the leaves, and the differences will appear.
Crepis capillaris, smooth hawkweed
Smooth hawkweed has many small flowers on a tall stem, and like common dandelion has smooth leaves with sharp points and deep scallops. Flower heads are yellow underneath as well as on top. The seed heads are also small, and the seeds are slightly curved and strongly ribbed. There is no long stem between seed and the bristles that carry it aloft; instead the lofting plumes attach directly to the seed at one end.
Hypochaeris radicata, hairy cat's-ear
Hairy cat's-ear has hairy leaves in large rosettes, forming large succulent mounds [excellent for sheltering baby slugs and snails]. There are 1 to several large yellow flowers per stem. The backs of the outer flowers are light purplish/brown. Seed heads are large, with a long stem between seed and lofting bristles.
Leontodon saxatilis, hairy hawkbit
Hairy hawkbit has low rosettes of hairy leaves, slightly scalloped, making small mounds in a lawn. Flowers are single, one per stem, and light purple on the underside of the head. Seed heads are small, and seeds are slim, slightly curved, with bristles attaching to one end without a stem.
Taraxacum officinale, common dandelion
Dandelions form rosettes of leaves, with thin leaves, deeply scalloped, single flowers per stem, and with green bracts under the outer flowers that persist on the seed head. There are two sets of green bracts, one large set under the flower, and a second smaller set that points down towards the ground. Even on seed heads, these 2 sets persist. There are long stems between seed and lofting filaments.
Mid to late summer is the time to look at all DYCs, because leaves, flowers, seed heads and ripe seeds are all present. We have other species here, both native and introduced, but these are the common species in most yards, called by most people "dandelions".