Fleabanes, of the Erigeron genus, are easy to identify when they bloom: they have small daisy-like flowers, usually with white or pinkish rays and a yellow center, but in contrast to daisies and asters, their flower rays are extremely narrow and very numerous.
Distinguishing between the different species of Erigeron, however, can be tricky. Where I live, there are four species, and the flowers all look similar. Erigeron annuus is easily distinguished from the other fleabanes native to Pennsylvania by the basal leaves, which are pictured below.
Dimensions and height
When I've grown this plant or seen it in the wild, it's tended to grow about 3 feet tall, but I've seen sources saying it can grow as tall as 5 feet. Under poorer conditions, it may only grow to about 2 feet.
The leaves are 3-5 inches long. The flowers themselves are small, usually around 10-14 mm wide.
Identification when not in bloom
When not in bloom, Erigeron annuus can be confused with a number of other wildflowers.
The basal rosettes of leaves, in particular, look a lot like the leaves of black eyed susan or purple coneflower, especially of first-year plants. The leaves are slightly less sturdy than those of black eyed susan or coneflowers. Also note that on Erigeron annuus, the base of the leaf gradually tapers to where it attaches to the stem or base of the plant. On a black-eyed susan, the leaf base curves inward, leaving a well-defined leaf stem (petiole) that connects to the base of the plant or the stem.