Certainly when they are in flower —and sometimes when they are out of flower —there will be little difficulty in separating these species. Reference to the photographs will show floral differences at a glance. When the plants are considered regionally, identification is often more simplified.
In the area of the northeastern border between the United States and Canada, there is only P. vulgaris. Our western readers will have to contend with the
"macroceras" hypothesis, if it is eventually established. Boreal botanists may run into P. vulgaris, the "macroceras" subgroups, and P. villosa. When the plants are in flower, identification will be easy, and the rounder leaves and hairy scape of P. villosa are also characteristic of that species.
On the Atlantic coastal plain of the Carolinas, there are only P. lutea, P. caerulea, and P. pumila, all with very distinctive flowers. Even out of flower, a stocky, tiny rosette is most likely P. pumila, which is generally uncommon in this area. Look for a maturing seed capsule to be sure that the plant is not a juvenile of one of the other two species! P. lutea and P. caerulea are common.
In the Gulf coastal region, especially upper Florida, there is the widest range of species, and identification by flower will be necessary in most instances, except when the red leaves of P. planifolia and the peripheral buddings of P. primuliflora are present.
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