Herbs suffrutescent, clumped. Leaves opposite only in proximal pairs, alternate or sometimes with fascicles of small leaves distally, caducous apical mucro prominent. Flowers actinomorphic; petals rose-purple. Seeds 1 or 2 per locule, obovoid to broadly obovoid.

Species 2: w United States.

P. H. Raven (1976), P. A. Munz (1965), and others noted the presence of an apiculus of brown oil cells at the leaf apex and suggested that this feature characterized sects. Cordylophorum and Xerolobium; however, further investigation of this character indicated that, while this feature is perhaps most prominent in these sections, similar apiculi occur in at least some taxa of all sections of Epilobium. Compared with the apiculi in subsect. Nuttalia, those in subsect. Petrolobium are particularly prominent and are a useful diagnostic character.

The two species of subsect. Petrolobium also characteristically have only four to eight seeds per capsule, which is fewer than the number found in any other species of the genus.