View photos of the medicinal plant Betula occidentalis (Water birch), profiled in Medicinal Plants of the Western Mountain States.

A passage from Medicinal Plants of the Western Mountain States:

For Birch’s topical application, make a base oil from the bark (and/or leaf) using the alcohol intermediate technique. Through this process the oil will contain fair amounts of betulin (and other triterpenes). It can then be used as is or converted into a salve or ointment for the betterment of eczema, psoriasis, and poorly healing wounds and ulcerations.