Betula: birches.

The key to Betula species relies heavily on the bark of mature trunks. Young plants may be difficult to distinguish, but with a little experience it is not as difficult as it may seem. To begin, the preponderance of mature individuals in the immediate area will often lend weight to a difficult decision. The wintergreen smell and taste of B. alleghaniensis will identify that species in many cases and the leaves tend to be longer in proportion to the width than those of the other common species, B. papyrifera. The irregular lobes of B. nigra will help to separate that species in lieu of mature bark. B. pendula rarely escapes and is not to be expected outside of residential areas. The most popular horticultural form of B. pendula is deeply lobed and has a pendulus "weeping" habit. Escaped plants reportedly do not develop lobes, but the long tapering tips should serve to distinguish this species from the remaining two white, papery-barked species. Immature plants of B. papyrifera and B. cordifolia basically offer the same challenges as their mature counterparts and some individuals will be difficult to place.

Betula populifolia of the northeastern U.S. eastern and canada does not occur naturally in WI, but is planted horticulturally. The bark does not exfoliate (peel spontaneously) as does that of B. paperifera and the leaves are more long-tapering to the tip than those of B. pendula, which are in turn more long-tapering than B. papyrifera. Twigs of B. populifolia and B. pendula usually bear many small resinous glands, but twigs of B. papyrifera bear no small resinous glands, or a just few widely scattered.

Birches have a reputation for hybridizing, but the species are also quite variable in form and it is not productive to designate hybrids every time an individual does not fit comfortably into the key. Collectors should take pains to assess the characteristics of the populations from which vouchers of questionable individuals are taken.