A number of species of crabapple and their hybrids are cultivated for their showy flowers and fruits. The spread of alien crabapples into the wild appears to be a recent phenomenon. But they are clearly increasingly spreading from cultivation and are now a characteristic feature of old fields near urban areas. They are also quite difficult to identify. Both flowers and fruits are very helpful, as in Crataegus, and notes on anther color and whether the leaves are folded (conduplicate) or rolled (convolute) in bud are useful. Collections should also include short (spur) shoots and elongate shoots, as the leaves may be quite different in form, being lobed in some species.
Thus far, only Malus baccata of the cultivated crabapples appears to be widely established throughout the state. More and better collections are needed to understand these introduced Malus, which are doubtless more diverse and common, especially in disturbed areas in and near cities and towns than the modest number of collections now in herbaria suggest. In addition to the species noted in the key, a number of individuals appear intermediate and are probably of hybrid origin (as are many cultivars). A pink-purple-flowered plant collected in Lenawee Co. by R. W. Smith in 1986 appears to be M. ×purpurea (Barbier & Cie) Rehder, as also a more recent Mason Co. specimen. Fruiting plants, with round, medium-sized yellow fruits, also collected in Lenawee Co., may be derived from M. ×zumi (Matsum.) Rehder.
1. Leaves all of similar shape, unlobed, closely and regularly singly or doubly serrate (or crenate-serrate); anthers yellow.
2. Undersides of leaves and floral tube glabrous or sparely pubescent even when young, calyx deciduous.
2. Undersides of leaves and floral tube densely pubescent when young, usually retaining pubescence into maturity; calyx persistent.
3. Leaves dentate-serrate; the underside pubescent even when fruits are mature; fruits green or yellowish, often flushed with red, rarely solid red, usually significantly more than 2 cm in diameter.
3. Leaves sharply acuminate-serrate; the underside quickly becoming glabrous after flowering; fruits solid red “crabapples,” ca. 2 cm in diameter.
1. Leaves on elongate vegetative shoots, usually with at least shallow acute lobes and sinuses, as well as coarse ± irregular teeth; anthers pink; fruit less than 3 (–4) cm in diameter.
4. Flowers with 3 styles; calyx lobes deciduous in fruit.
4. Flowers with 5 styles; calyx lobes persistent in fruit.
5. Calyx lobes (outside), pedicels, petioles, and leaf blades beneath glabrous, or if tomentose becoming glabrate toward end of anthesis.
5. Calyx lobes (outside), pedicels, petioles, and leaf blades beneath retaining tomentum well past flowering time, even into fruit.