The wild crab is more or less thorny, with leaves entire, and monoecious (i.e. bears both male and female flowers on the same tree). The white flowers often appear in groups of five, with no contact between the petals. On careful study, the flowers can be seen to have five styles grown together, but at the tip all five can be distinguished. The green fruit is at most 3 cms in diameter, hard and bitter, with remains of the calyx lobes persisting.
Because the wild crab crosses easily with domesticated varieties, its identification is always rather problematic. One tip is that the young stems and leaf undersides of the hybrids classed under the catch-all M. pumila are downy, unlike the wild crab.