Varieties of crab apple are selected to bloom at the same time as the apple trees in an orchard , and the crabs are planted every sixth or seventh tree , or limbs of a crab tree are grafted onto some of the apple trees . Varieties of crabapple are selected to bloom contemporaneously with the apple variety in an orchard planting , and the crabs are planted every sixth or seventh tree , or limbs of a crab tree are grafted onto some of the apple trees .