Crabapples serve decorative functions in landscaping--the most important crop is the dense spring bloom, not the tiny bitter fruit. Some of the methods employed in apple orchards also apply to this ornamental tree, since both types of apple trees share many of the same diseases and problems. Pruning techniques focus on shaping the crabapple and its canopy to promote healthy growth.
Prune crabapple branches in late winter during the tree's dormancy. Seal large pruning cuts with pruning paint. Clean cuts made close to the branch collar harden off in time to resist springtime fungal infections. Neat work protects the trees better than pruning paint. Remove crossed branches and damaged limbs, but avoid disturbing the natural shape of the tree. Thinning branches increases the yield of edible apple varieties, but reduces the spring bloom of crabapples without improving the fruit.
Spray dormant oil over the bare limbs of crabapples as soon as buds turn green in early spring. This mixture of oil and insecticide covers and suffocates clusters of insect eggs glued to twigs and bark, ending many insect threats before any visible damage occurs. Raking leaf litter and fallen fruit from beneath the trees before weather warms removes fungal sources and cover for over-wintering insects. Control outbreaks of summer insects with insecticide spray or dormant oil rather than by pruning out infested limbs. Defoliated limbs usually recover. Pruning unnecessarily could do more damage than the insects would.
Spray vulnerable crabapple varieties early and regularly with fungicide through the growing season to prevent common diseases like cedar-apple fungus. Control cedar-apple fungal infections at the source by inspecting neighboring cedars and junipers for warty growths and removing the tumors before they release infective spores. Blackened and blistered crabapple twigs and branches could indicate fireblight, a disease which responds to antibiotic spray.
Protect the grafted portion of the crabapple by removing any sucker shoots from roots or trunk as soon as they appear, throughout the growing season. Most crabapples grow from grafted scions of trees specially selected for vigor and beauty. The supporting rootstock quickly outgrows and even chokes out the graft if not pruned back.
Fertilize crabapples once a year. For best growth apply fertilizer in late fall at least a month after the first hard frost. Roots grow the most in late fall and early spring. Alternatively, fertilize in early spring to boost the first cycle of woody growth. Treat an area roughly three times the diameter of the tree's canopy to cover the entire root zone. Don't fertilize areas already fed by lawn or shrub treatments.