The sight of a crab apple tree (Malus spp.) in flower in spring is the main reason it is so highly regarded in cold winter regions. A lack of blossoms is troublesome, and can be caused by any number of factors. Lack of adequate sunlight, warm winters, drought stress, inappropriate planting, tree age and disease all can play roles in diminishing flowering displays or, in severe cases, prohibit any flowering.
Lack of Sunlight
Crab apple trees that do not receive at least 6 to 8 hours of direct sunlight daily will not be healthy, robust plants. Lack of sunlight will diminish production of flowers, even across the canopy of branches on the same tree.
All apple trees (Malus spp.) must undergo an annual process called vernalization, which is the exposure to prolonged temperatures below 40 degrees Fahrenheit. Usually measured in "chilling hours", if a crab apple does not endure a winter cold dormancy, flower buds will not be formed, regardless of tree health or other factors.
In regions that experience early, warm springs, some varieties of crab apples may be susceptible to flower bud kill if late frosts or freezes occur. Once the crab apple forms and exposes its flower buds and new merging leaves, subfreezing temperatures can fully burn and destroy the flowers, whereas new foliage will be rejuvenated by the tree.
Trees that are unhealthy or stressed from drought may have diminished flowering displays. Although flowering is in spring, an unusually dry growing season last year can cause the tree to redirect its energies into retaining foliage or growing roots rather than flower bud formation.
Newly planted trees that have not been adequately watered during establishment, the first 1 to 3 years, may also diminish their flowering.
Very young trees, saplings, may be too young to produce large amounts of flowers. Conversely, aged crab apple specimens may also reach a point that their vigor and productivity precludes springtime flowering as the old plant slowly degrades.
Trees planted too deeply, with their trunk flares under the soil line, are often stressed and will not perform typical healthful events, including flowering. The trunk flare--the part of the trunk that widens just before transitioning into the roots at the soil line--needs to be slightly above or at the same average soil line. Deep-planted trees slowly die as oxygen is deprived from the roots and soil moisture leads to fungal and rot diseases on the buried tree bark.
Crab apples traditionally have been susceptible to blight, scab, rust diseases as well as attacks by borers or other insects. A healthy tree will stave off a casual encounter by a pest or disease, but compounded attacks can weaken a tree. These stresses, just like mentioned with drought, can cause a crab apple to focus its energies into combating the disease or pest rather than flowering, fruiting or even producing much new stem growth.
Modern hybrids and selections of crab apples have been developed with greatly improved resistances to common diseases. Whereas old, "heritage" crab apple varieties may still remain highly susceptible.