Gardeners across U.S. Department of Agriculture hardiness zones 3b through 8a rely on crabapples (Malus spp.) to provide an ornamental display of flowers in spring. In many varieties, crabapples later form small fruits that are decorative and provide a food source to songbirds. Crabapples must endure a winter dormancy for vernalization to occur--the process of exposure to chilly temperatures to initiate formation of flowers the following spring. Bud break is a term to measure end of dormancy.
Bud break refers to the swelling of the dormant buds on crabapple tree branches. Normally they are protected by a scale, but once temperatures warm and sap again flows in above-ground tissues, buds begin to grow. Break can be considered when the protective scale coating is shed and tender new growth tissues of flower buds or leaves are first exposed to air and light.
According to Colorado State and Clemson Universities, there are varieties (cultivars) of crabapple that bloom at different times in spring. They are conveniently referred to as early-, mid- or late-season crabapples. Most crabapples expose their flower buds and bloom before the majority of leaves mature. Local weather conditions can affect when bud break begins; chilly weather prolongs emergence of flower buds, while no frost and warm days tend to make bud break occur earlier than usual in a region.
Latitude and elevation play part in when crabapple trees leave dormancy and begin breaking buds in spring. In USDA hardiness zones 7 and 8 (southern U.S.) where winters are short and milder, crabapples begin breaking buds as early as late March and continue into mid April. By contrast, USDA zones 3 and 4 (northern U.S. and Canada) have longer winters and bud break is expected by early to mid May. Higher elevations have colder winters and later springs, and correspond respectively with later bud break on crabapples, just as if they were at a higher latitude.
Once bud break occurs and tissues of flower buds or leaves are exposed to the air, they are increasingly susceptible to frost damage. Bud scales provide limited but effective shelter and protection from light frosts that are untimely and late in spring. Doug Akers, extension educator with Purdue University, says that the bud or "balloon" stage of flower buds, when buds are big and just ready to open, are potentially damaged when temps drop below 29 degrees Fahrenheit.
Young, tender leaves and petals turn limp, become brown or fully drop off the crabapple tree branches when exposed to too low of subfreezing temperatures. The trees must rejuvenate new buds and break bud again quickly once warm temperatures return. In crabapples, leaves are immediately grown when the initial flower buds that broke are destroyed by frost.