Modern meteorology predicts and explains weather systems, including that last cold snap that interrupts the first sunny, breezy days of spring. Nevertheless, folklore remains, giving colorful names to weather phenomena. In the sunny South and mild Midwest, blackberries (Rubus spp.) are one of the first fruits to blossom, often coinciding with the "return of a continental polar air mass after maritime tropical air masses have begun to dominate," explains Kentucky Climate Center. In regional folklore, Blackberry Winter refers to this predictable weather event.
Native to U.S. Department of Agriculture plant hardiness zones 7 through 9 where temperatures generally stay above 10 degrees Fahrenheit, blackberries grow in well-draining, acidic soil in full sun. While the root system is perennial, blackberry canes are biennial, with two kinds of canes: primocanes, the first year canes, and floricanes, second year, flowering and fruiting canes.
Cold Weather Susceptibility
Resembling their cousin, the wild rose, blackberry blossoms range in color from white to pale pink, and grow in clusters, with a "king" or main flower surrounded by subordinate flowers. Although the canes are winter hardy, buds and blossoms are susceptible to frost or freeze during a blackberry-winter cold spell. Many variables affect the amount of damage from frost during a blackberry winter, including the stage of flowering. Closed buds will be the least damaged, whereas fully open flowers sustain more injury than buds or popcorn-stage flowers.
Rescue From Blackberry Winter
Early season blackberries, such as 'Arapaho' (Rubus 'Arapaho') are more susceptible to frost damage than varieties that flower later, such as 'Navaho' (Rubus 'Navaho'), 'Chester (Rubus 'Chester') and 'Triple Crown' (Rubus 'Triple Crown'). If frost does injure your blackberries, you can still harvest a crop of berries. Take measures depending upon the degree of frost damage. If only the main bud, which opens first, is blackened, prune it out, leaving 1/4 inch of stem to avoid damaging secondary buds. If secondary buds are also damaged, remove all the dead flowers and their canes. Removal of damaged flowers and buds allows the plant to send energy to the remaining viable canes and buds.
Frost-injured blackberry plants with dead and damaged tissue are more susceptible to fungal diseases than non-injured plants. Begin fungicide use at bud break and continue throughout the season according to the product label directions. Use two or more types of fungicide, for alternate applications, for broad spectrum control of blackberry fungal and rust diseases.