Why Do My Thornless Blackberries Freeze Off to the Ground?

Blackberry Types

• There are two physical types of blackberries, those with "erect" fruit-bearing canes and those with a more trailing habit. Trailing types, known as dewberries in the eastern United States, include all thornless cultivars. Erect types, with stiffer, arching canes, are generally more cold hardy. The most probable reason for thornless blackberries freezing all the way to the ground is because they simply could not survive the temperature drop.

Blackberry Canes

• Blackberry plants are perennial, meaning that roots continue to live and generate new fruiting canes for many years. The canes are biennial, living for 2 years. First-year canes or primocanes generally produce fruit the following year, when they are known as floricanes. These fruiting canes die after they bear fruit and need to be removed after harvest. Any cane that bears fruit will die back to the ground even if you don't remove it, a process that may coincide with the year's first freeze.

Fruiting Canes

• When establishing a new blackberry bed, it's essential to cut back the tops of transplants, so that all new growth arises from primary buds just below the soil line. All growth above that point is starting its second year and ready to produce flowers and fruit. If this old top growth isn't removed at planting, the plant will exert immense energy fruiting and also producing new canes before plant roots have been established--stress that might cause cane loss, if not loss of the entire plant.

Mulching

• Trailing blackberries can be grown in colder areas if you leave the canes on the ground--they tend to grow flat on the ground unless trained onto a trellis--and mulch them heavily in winter. The normal practice otherwise is to prune and train these primocanes--next year's floricanes--onto the trellis at the end of harvest, in preparation for next year's growth. But for more vulnerable thornless blackberries, such above-ground winter exposure is fatal in cold climates.

Freeze-Proofing

• A number of thornless blackberry nurseries promote and sell a product called FreezePruf, an environmentally safe spray-on "plant antifreeze" developed by botanists that is said to improve a plant's cold tolerance by 2 to 9 degrees F, depending on the plant and the intensity of the freeze. This type of product used in conjunction with other preventative measures could help boost thornless blackberry survival rates in borderline climate zones.