Winter injury is one of the limiting factors for blueberry growing in Minnesota. Although blueberry cultivars released by the University of Minnesota are all rated for Zone 3, there can be significant winter injury following relatively mild winters, especially winters with little or no snowfall. Young plants usually have significantly more damage than plants that have been in the ground for ten or more years.
Branch tips are the part of the blueberry plant most susceptible to winter injury. Mild winter injury results in only a few scattered dead branch tips. Since blueberries form flower buds at the tips of the branch, mild or moderate winter injury can reduce the amount of fruit produced the following summer. With severe winter injury, the plants will die down to within an inch of the soil. Vigorous stems that grew one or two feet during the summer show more injury than slow growing stems. Sometimes one or two blueberry branches grow late into the fall, and still produce new leaves into October. Branches that are still producing new leaves in October do not harden off properly and will be damaged during the winter.
Branches showing winter injury should be removed or cut back to living tissue. Wait until the leaves have emerged in late spring before pruning the plants to avoid cutting off living parts of the branch. In blueberries, branches that have brown bark are often still alive.
Avoid fertilizing blueberry plants with nitrogen after the middle of June. Branches are more likely to keep growing into the fall if the plants receive nitrogen in late summer. Make sure that blueberry plants have sufficient water when they go dormant in November either with irrigation or rainfall. Winter injury can also be minimized by planting blueberries in areas that are protected from winds during the winter. Blueberries grow particularly well next to conifer windbreaks. Always plant cultivars that are suitable for Minnesota winters.
Young plants can be protected from winter injury with mulch, snow or a breathable row cover. Row covers should be put over the berries in November and removed shortly after the snow melts in early spring. Be careful when using straw mulch because straw mulch attracts mice, which then can feed on the blueberry bark.