Grapevines (Vitis spp.) are hardier than most fruit crops, but they're not impervious to the wiles of Mother Nature. They are susceptible to freezing, which can ruin one season's crop or kill the vine. In most cases, the damage isn't permanent, especially if you've selected a vine that's hardy for your region. Wait a few weeks to prune the vine until new growth emerges and you can accurately access what's damaged.
Grape vines go dormant in late fall, like all deciduous vines and shrubs, but they are subject to winter damage in cold areas. This is why it's so important to choose a variety adapted to your area. American grape varieties, such as the classic "Concord" (Vitis labrusca "Concord") are adapted to cold climates and grow in U.S. Department of Agriculture plant hardiness zones 2 through 6, while European wine grapes (Vitis vinifera) are hardy only to USDA zone 6, depending on the variety. Native to the southeastern states, muscadine grapes (Vitis rotundifolia) aren't winter hardy and grow best in USDA zones 7 through 10. Grafted vines usually suffer more winter damage than those on their own rootstock. Mulching them with straw or soil can help protect them.
Regardless of their winter hardiness, all grape varieties can be damaged by late spring frosts and freezes. If a frost occurs after the new leaves and buds start to open, the leaves and canes can be damaged and the vines might produce no grapes that year. Damage can occur when temperatures fall below 32 degrees Fahrenheit.
Preventing Frost Damage
If you've planted a variety adapted to your area, it will likely survive winter temperatures, but late frosts are another story. To minimize damage, especially if you live in a cold climate prone to late spring frosts, plant grape vines on a slight slope, if possible. Cold air pools in lower areas and valleys, so it's best to avoid those. The buds nearest the trunk typically open later than those at the ends of the canes, so waiting to prune until late spring can offer some protection. If a frost occurs, the buds on the ends of the canes might be killed, but those closer to the trunk will be saved. If you prune early, you'll probably lose all the buds. Grapes grown in containers are more prone to root damage and more likely to die during the winter because pots don't stay as warm. Move container plants to a shed or protected area.
Planting grapes in the right spot and pruning them at the right time can help prevent damage, but if you know frost is imminent, take a few extra steps to protect your vines. The vines can be covered with light blankets, floating row covers or sheets, but be careful not to knock the buds off as you work. Another trick, which vineyards use, is to run sprinklers over the vines continuously during freezing weather. As the water freezes around the developing buds, the energy needed to freeze the water creates heat. This approach sounds counterintuitive, but it can save your grapes.
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