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If you are growing grapevines in the warm commercial regions of southern Europe, then issues with overwintering grapevines are not something you would usually need to worry about. However, northern European growers with have more work to do, depending on how far north they are and the specific cultivars grown. Of course if your grapevines are grown within the protected environment of a greenhouse, you will have no choice other than to make preparation for the oncoming freezing weather - usually shutting the windows and doors and adding heat depending on the varieties grown.
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To protect the roots of grapevines in cold conditions, cover the base of the plant with approximately 8 inches of mounded soil. If you are only growing a few plants then consider covering the vines with horticultural fleece. Regions that experience freezing temperatures will require a taller mounding of soil of at least a foot or so or a further insulation of a dry mulch such as straw or shredded cornstalks. Areas that are prone to snowfall will require less work as the snow itself can insulate the vines, particularly from lower, overnight temperatures and freezing winds.
Under cold, wet conditions, mounded soil can still freeze as it is exposed to the weather. However deep ditch cultivation can prove to be a more successful in this instance. Prior to planting out the vines, dig ditches approximately 4 ft deep and 3 to 4 ft wide. The vines are planted at the bottom of the ditch and soil is added progressively as they grow until the original soil level is reached.
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Come the spring, and once the new buds begin to emerge, the grapevines can be uncovered and re-attached to their support structure. Be aware that light protection (horticultural fleece or suitable geotextile fabric) may need to be put in place to prevent damage from late frosts.
For related articles click onto the following links:
HOW TO GROW GRAPEVINES IN A GREENHOUSE
HOW TO GROW VITIS VINIFERA 'PURPUREA'
HOW TO OVERWINTER GRAPEVINES
WHAT IS GRAPESHOT?