Factors that Influence Vine Hardiness
Many factors other than species and cultivar influence grapevine hardiness. In one study, winter hardiness varied up to 22°F (12°C) on different canes of the same vine (Howell and Shaulis, 1980). The presence of the woody covering on a mature cane, cane color, leaf exposure to the sky, and cane diameter influenced cold hardiness for "Concord". Dark colored canes of medium diameter exposed to the sun were hardier. Canes with deciduous laterals (side shoots) or small persistent laterals had the best mixture of cane and primary bud hardiness when the cane diameter was in the medium range. These observations should be kept in mind when pruning or sampling for evaluation of winter injury.
Heavy fruit load and excessive shoot growth that continues into the fall are the factors that enhance the probability and severity of cold injury. "Chelois", for example, is subject to winter injury and Gloor stated that most of the problems with this variety can probably be attributed to over-cropping (Gloor, 1983).
Site selection is extremely important when considering cold injury. A site with both adequate air and water drainage is recommended. In general, cold air pockets and excessive moisture can be associated with low areas. When judging the air drainage situation, note the area surrounding the site as well as the site itself. A dense wooded area above the planting can divert cold air away from the site, thus keeping it warmer. A wooded area below the site may impede cold air drainage away from the site, thus increasing the danger of cold injury. Drainage ways can be cut through wooded areas in order to allow the cold air to drain through.
Internal water drainage should be assessed prior to planting. To do this, dig holes a foot or so deep in representative areas on the site, fill them with water and note the amount of time they take to drain. If a substantial amount of water (about 4-6 inches) remains for more than a day, a drainage problem is indicated. If the hole is dug in dry soil, keep the hole filled with water a day before testing in order to saturate the surrounding soil. Grapevines on poorly drained soils do not harden as well as vines on well drained soils in most cases. Hardy tissue is more dehydrated than tender tissue.
Basically, cultural practices that promote healthy vines that bear a consistent crop will promote winter hardiness as well. Additional cultural modifications are discussed with the treatment of specific types of cold injury and are summarized below (Dethier and Shaulis, 1964; Jordan, Pool, Zabadal and Thomkins, 1981; Shaulis, Einset and Pack, 1968; Stergios and Howell, 1977).
- Select a site that has adequate air drainage.
- Practice delayed pruning or double pruning.
- Multiple trunk training will allow the grower replacements for winter injured trunks of tender varieties.
- If replacement trunks are not available, allow basal shoots to grow and replace injured trunks.
- Promote development of a deep, healthy root system by adequate soil preparation.
- Maintain adequate vine size with sound consistent cultural practices.
- Avoid late harvest.
- Maintain healthy foliage.
- Assure proper exposure of leaves to the sun with a suitable training system.
- Manage to promote early and complete vine maturity. Do this by allowing under-vine cover to grow at the end of the growing season. Also, avoid cultivation, nitrogen fertilization, excessive irrigation, and pruning late in the season.
- Keep cover crop height low, especially in areas where cold air tends to settle.Avoid over-cropping and under-cropping.
- Avoid defoliation of vines.