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Early pruning: good or bad?

Does early pruning decrease cold hardiness?

David V. Peterson, of New York State's Finger Lakes grape extension program, posed the question particularly for cold-climate vineyardists. His conclusions follow:

Many sources in the literature have recommended waiting until the danger of severe winter injury is past before pruning. A major advantage is that the grower can determine the extent of winter damage and compensate by leaving extra nodes when pruning. Waiting until late winter is not practical for large growers on the entire acreage. Therefore, the hardiest cultivars are generally pruned first, leaving those that are less cold-hardy until the end of the winter.

Another possible advantage to late winter pruning is related to the effect of pruning on cold hardiness. Several studies suggest that pruning may cause a reduction in hardiness of grapevines. The research on this subject has produced some inconsistent results, however.

Water content of grapevine tissues (as well as other plants) is correlated with the degree of hardiness. As the water content in the buds, for example, decreases, hardiness generally increases. The general relationship of water content and cold hardiness is supported by a study with Concord in Michigan, but it also was found that a change in water content in the buds was not always associated with a change in hardiness. Most studies indicate that pruning has little but sometimes inconsistent effects on water content in buds.

Artificial freezing tests on both native and vinifera species have generally not supported the idea that pruned vines are less hardy than unpruned vines, although pruning did delay acclimation (development of hardiness) of Baco noir in one study. These vines, however, did eventually achieve the same level of hardiness as the unpruned vines.

Even though a loss of hardiness with pruning has not been documented by the artificial freezing tests, field studies as well as grower observations have often shown greater levels of cold injury on vines pruned early in the season. The importance of the possible loss of hardiness that is associated with early pruning is controversial. The lack of evidence in the artificial freezing trials may be related to the timing of when the hardiness was tested. As previously pointed out, acclimation may be delayed by pruning. Whether or not the pruned vines in this study would have been more severely injured than unpruned vines may have depended on when a cold spell occurred. If it occurred before acclimation of the pruned vines caught up with the unpruned vines, then greater cold injury may have resulted. If the cold spell occurred later in the season, there may have been no difference.

Regardless of the importance of any possible changes in hardiness due to pruning, late winter pruning of cold-tender cultivars is recommended since the grower can compensate for bud mortality that has occurred. Growers on sites that routinely experience severe cold injury may even wish to wait until the buds begin to break before pruning. This approach may additionally offer some protection against spring frost since unpruned vines have slightly later budbreak than pruned vines. In doing this, however, the brush must be carefully removed to avoid breaking off buds. A more practical approach for larger plantings is to leave extra nodes when pruning in the winter, and then come back after budbreak to remove any extras. How early a grower should start pruning may be more of a practical problem dependent on the labor available and the number of acres to be pruned. Vines pruned soon after leaf fall, however, are likely to be at greater risk to cold injury than those pruned later since the early pruned vines may acclimate more slowly. Pruning in December and January presumably would have less impact on any potential loss of hardiness.

Obviously, the subject is confusing and without a clearcut answer.
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Publication:Wines & Vines
Date:Mar 1, 1991
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