Double Pruning to Avoid Spring Freeze Injury in Grapes – Facts for Fancy Fruit

Double Pruning to Avoid Spring Freeze Injury in Grapes

Spring freeze damage can be a significant economic problem for Midwest grape growers. Widespread damage occurred in 2007 and 2012 when warm temperatures in March were followed by freezing temperatures in April. Obviously this year we have had a warm February and March so it is very possible that we will have early bud break and the potential for freeze damage. The average date of last spring frost for central Indiana is about May 1. Grapes pruned recently are bleeding, meaning that bud swell is beginning to occur and bud break will not be far behind.

Varieties differ considerably in the amount of heat units (growing degree days -base 50˚ F) needed to cause bud break. Exact figures are not well established, but for early grapes such as Foch, Marquette and Brianna, I think 100-150 GDDs is sufficient to lead to bud break. For late varieties such as Vidal and Chambourcin, it is likely 250 or more. We normally start counting GDDs on April 1, but by then, we are already at bud break in the south. So it makes sense to consider those that occur earlier. In Indiana, we normally only accumulate about 25-30 GDDs by April 1. I checked this week and we have accumulated 150 in southeast Indiana, about 50 in central Indiana, and 30 in northern Indiana. There are a number of web sites where growers can find this information. I regularly use the Midwest Regional Climate Center and the GDD Tracker out of Michigan State.

Fortunately growers have options to manage freeze risk. A technique called long pruning or double pruning helps avoid spring frost and freeze damage, especially on varieties that tend to bud out early. The procedure utilizes the apical dominance of buds on a cane. The first buds to begin growing are those on the tip of a cane, while buds closer to the cordon begin growth later. Additionally, if more buds are left on a vine, the rate of bud development for all buds will be delayed.

To perform long pruning, select canes to be used for fruiting spurs during the normal pruning practice, but leave those canes long, with 10- 15 more buds than desired. Spurs are normally pruned to 3 to 4 nodes for fruiting, but if they are not cut back, then the extra buds will help delay the development of the desired basal 3 to 4 buds, which helps avoid frost injury. After the date of the last probable spring freeze has passed, the canes are shortened to the desired length to properly adjust the shoot number for the vine. Growth of the basal buds can be delayed as much as two weeks if weather conditions are favorable.

Another advantage of double pruning is that if frost damage occurs to primary shoots, the large number of buds retained will result in many secondary shoots. Even though secondary shoots are not as fruitful as primaries, the large number can result in near normal yields. This was the case in our research plots in 2012 and we were able to produce a full crop on most varieties, despite essentially complete loss of all primary shoots. While this procedure may require an extra trip through the vineyard, it can mean the difference between a full crop and little or no crop

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