Avoiding 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 very warm February, but more normal March. So far it looks like bud swell is on track for a normal year. Most growers are reporting that their grapes are still dormant, or mostly so. Grapes pruned recently are bleeding, meaning that activity is beginning and bud break will not be far behind. There is still the potential for freeze damage. The average date of last spring frost for central Indiana is about May 1.  At early stages of swell, buds can tolerate temperatures in the single digits or low teens, but as they progress they rapidly loose hardiness. A bud at full swell can be damaged by temperatures about 20˚F. Once bud break occurs, damage happens at 28˚F.

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 120-150 GDDs is sufficient to lead to bud break. For late varieties such as Vidal and Chambourcin, it is likely 150-180 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 accumulate only 25-30 GDDs by April 1. We also have to take into account the accumulation of chilling hours. Plants require a certain amount of chilling (temperatures between 32 and 45˚F) to satisfy their “rest” period, or dormancy, before they will respond to warm temperatures. So warm temperatures in January or February don’t necessary mean cause for alarm as chilling requirement may not have been satisfied. Grapes don’t need as many chilling hours as some fruit crops, but probably at least 1,000. We accumulated 900-1,000 chilling hours in mid February this year. So vines have been ready to respond to the warmer temperatures. If we look at GDDs since Jan 1, it gives us a good idea of where we are in terms of heat units needed to cause bud break. I checked this week and we have accumulated 250 in southern Indiana, about 100 in central Indiana, and 50-75 in northern Indiana. Some of those likely occurred before chilling was satisfied.

Fortunately growers have some 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. Dr. Imed Dami presented his research on this topic at the Indiana Horticultural Congress this year as well. While this procedure requires more labor, it can mean the difference between a full crop and little or no crop.

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