Spring freeze damage can be a significant economic problem for Midwest grape growers. Widespread damage occurred in 2007, 2012 and 2017 when warm temperatures in February and March were followed by freezing temperatures. Obviously this year has been very cool so far so there is hopefully less risk of early budbreak and frost damage. But, considering that we had fairly significant winter injury in the northern half of the state, we need to do all we can to avoid further damage.
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 frost and 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 seasonal 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 when determining date of bud break. 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 over 1,000 chilling hours by late February this year, so vines are 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 about 100-130 in southern Indiana, about 50-60 in central Indiana, and 20-40 in northern Indiana. Some of these GDDs may have occurred before chilling was satisfied.
Fortunately growers have some options to manage freeze risk and adjust for winter injury. 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. Since many northern growers have 50% or more winter injury this year, they may already be pruning minimally (see related article). But for others, this technique may offer a method or risk management.
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. For vines with winter injury, final decision on pruning can be done once fruitfulness of shoots can be determined.
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. While this procedure requires more labor, it can mean the difference between a full crop and little or no crop.