I’ve had some growers inquire and there has been general chatter lately about the risk of pruning apples and grapes now, given the warm weather we’ve been having. My advice will apply equally to apple and grape growers, with lessening degrees of caution: hold off as long as you can.
The danger is two-fold. Plants may begin growth if warm weather continues, since most some (i.e., grapes, but not necessarily apples) may have seen the required chill hours needed to break dormancy. Continued, extended warm weather could cause buds to swell or break which would be very susceptible to winter cold. Even if buds stay closed, cold hardiness acclimation is a two-step process initiated by shortening day length (beginning back around August) and gradually decreasing temperatures. It is that last part that likely has perennial plants like apples and grapes in a less than fully acclimated state right now. Past research on pruning apples, grapes, and peaches in fall, and meteorologically, we’re still in late fall, are less hardy to deep cold in mid-winter. I know many growers are going to be ready to get out in the field after the holiday, but if at all possible, wait as long as possible, or at least after a (hopefully gentle) cold snap. Gradually decreasing temperatures will allow plants to harden off better to withstand any cold that may come. Fresh pruning wounds not only are more cold-tender, but also may stimulate bud break if conditions are right.
Thankfully the 10-14 day forecast is calling for just what we need: daytime temperatures (after the Christmas eve warmup) dropping from the 40’s through the 20s and 30s and nighttime temperatures in the teens and twenties. It’s best to leave your trees and vines be until at least the first week in January. I wouldn’t touch a grapevine right now. On apples, the usual advise is worth following: start on your largest semi-dwarf trees and trees with lighter crop load and good nutrition/management in the past season, then move on to smaller, younger, ad more crop-stressed trees.
Enjoy the holidays. We’ll get through this.