Every year as the holidays approach, I get inquiries as to whether it is O.K. to start pruning grape vines, fruit trees and other fruit crops. My feeling is that we should hold off until after the first of the year or later. My general recommendation is to wait as late in the dormant period as possible before pruning, yet early enough to get it all done, to lessen the potential for cold injury and to be able to react to damage done by cold events earlier in the winter.
This year, we have been experiencing unusually warm weather for an extended period. This is expected to continue for a while longer, with record high temperatures predicted for Christmas day. When it finally does turn cold, it could happen rather quickly. Temperature drops in the range of 50 to 60 degrees F within a 24 to 36 hour period are not unusual. In January, 2014, we had three such events and, as a result, cold damage was substantial.
We may not have had enough consistently cold weather to enable plants to enter dormancy, which will impact the plant’s ability to tolerate cold temperatures. Even if plants are fully dormant, they will lose hardiness with warm temperatures like we have been experiencing (deacclimation). A dormant plant will reacclimate (regain cold hardiness) when temperatures drop. However, the rate of reacclimation is much slower than the rate of deacclimation and a sudden cold event does not allow for plants to regain sufficient hardiness to avoid damage. If a plant has not gone fully dormant, it still can reach that stage, but it is not as capable of tolerating cold temperatures as a dormant plant.
In Tennessee we frequently see winter injury in fruit crops (and in other deciduous plants) at temperatures well above those regarded as being the critical point for cold injury for that plant. For example, if a nursery catalog states that a “hardy” variety will withstand temperatures down to -20 degrees F, they are assuming that the plant is healthy, that it is fully dormant and that the temperatures leading up to the cold event are consistently low and that the duration of the cold event is relatively short. With severe temperature fluctuations, that same plant may be damaged at 10 or 15 degrees F or higher, depending on the circumstances.
With the above in mind, growers are well-advised to delay the start of pruning and to consider the weather patterns experienced prior to the time they prune, as modifications in the type and severity of pruning may be needed. For most of our fruit crops, fruit buds were initiated this past summer. They are not as cold tolerant as leaf buds in winter and spring. In some cases, no apparent cold damage may be experienced by the leaf buds, but the level of fruit bud mortality may be severe. Knowing this prior to pruning will allow growers to modify their pruning practices somewhat to leave more fruit buds than they normally would have left. While pruning is an effective way to thin down a fruit crop to a more manageable size, if the crop potential is already short, further thinning is not desirable.
W. Alan Bruhin