Discovery of blackheart in the sapwood, without any bark or 

 cambium injury would indicate severe midwinter cold. Many New 

 England Baldwin trees have blackheart as a result of the extremely 

 low temperatures of 1933-34. Finding injured cambium or bark but 

 normal sapwood points to fall or early winter cold. Injury to both 

 cambium and sapwood could result from unusually low temperature at 

 any time in trees that were not well acclimated. Southwest trunk 

 injury could also occur at any time in the winter months. 



In summary, the factors that seem to have been most involved 

 in the 1975-76 winter injury seen in Southern New England were: 

 (1) a mild fall encouraged late growth activity, and discouraged 

 acclimation; (2) warm temperature in mid-December may have deaccli- 

 mated the tissue just prior to a period of low temperatures; (3) 

 a very heavy 1975 crop load on some trees limited the development 

 of cold hardiness; (4) pruning of trees prior to occurrence of 

 critical temperatures reduced the trees' ability to withstand cold. 



From the experience of 1975-76, and other winters, it should be 

 reasonable to conclude that pruning before late February entails risk 

 any year, but when conditions have not been favorable for development 

 of cold acclimation by late December, early pruning is especially 

 hazardous. Growers should learn to recognize seasons in which early 

 pruning must be avoided. A suggested guide, until something better 

 is developed, might be: 



(1) Don't prune before Christmas, because mild temperatures 

 are likely to occur before then that can deacclimate the 

 trees. 



(2) Keep a record of minimum and maximum temperatures, begin- 

 ning November 1. Delay pruning until February if there 

 have been 2 5 days with minimums of 28°F or lower by 

 December 25. 



(3) Don't prune within 10 days following maximums of 55°F or 

 more that occur before Christmas. 



(4) Leave for late February and March, the pruning of all 

 trees that bore especially heavy crops and those that 

 were weak or had reduced leaf surface for any reason. 



Use of a guide such as this will not eliminate the possibility 

 of injury due to unusual temperature extremes, but it should minimize 

 the risk of cold injury that is associated with pruning. 



