Researchers in Maine, USA, are concerned that low temperatures are injuring apple rootstocks. Consequently, they set up a series of experiments to test the cold-tolerance of ungrafted M26 rootstocks (believed to be cold-tolerant) and various 'Geneva(R)' ones.
In some experiments, whole trees were subjected to controlled low temperatures for one hour, while in others, the roots were insulated from the cold and the cold-hardiness of the trunks was assessed. In yet others, it appears that the roots alone were treated.
In general, M26 trees were tolerant of air temperatures down to -8 degsC, but following exposure to temperatures of -12 degsC or lower, shoot growth following pruning was retarded. Many of the roots of M26 trees could tolerate temperatures down to -14 degsC but trees died if the whole tree was subjected to -16 degsC or less.
Most of the 'Geneva(R)' rootstocks were no more tolerant of cold than M26 but many plants of the G935 rootstock survived having the whole tree subjected to -16 degsC, although none survived exposure to -21 degsC. Grafting apples to the G935 rootstock may become necessary if winter temperatures regularly fall to levels that harm the M26 rootstock.
Dr Ken Cockshull, Associate Fellow, Warwick Crop Centre, University of Warwick
Cold Temperature Tolerance of Trunk & Root Tissues in Oneor Two-Year-Old Apple Rootstocks by Moran, Sun, Geng, Zhang and Fazio (2011). HortScience 46 (11): 1460-1464. The author's abstract can now be seen in full at http://hortsci.ashspublications.org but ISHS members can view HortScience at www.ishs.org.