The capacity of apple trees to survive low temperatures is influenced by the scion cultivar and the rootstock. Rootstocks vary in their low temperature tolerance. Some rootstocks cease growth earlier than others, which results in high levels of resistance to early winter freezes. In the fall, with shorter day lengths and cooler temperatures, trees start to acclimate to low temperatures at the outer tips of branches. This process of acclimation (the ability to adapt to colder temperatures) progresses toward the inner part of the tree and downward, with the trunk area just above the soil surface, the last tissue to acclimate or “harden off.” For this reason, the rootstock tissue just above the soil surface on young trees is susceptible to low temperature injury or tree death in late fall.
Rootstocks also differ in their tolerance to low temperatures at various times during the dormant period. For example, in Missouri, M.7 rootstock induces early vegetative maturity in the fall and has good resistance to injury with a sudden drop from 70°F to < 20°F in November. However, M. 7 and M.9 rootstocks have poor midwinter hardiness as compared to B.9, P.22, P.2, Antonovka 313, and most Geneva rootstocks. Unfortunately, rootstocks that survive any type of low temperature event during the fall or winter months are often judged “cold hardy.” In any case, the temperatures recorded about two weeks preceding a freeze event and the number of degrees to which the temperature drops often determine tree survival because trees can acclimate and de-acclimate in response to fluctuating temperatures.
North America has been divided into hardiness zones. These zones are based on the average annual minimum temperatures recorded between 1974 to 1986. You can use the USDA Plant Hardiness Zone Map (http://www.usna.usda.gov/Hardzone/) as a guide to determine the minimum winter temperature in your area. Often plant recommendations are listed as “hardy to zone 4,” which means that if your site is located at that zone or one numbered higher, the plant will likely survive the winter with proper care.
Dr. Emily Hoover, University of Minnesota
Dr. Michele Warmund, University of Missouri