On an average of every eight years, winter temperatures in Washington drop to a level low enough to cause severe damage to fruit trees. Almost every year phenomena occur in combination with low temperatures that cause localized damage to fruit trees. To better understand and alleviate the problem of winter injury, the research center has used several different approaches.
Various cultural practices such as manipulating nitrogen fertilizer application and irrigation practices can increase freeze resistance of fruit trees. However, any change in cultural practices that favors an increase in winter hardiness results in a less efficient tree in terms of growth and productivity.
A few guidelines that have been determined which are not detrimental to the productivity of the tree are the following:
do not apply nitrogen fertilizer from mid-June until after vegetative maturity in the early autumn
make sure soil moisture is at field capacity going into the winter
do not let the trees come under moisture stress in the summer and then apply water to break the stress before vegetative maturity
Cryoprotectant chemicals and certain manipulation of hormones will increase freeze resistance in fruit trees. The application of these is so precise and dependent on weather conditions that any variance in application or misinterpretation of the weather will result in adverse effects on the trees.
Since the ability to develop freeze resistance is a genetically inherited trait in apple trees, it has been advantageous to determine the winter hardiness of different varieties. A tender variety, Golden Delicious, and a very hardy variety, Antonovka, have been used as a reference in characterizing the hardiness of different varieties. Golden Delicious and Antonovka gain cold resistance when the trees stop growing in the summer although Antonovka precedes Golden Delicious by three weeks. They retain this immediate hardiness until vegetative maturity is reached in early autumn. Both varieties again gain immediate additional cold resistance, but Golden Delicious at this point lags behind Antonovka by 5°C. After the first solid frost, both varieties begin acclimating, with the rate being dependent on temperature. At the end of winter dormancy, which is about mid-January, both varieties begin to deacclimate again, the rate depending on temperature. Through the whole process, Golden Delicious is approximately 5°C less resistant than Antonovka.
Other apple varieties
Red Delicious follows the Golden Delicious curve quite closely except it is 1 or 2°C more resistant. Gala lags behind Golden Delicious in acclimating in the autumn but becomes equal in resistance about Christmas. Fuji follows the Antonovka curve very closely except it is about 2°C less resistant in the autumn but catches up with Antonovka about mid-December. Braeburn acclimates very slowly, even compared to Golden Delicious and Gala. There are only a few days between the time the tree stops growing and vegetative maturity. It deacclimates very easily even during the winter dormant period and deacclimates very rapidly in the spring.
Pears respond to nurture much earlier than most apple varieties. Anjou, Bartlett and Bosc pears end their growth flush in mid- to late June and immediately acclimate to -13°C. All three varieties reach vegetative maturity approximately the middle of August and gain another 5°C hardiness. Acclimation continues when temperatures of -1°C or lower are experienced. Anjou and Bartlett respond to low temperature more than Bosc and therefore attain more hardiness.
Text and data for graph were provided by Dr. Del Ketchie, retired WSU-TFREC horticulturalist