Stephen, I believe that spring, summer is fine.....right now I am starting some air layers, believe it takes 2-3 months for good roots. Its sunny here until Dec.15th....I am in Sunny California. I started these air layers to save some varieties that got infected with RKN in the pots. Looks like I will do OK.....On your area the winter is going to come upon you fast. You can start an air-layer with skimming a little of the bark and if it roots hooray. If it does not root well, then take it off later. You will get better opinions from people in your same climate.