With the way the climate has been changing, its really hard to give specific times, but all I can say is early! The earlier the better. As the low temps here begin to be consistently in the 60's, going down, seems to do well for me. In the past anyway....Who knows what our future will bring...We didn't have any winter this year, so I'm not sure what the future will hold. Sadly, I'm not feeling optimistic. All I can say is try to take advantage of your coolest temps. They seem to be fewer and farther between lately. The cooler your temps, the better!