This year, however, a long, warm spell in late September seems to have fooled a lot of trees into thinking that fall wasn't on its way. Even by the beginning of October, most trees were still wearing plenty of green. So does this mean that it isn't daylength that causes leaves to drop, but the cold weather that usually begins in September?
Many trees that we grow are not native to our climate. These trees may not be as dependant on daylength as our native trees are, because the variations in daylength are different in their native latitude. They might be able to grow well into the fall, if other conditions are right. These trees may generally use temperature cues to begin dormancy in cold climates, rather than light, while native trees are more likely to lose their leaves at the "correct" time, regardless of the temperature. This is probably why, in Edmonton this year, you could see a tree with all its green leaves growing right beside a tree with none at all. Of course, with all the snow and freezing weather we've had, it probably won't be long before all the leaves are down.