Why Trees Lose Their Leaves in the North

It is generally said that deciduous trees lose their leaves because of shortening daylengths, or, to be more accurate, lengthening nights. As the nights get longer, it signals to the trees to begin preparing for dormancy. The green chlorophyll fades from the leaves, leaving behind the yellows, reds and oranges that weren't visible before, and that signal fall to us. The leaves then fall, and the dormant tree is prepared for winter.

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.


  1. Hi Far North Garden,

    I disagree with your hypothesis for a number of reasons, but here's a couple:

    1. Although below freezing night temperatures are common this time of year, an extended period of days in the first half of Octbober when temperatures do not rise above freezing is very rare. Based on the Environment Canada temperature history for Edmonton (which goes back to 1937), this has only happened twice before - 1959, 1957. Even in the second half of October long periods of below freezing days are not the norm. I think the early cold simply caught the trees napping.

    2. Unlike most of the boulevard trees, the green ash in front of my house dropped its leaves well before the cold snap. I think it is less tolerant of drought than the elms and burr oak, not more in tune to cold fronts.

    3. My wife and I were out at our land near Elk Island on Sunday and the aspen have as many frozen green leaves as many of the City trees. Looks like they expected a better Fall too.



  2. Dave,
    I agree with you that trees will lose leaves for all sorts of reasons, including drought. The thought I was trying to convey in my post was that daylength is not, as is sometimes said, the only deciding factor in leaf drop.
    Anyway, the trees are probably smarter than me, the forecast is calling for better weather towards the weekend. Maybe we'll get some fall colours after all.


  3. Hi Cassandra,

    Well, I agree with you that day length is not the only deciding factor in leaf drop, and plants can certainly vary their response to take advantage of good or bad conditions. Too bad they got caught being overly optimistic this Fall - I suspect they will lose a lot of nutrients with every green leaf that falls off. I'm wondering what my Evans Cherries will do. They've put on a very nice, mostly red display the last couple of autumns, but look pretty forelorn at the moment. On the otherhand, the rosemarie and sage looked pretty sad too, but when I clipped some for seasoning on the weekend, they actually seemed to be in good shape.


Comments are somewhat moderated.