Fellow blogger Jodi Torpey asked an excellent question about the speed with which her ash tree dropped its leaves this weekend. Reading her post on this in her blog, I couldn’t help but comment to her that my black walnut had done the same thing – dropped all of its leaves in a great, big circle all around its base.
Oddly, my burr oak still holds its leaves despite the frigid temps, something Jodi has noticed going on all over the area; some trees holding onto their canopies, others shedding them fast. Her question “is it normal for some trees to experience sudden leaf drop after subfreezing temps?” had me diving into the research to find the answer.
Deciduous trees lose their leaves in fall by ‘abscission’.
Taking their cue from changes in temperature, length of daylight, and natural aging, trees form an abscission layer between leaf and branch with cells that get larger and harder, shutting off water flow to the leaf (also shutting off chlorophyll, which is part of the reason leaves change color in fall).
Eventually the leaf is completely separated from the tree and falls gently to earth. Every species of tree will do this on its own schedule; some early, others late.
When temperatures drop below freezing, the abscission layer hardens more rapidly, cutting off the leaf’s connection to the tree. Any weight, such as snow or movement from wind, will make these leave drop from the tree. Some of our trees were at the right stage in their fall abscission to lose every leaf to the freeze all at once.
The good news is that this shouldn’t harm them. Yes, left on their own more of the potassium and phosphorous in the leaves would have been absorbed back into the plant, but the trees can take a small loss of these nutrients if they were healthy.
A different result from this cold snap will have the opposite effect: when we get a hard freeze in early October, some trees haven’t had time to develop the abscission layer. On these trees the leaves freeze and remain attached to the branches.
Eventually wind and snow will force those leaves to drop, but in the meantime there is a danger of branch breaking if we get a wet, heavy snow, where the added weight held by the clinging leaves can be a problem. Keep an eye on your branches when we get those wet snows, and be prepared to prune should damage occur.