Question: My husband and I were discussing what caused the trees to lose their leaves. We decided it wasn’t the decreasing temperatures because trees lose their leaves in colder regions (and warmer ones) at about the same time. Do you know why they all lose their leaves at about the same time?

Answer: Trees drop their leaves in response to decreasing day length. Day length also tells them when it is time to develop new leaves in the spring. In fact, day length controls other plant processes, notably flowering and fruit production. I mentioned in a previous column that greenhouse growers shade poinsettias so they will be in full bloom by Thanksgiving when people want to buy them. Poinsettias bloom naturally at Christmas time, but not a month before that.

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