What causes leaves to turn and drop? Is is the shorter days, the soil temps or the air temps? If it was just light then it would be the same every year. Wondering if there is anything definitive so we can plan our fall better (i.e, if its a certain temp threshold then we'll know when they start coming down). It feels like I'm 1-2 weeks behind last year.

I would think water and fertilizer would have an effect too.

This is really to answer. GOOGLE is your friend.

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I can state that I can prove definitively and easily that it's not this simple (but the article does state that). I have exactly the same species of street trees around here that everybody else does (we vary through about 12 of them as you move through the neighborhoods for variance and to keep diseases from running rampant too easily). Everybody else's are turning or dropping. Mine are still dark green and doing just fine, although one's showing signs of starting to turn.