GRAND JUNCTION, Colo. — There's a whole complicated science and biology as to why leaves change colors. But leave it to Susan Rose, horticulture educator at the Colorado State University Extension at Mesa County Fairgrounds, to give us an explanation that we can all understand.

FP: How do trees, bushes and shrubs know when to begin getting ready for winter?

ROSE: They tell by day length - or more accurately, by night length. They actually begin the hardening off process in July.

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FP: What causes leaves to change colors?

ROSE: The “fall” colors are actually there all along, but they are masked by the green chlorophyll. As the chlorophyll stops being produced (in response to night length), the other colors can shine through.

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FP: Why do maple leaves turn red, yet other leaves turn gold and orange?

ROSE: The two primary other color agents — they are actually anti-oxidants and are protective in plants just as they are in people — are carotinoids and antho-cyanins. The more carotinoids, the more yellow; the more antho-cyanins, the more red. It is determined by the plant's genetics.

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FP: Does it seem like we are have a “late” turning of colors this year?

ROSE: Seems to me we're right on schedule. The colors in GJ typically peak in mid-October. The lateness in the mountains may have had something to do with the really warm late summer and early fall, but I'm afraid that's just a guess.

Also in the mountains, the protracted, long, wet, cool spring set up the perfect conditions for marssonina leaf spot, a common and relatively harmless disease of aspens, which will turn the leaves brown before they have a chance to turn gold.