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Why are fall leaves bright red in some years and dull colors in other years?

Two factors are necessary for the production of red autumn leaves. First, there must be warm, bright, sunny days, during which leaves manufacture sugar. Second, the warm days must be followed by cool nights with temperatures below 45° Fahrenheit. During the cool nights that follow warm days, sugar and other materials become trapped in the leaves. This results in the manufacture of the red pigment anthocyanin.

Warm, cloudy days inhibit the formation of bright colors. With decreased sunlight, sugar production is decreased. This small amount of sugar is then transported back to the trunk and roots where it has no color effect.

Source: Wyman, Donald. Woman's Gardening Encyclopedia, 2nd ed., p. 99.

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