One of the nicest things about living in northern climes is the ever-changing seasons. And, for a short week or so, nature puts on one of its most spectacular displays as native trees and shrubs finish out the growing season in a potential spectacular display of fall colors.

"Potential" is the key word. There are all sorts of indicators on whether the autumn will be colorful or not, but it's not an absolute guarantee. One part of the state or Midwest may be colorful, while another part is not.

Both the plant's genetic factors and the environment, according to David Robson, University of Illinois Extension horticulture educator, Springfield Center, control fall color.

Carotene and xanthophyll are yellow pigments produced in foliage all year–along with chlorophyll, the green pigment. In autumn, when short days and cool temperatures slow down the production of chlorophyll and the remaining chlorophyll breaks down and disappears, the yellow pigments which have been masked by chlorophyll show up.

These pigments give the ginkgo its clear yellow color. Redbud, larch, hickory, birch and witch hazel turn hues of yellow and gold. Some plants produce anthocyanins (red and purple pigments) that may mask the yellow pigments. Some maples, dogwood, black tupelo, oaks and winged euonymus seem to be on fire with red and purple.

Anthocyanin production increases with increased sugars in the leaves. A fall season with sunny days and cool nights increases sugar content of the leaves and intensifies fall reds.

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This also explains the two-tone effect on green ash, which exhibit yellow on the inside leaves and purple on the outside leaves where they are exposed to sunlight, and on the Amelanchier, which may be red on top branches and yellow on bottom branches.

Tannins that accumulate as the chlorophyll disappears cause the tans and browns of oaks.

The amount of rainfall throughout the growing season also has an effect. Plants that are stressed by droughts usually don't produce lots of sugars, which mean the brilliant reds aren't potentially there.

And then there's cold temperatures–how much cold and when does it come? Lingering cool days and nights bring more color. A hard frost or freeze will zap the leaf and any potential color.

Since most plants are individuals having come from seeds, plants of the same species have different genetic programming for producing pigments. So, not all sugar maples might turn the same color, for instance.

As a result, nurseries often select and name varieties with exceptional color. These are propagated vegetatively to insure each plant will perform as expected.

Fall color starts in September with poison ivy, ashes and walnuts and ends in November with the larches and weeping willows. Frost and freezing temperatures will stop the coloration process and blacken the leaves.