Cool nights, warm days and brilliant colored leaves - fall is upon us here in the Lehigh Valley.

The fall color of many trees and shrubs in the northeastern United States provides much more color to the landscape than spring flowers. Its effective period is also generally longer.

This characteristic of plants should receive more consideration than it now does in the landscape planning process. Plants with vivid fall color should be placed in prominent locations that can be seen both from the street and the inside of our home or buildings. Plants should be grouped by fall color whenever possible to provide the mass impact effect.

Background colors should be evaluated prior to plant selection. For example, the effect of a small tree or shrub with red fall color will be greatly lessened if it is planted in front of a building with red siding. In this situation, a plant with good yellow or orange color would be better.

Since fall color can have such a dramatic effect on a landscape, it is important to select plants that provide the desired color. This can be tricky because some plants like red and sugar maples and sweetgum exhibit a lot of variation in fall color.

Some trees are yellow each year; others may be red or orange. The only way you can be sure of getting the fall color you want from these trees is to select them in the fall when they are showing their color.

The fall color of many plants, particularly those that are vegetatively-propagated, is much more predictable. This information can be obtained from reputable garden centers or reference books.

The process by which yellow fall color develops is quite different than the way red or orange color develops. Yellow color is due to two yellow pigments carotene and xanthophyll. They are always in the leaves, but their presence is masked by the dominant green of the chlorophyll pigment.

In the fall when chlorophyll manufacture slows, then stops in the plant, the yellow pigments are exposed. This occurs regardless of the weather conditions.

The red in leaves is caused by the pigment called anthocyanin. Two factors are necessary for its maximum production in the leaves. The first is light. There may be warm, bright sunny days in the fall during which time the leaves naturally manufacture a great deal of sugar.

Such days must be followed by cool nights with temperatures below 45 degrees Fahrenheit. Under these conditions, there is little translocation of sugars from the leaves. The accumulation of these sugars in the leaves results in the manufacture of the red anthocyanin pigment.

Because the anthocyanin production depends on two environmental factors, red fall color may vary from year to year, plant to plant, and even on the same plant in the same year. Plants located where they receive full sunlight, especially in late afternoons during early fall, should be expected to show outstanding color if weather conditions are favorable.

On the other hand, if a plant is growing in shade where it receives no direct sunlight, it cannot be expected to produce good red fall color. In extreme cases, some plants may develop good red fall color only on the side exposed to the sun. The shaded side may be green or yellow.

Even evergreens turn color in the fall, but not the yellow or red of deciduous trees. If your evergreens are not all green this fall, don't panic. The color change is not the result of a disease; it's nature. In the fall, evergreens turn color and drop their needles in much the same way hardwoods do. With evergreens, the change is more subtle and few species drop all their needles at once.

The calendar for discarding needles varies with the species. Spruces keep their needles for about nine years, while pines hold onto needles for only two or three years. This means that every autumn pines drop about half their needles.

So, if you evergreen starts shedding, relax. At this time of year it's probably just nature doing some maintenance work.

For answers to your garden questions, call the Lehigh County Cooperative Extension Office, 610-391-9840, or Northampton County Cooperative Extension Office, 610-746-1970, and ask to speak with a Master Gardener. Volunteers staff phone lines several days a week, Monday - Friday.

Growing Green is contributed by Lehigh County Cooperative Extension Office Staff and Master Gardeners.