Fall colors won't reveal their timeless mystery

In the fall when days get shorter, the diminishing amount of sunlight triggers a process that turns summer-green leaves into a kaleidoscope of color. While the process by which leaf colors change is understood, the reasons why they do are not.

Some scientists believe fall colors help plants deter pests; others believe it protects leaves from drought- or season-ending, frost-related damage; while still more think the color-producing chemicals of decomposed leaves interact with soil and help to prevent seeds stored there from growing and competing with the plant for water and nutrients.

No matter what "survival of the fittest" benefit fall color provides, it helps to soften disappointment that another season has past.

In spring and summer, leaves are green because of chlorophyll, a chemical needed for photosynthesis - the process plants use to turn sunlight, carbon dioxide and water into carbohydrate-energy they use to grow. Plants use some of this energy right away. Most of the energy is transported from leaves to branches, roots and buds, and stored there until the following spring when awakening plants need it to grow.

In fall, shorter days signal deciduous plants to begin preparing to shed their leaves. A protective barrier slowly forms between each leaf's stem and the branch on which it grows. The barrier causes chlorophyll production to stop and eventually blocks carbohydrates from exiting leaves. That is when the color show begins.

Without chlorophyll's green there to mask them, the yellow and orange pigments that have been there all along become visible. At the same time, carbohydrates trapped behind the barrier produce new pigments in shades of red and purple. Which of these colors predominate -yellow, orange, red, purple - vary by species, by individual plants within a species, and even leaf by leaf. When these colors fade over time or after a frost, the only pigment left is brown.

The best and longest lasting fall color occurs after a growing season that has enough moisture to protect plants from drought stress, and fall weather that is dry, cool, sunny, and has a later, first hard frost.

This month, be they fiery or subdued, enjoy all of the end-of-season colors. Each one helps us to celebrate the intricate beauty of the natural world.

Trees and shrubs

Apply fertilizer to trees or shrubs that have demonstrated a need for nutrients, e.g. stunted growth, failure to flower or leaf out, undersized fruit, off-color foliage, and recovery from disease or pest damage. A layer of organic mulch also helps to maintain soil fertility.

Continue to water newly planted trees and shrubs, especially evergreens, up until ground freezes. Milder temperatures can fool gardeners into thinking plants need less moisture in fall.

Plant many deciduous trees and shrubs when leaves show color and drop, signs the plants are becoming dormant. Water well and mulch with 2 to 4 inches of decomposed organic matter or shredded bark. Wait for one year before applying a slow-release 10-10-10 granular fertilizer. Trees that are better planted in spring include redbud, Japanese maple, dogwood and other woody plants considered marginally hardy in Zone 5.

Take precautions now if wildlife has been a problem in past winters. Physical barriers are the most effective method for preventing damage. Spread garden netting around trees to keep deer from rubbing antlers on trunks. Deter rabbits and rodents by cutting down ornamental grasses and other perennials. Protect tree trunks from gnawing rodents with hardware cloth tacked into soil 3 inches from the trunk. Discourage all wildlife from forming browsing habits.

Flowers

After a killing frost, remove annual plant material and add to compost pile.

Plant spring-flowering bulbs.

Soilless mix from window boxes or containers can be saved for use a second time. Before using again, mix equal parts old mix with fresh mix.

Lift and store tender bulbs a few weeks after the first killing frost, which might be as late as November:

• Dahlias/Cannas - After frost has blackened stems, gently lift tubers with a pitchfork. Shake off excess soil and dry in a warm dry place. Do not separate roots at this time. When dry, place labeled tubers in cardboard boxes lined with newspaper and filled with barely moist wood shavings, peat moss or vermiculite. Store in a dark room between 40 and 50 degrees. Check periodically; throw away rotted tubers and sprinkle water on those that are beginning to dry out.

• Begonias/Caladium - Remove all foliage and stems and store in a cardboard box lined with newspaper and barely moist wood shavings, peat moss or vermiculite. Store in a dark room between 45 and 55 degrees.

Lawns

Continue to mow lawns at 21/2 to 3 inches high. Dry clippings may be added directly to compost pile. Avoid adding soaking wet clippings to pile.

If not done in September, fertilize lawns with a slow-release 4-1-2 or 3-1-2 organic fertilizer.

Edible plants

Plant garlic for harvest next summer.

Cut back any remaining herbs and bring indoors to use fresh or dried.

Protect tender plants from light freezes at night by covering them with sheets, plastic or upturned bushel baskets.

Apply a heavy mulch layer over leeks, carrots, beets and turnips to continue harvesting into early winter.

If a hard frost threatens, pick all tomatoes, including unripe ones and store in cardboard boxes or paper bags.

After a hard frost, remove all dead plant material and add to compost pile. Till 1 to 2 inches of organic material, composted manure or shredded leaf mold into garden soil.

Houseplants

Gradually acclimate houseplants that summered outdoors to indoor conditions before the first frost. Monitor plants carefully for pest or disease problems before moving them inside. Sequester outdoor plants to prevent contaminating those that remained indoors with disease or insects.

• Denise Corkery is a horticultural writer at the Chicago Botanic Garden.