What causes leaf color to change from green to yellow or red in the fall?
It’s a combination of light, temperature and available water. With less light on short days, trees and shrubs return to an energy-saving mode to survive the winter. Short days and falling temperatures signal trees and shrubs to quit producing chlorophyll, which converts sunlight into sugars through a process called photosynthesis.
As chlorophyll breaks down, leaves reflect the remaining leaf pigments that were there all along. Yellow is produced by carotenoids, the same pigment that gives carrots their color. The red color is produced by anthocyanin.
These pigments, normally masked by chlorophyll, help protect leaves from excessive sunlight. Anthocyanin are actually toxic, and contribute to a plant’s natural defense against insects.
Many of our native trees, and trees introduced to Colorado, turn yellow in the fall. Aspen in the high country can turn entire mountainsides golden-yellow. Huge areas are actually clones of the same tree that have spread via underground root suckers.
Moisture during the fall keeps leaves bright and colorful for a longer period of time. When conditions are dry, leaf color fades to brown, and leaves drop quickly. Cooler temperatures also support a longer period of color, which explains why fall color lasts longer in the mountains than along the Front Range.
The normal season for autumn color is September and October. On average, the best color occurs in late September and early October. Of course, with Colorado’s temperamental weather, fall color can occur earlier or later.
Lawn watering is often stopped in early fall. Conventional thinking is that because evapo-transpiration (ET) rates are low and the turf isn’t growing much, it is OK to stop watering. However, historic ET and rainfall data for most of Colorado shows a need of 0.5 to 1.0 inches of irrigation per week during September and October.
While mowing isn’t needed as frequently during fall, the turf does continue to grow – but in ways that differ from spring and summer. Turfgrasses form tillers (side shoots) and rhizomes that increase the density of fall turf. This is an important time for turf to “heal” after a stressful summer, especially if it has been worn down by traffic or suffered from disease or insect problems.
Fall watering is essential for late season nitrogen applications to work most effectively. Fertilizer applied to dry turf is less likely to enhance fall rooting and increase energy storage.
Fall is the best time of year to control perennial broadleaf weeds – dandelion, clover, bindweed, plantain, and thistle, to name a few. Fall herbicide applications are more effective when applied to healthy, green, actively growing weeds. The herbicide is more easily absorbed and translocated to weed roots resulting in better control.
Finally, fall watering of lawns that were damaged by winter mites (clover mites, Banks grass mites) is essential for discouraging mite activity this upcoming winter and reducing potential mite problems.
Garden Vegetables and Pests:
Cleaning up vegetable garden in fall following a hard freeze can pay off next year. If gardeners have had problems with the following diseases or insects, do encourage follow-through with a thorough vegetable garden cleanup this fall, soon after harvest is complete. A thorough cleanup consists of complete removal of all infested plants including weeds in large plastic garden clean-up bags.
Early blight – The fungus over winters on diseased plants and perennial weeds such as horse nettle and nightshades. Remove diseased tomato plant debris and clear weeds. Spores are spread to new garden plants by splashing water, wind and insects. Consider moving tomatoes to a new location next year if possible.
Viruses – The most noticeable symptom of TSWV and INSV (Tomato spotted wilt and Impatiens necrotic spot) on tomatoes are yellow rings or spots on fruit. They also affect lettuce and pepper, as well as many weed species, such as bindweed and nightshade. Viruses survive on plants, not in the soil. They are carried to vegetables by Western flower thrips that fed on infected plants. Clean up affected perennial weeds.
Thrips – Onion thrips affect onion, cabbage and beans and are often the most common type of thrips on vegetables and flowers. Because thrips pupae over winter in the crevices of plants or soil, cleaning up infested weeds as well as vegetables in the fall is very important. Western flower thrips are native to warmer areas of the Mountain West and have many weed and crop hosts. They over -winter in a similar fashion as onion thrips.
Flea beetles – flea beetles spend the winter as adults hiding under leaves, dirt clods or in other protected sites. Fall cleanup of plants may eliminate some but not all hiding places. Use additional means such as trap crops during the growing season for control.