Why Leaves Turn Color in Fall
When Daylight Saving Time ends on November 5, we’ll fall back an hour. These time switches can be hard on people, but plants don’t rely on clocks to know that fall is here. But the shorter hours of daylight definitely do have an impact on our plants. Temperate species, like our native deciduous trees, shrubs, and perennials, keep track of time biochemically, through processes that are affected by air temperature and the number of hours of daylight they experience. The arrival of shorter days and cooler temperatures announces that it’s time to sleep.
Probably the most widely recognized biochemically-induced change for plants in the fall is leaf-drop for deciduous species. If freezing temps are on the way, soft, green tissue can be more of a liability than an advantage, so deciduous trees and shrubs reabsorb the nutrients in leaves and then drop them, as a means of self-preservation. Natural biochemical changes in leaves lead first to a color change, then to death, and eventually to leaf drop. Autumn biochemical changes also induce other plant responses, like blooming, such as in Chrysanthemums, or fruit-set, such as the ripening of berries.
In plants whose leaves change color before dropping in the fall, the interplay of weather and biochemical processes has an effect on autumn leaf color. As leaves begin to die, green chlorophyll is broken down, making way for red, orange, and yellow pigments. If plants are stressed and natural biochemical processes are disrupted, due to drought, perhaps, dead leaves may cling on unnaturally for most of the winter, only being removed by the arrival of winter winds strong enough to tear them violently away from limbs.
But once leaves do finally fall to the ground, consider leaving them in your yard, rather than raking them up and putting them by the curb. Leaves aren’t trash, they’re treasure! You can use them in compost piles as a source of “brown” to balance out the “green” of lawn clippings and food waste. Or, you could mow them up, allowing the mulched clippings to fall back to the ground and work their way down into the root zone, where they add precious organic matter, along with a small amount of nutrients, building a healthy soil structure for roots to grow in. Or, you could simply rake fallen leaves into garden beds and use them as free mulch, piling them around your plants to protect from the coming cold of winter.