The Leaves of Fall
Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent
Falling Leaves
Trees, shrubs, and vines are also amazing in the fall. They take cues that winter is on its way from the cooling temperatures and shortening days. Nature has programmed them to avoid cold weather by stopping growth and becoming dormant. This is a physiological process triggered by hormonal changes in the plant. Photosynthesis (the production of carbohydrates using the sunlight for energy) stops. Nutrients from the leaves are moved to the twigs, buds, stems, and roots.
Eventually, at the base of the leaf stem a layer of cells forms, stopping all movement of water and nutrients in and out of the leaf. This layer is called the abscission layer and it’s at this point where the leaves break off and fall from the tree.
Unseen are the internal changes in the buds, stems, and roots, which are becoming less susceptible to damage from freezing temperatures. Sugars increase to lower the freezing point of cell contents and the cell walls become stronger, along with other protective changes. During the fall as temperatures continue to decline, plants become “acclimated” to cold temperatures until they achieve their maximum winter hardiness in mid-winter. A plant’s maximum potential hardiness is dictated by its genes. In the spring, a tree or shrub will start to de-acclimate and break its dormancy in response to warming temperatures and longer day length.
It’s interesting to note that many trees and shrubs also have a chilling requirement, much like the bulbs. They will not readily break their dormancy and resume their growth in the spring unless they’re exposed to enough chilling temperatures. A good example of this is peaches. Peach trees need from 700 to 1000 hours of temperatures between 32 degrees and 45 degrees. That’s why it’s hard to grow peaches in very warm climates, because they don’t get enough winter chilling to break dormancy.
Gardeners may have noticed certain varieties of peaches advertised by mail-order nurseries as “low-chill” varieties. These require less chilling than the standard varieties used in cooler areas of the country. What happens if there isn’t enough of a winter chill to meet the tree’s requirement? Leaf development will be delayed and flower structure will be defective, resulting in poor fruit set.
Fall Color
Fall color is another amazing facet of nature that accompanies the onset of the dormancy process. I grew up in upstate New York and have always been intrigued with the colors of autumn leaves. I’ve told the story before of why leaves turn such beautiful colors, but it’s worth repeating again. As the weather turns cool and the abscission layer starts to form, the leaves stop producing chlorophyll and the green color of the leaves disappears. This reveals the more stable orange and yellow pigments (carotene and xanthophyll).
Some trees and shrubs also exhibit wonderful red colors. These species are genetically capable of taking the carbohydrates left in the leaves and converting them to anthocyanins. Anthocyanins are red pigments responsible for the reds, pinks, and purples seen in autumn leaves. As the chlorophyll begins to break down, the newly formed anthocyanins become apparent. Brilliant oranges come from a mix of red pigments along with the orange and yellow pigments. Of course there are trees that aren’t very spectacular and lose all their pigmentation at once, turning from green to brown over a short span.
Autumn weather that favors good production of sugars in the leaves will lead to more splendid fall coloring. Dry, sunny, and cool fall weather, but not lots of heavy frost, are best for bringing out the vibrant reds, smashing oranges, and brilliant yellows.
Plants with Fall Color
Trees and shrubs that commonly provide our landscapes with fall color include red maple, Amur maple, Autumn ashes, red oaks, flowering pear, blueberry, sumac, sweetgum, dogwood, burning bush euonymus, Persian parrotia, and viburnums. Bright yellows come from birch, aspen, and gingko. They are glorious!
A lack of fall color in a species or cultivar that is supposed to provide a good display can be attributed to its location, growing conditions, or general health. Full sunlight is needed for the best development of fall color, especially the reds. A plants that’s shaded, will probably not develop good fall color. Plants experiencing drought stress or other problems may also be color deficient in the autumn. An extremely vigorous plant that’s still actively growing in the fall due to late season pruning, fertilization, or excessive watering will also fail to color up nicely. This is also a clue that the plant is not becoming dormant and acclimated to cold temperatures. These plants are more likely to sustain winter injury due to freezing temperatures, especially severe cold early in the winter.