Why Leaves Change Color
by David Alosi, U.C. Master Gardener
Pistacia chinensis
In preparation for this workshop, I would like to offer a little science about this process of color change and a list of good trees and shrubs that will provide autumn color in your gardens.
Leaves have three basic types of color pigments: chlorophyll (which make leaves green), carotenoids (which produce the yellow, orange and brown colors) and anthocyanins (source of the purples, reds and bronzes).
All through the spring and summer, the leaves contain a lot of green chlorophyll, which is used for photosynthesis. That's the process plants use for making food from the sun's energy. The yellows, oranges and browns are also present in the leaves all summer, but they are hidden by the chlorophyll. The purples, reds and bronzes don't appear until late summer, when sugars and other chemicals start to build up in the leaves.
The real changes in leaf colors are almost always caused by shorter day lengths, not by cold. This response to changes in the length of light and dark periods is called photoperiodism. The longer nights signal the tree to enter dormancy for winter.
The longest day occurs on the summer solstice in June, then days get gradually shorter until days and nights are equal in late September on the equinox. After that, the nights become longer until the winter solstice in December, the shortest day of the year.
In preparation for winter, a tree undergoes a process called senescence. The veins in its leaves begin to contract, beginning dormancy and decreasing the use of water. Chlorophyll production slows and the other pigments present in the leaves (yellows, oranges and browns) start to show. Sugars and other chemicals start to build up in the leaves at the same time, producing the purple, red and bronze hues.
Different trees of the same species may be inconsistent in the timing of senescence due to variations in the amount of light, shade and moisture and to variations in weather conditions.
Color differences from year to year do depend on the weather. Colors are better after warm, sunny summers with plenty of water. If fall days are sunny and cool but not frosty, sugars build up in the leaves and the pigments that produce purples, reds and bronzes will dominate. A heavy frost kills the cells and the leaves turn brown immediately, quickly ending the color show.
Fall Color In The Bay Area
If you would like to plant trees and shrubs with great fall color, now is the time to shop around and look around your neighborhood. The cooler temperatures and coming rainy season make fall a great time to plant. Here are a few of the best plants for fall color.
Note: Always be careful when selecting a tree or shrub. You are forming a long-term relationship. Read about each species ahead of time in a good reference book such as Sunset's Western Garden Book or the Napa County Master Gardener's Trees of Napa Valley, or talk to someone in your local nursery or Master Gardeners. Some trees or shrubs may not be suitable for a small garden, or they may have other problems associated with them that make them unsuitable for your intended location.
TREES
- Big leaf maple (Acer macrophyllum)
- Black locust (Robinia)
- Chinese Pistache (Pistacia chinensis)
- Chinese tallow (Sapium sebiferum)
- Crabapple (Malus)
- Crepe Myrtle (Lagerstroemia indica)
- Dogwood (Cornus florida)
- Fruit trees (apple, cherry, peach, pear and plum)
- Ginkgo (Ginkgo biloba)
- Hawthorn (Crataegus)
- Japanese maple (Acer palmatum)
- Japanese persimmon (Diospyros kaki)
- Liquidambar (Liquidambar styraciflua)
- Sour gum (Nyssa sylvatica)
- Trident maple (Acer buergeranum)
- Tulip tree (Liriodendron tulipifera)
SHRUBS
- Deciduous azalea
- Barberry (Berberis)
- Blueberry (Vaccinium)
- Deciduous euonymus
- Heavenly bamboo (Nandina domestica)
- Oakleaf hydrangea (Hydrangea quercifolia)
- Pomegranate
- Viburnum
- Sumac (Rhus)
- Western redbud (Cercis)
Join Napa County Master Gardeners on a free tree walk on Saturday, October 15, at the Veterans Home in Yountville. The guided walk will begin at 9:30 a.m. at the Napa Valley Museum. Reserve a space through the Master Gardener hotline at 707-253-4221 or toll free from Upvalley and American Canyon at 877-279-3065. Information is also available online at http://www.mastergardeners.org.