How do I get fall color?

Answer: This is a complicated subject. Factors which are involved in development of fall color include the genetic capacity to develop the colors, proper growing conditions and location, and the absence of leaf diseases. Development of autumn color really means that the chlorophyll in the leaves has degraded and the underlying colors hidden by the green become revealed. Yellows were there all along, sometimes purple is present throughout the growing season as well, and is apparent as dark green leaves until the chlorophyll degrades. Then the purple remains. Reds often develop in the autumn under the influence of sunny days. The sunny days allow the plants to produce sugars. The cool nights prevent the movement of sugars from the leaves to other parts of the plant. The sugars accumulate in the vacuoles, storage organs in the cells, and are converted to anthocyanin and other pigments. When the green chlorophyll degrades, these stored pigments become visible.

Genetics, or the innate ability of a plant to develop fall color is important. Some species of trees don't have the ability to develop color. Other species have that capacity under some environmental conditions. Within a species of trees which can develop fall color there are individuals which lack the ability to develop color. If you want to purchase a tree that develops fall color, it is often wise to purchase the plant in the autumn when you can see its color. Within the nursery I have observed tremendous variation in leaf color of some trees and shrubs. Within a species I have seen the colors range from no color, green leaves until they drop, to bright yellow, red, orange, and mixtures of yellow and red. Some will develop color early, others may delay color development.

Even if the tree has the genetic ability to develop beautiful fall colors, it may not do so if it is not grown properly. Irrigation or precipitation late into the fall may delay or prevent color development. By irrigating the tree well through the growing season and withholding irrigation the last month before frost, we can encourage the tree to prepare for autumn and often to develop color. This also prevents the tree from being injured by a warm fall followed by a sudden change to very cold weather. Fertilization late in the summer can also prevent or reduce fall colors. This is also a factor which can cause injury by a sudden fall freeze as the tree has been encouraged to remain actively growing and has not hardened-off in preparation for fall.

Some trees such as the maples, famous for their contributions to the Northeastern autumns, do not do well in the West. They will grow here, but many of them cannot tolerate our hot, dry, windy summers. Their leaves are injured by our summers and often not able to provide the colors we desire. There are some maples which do well here as evidenced by the Big-tooth maples native to the Rocky Mountains. I have even seen sugar maples in very protected locations and sometimes, unexplainably, in exposed locations exhibiting spectacular fall color. They can provide bright autumn colors if other conditions are correct. There are also oaks which can give us good fall color. The Chinese Pistache and other trees and shrubs adapted to the soils and environment of the Southwest are also available, so by choosing adapted plants which have the ability to develop fall color, you can accomplish what you wish.

So, your tree may not change color in the fall because it is not able to, or because you have been too nice to it, watering, fertilizing, and discouraging preparation for autumn. However, even when everything else is correct, a wet monsoon season and fall can result in leaf spot fungi which cause early leaf drop, preventing color development.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

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