In many parts of the United States, fall color is as automatic as the changing of the seasons. Some years the colors are more spectacular than others, depending on weather conditions, but none the less it occurs.

Here, in Las Vegas, we’re not so lucky when it comes to leaf color variation in the fall. Colors other than shades of yellow are not very common here. So what’s the deal?

Well, there are several contributing factors to Autumn Leaf Color. Here’s how it works or at least what is known to date.

Due to genetics, some tree varieties contain more leaf pigment than others. Unfortunately, most of the tree varieties that will grow well in an arid climate and a high alkali soil, like here in the desert southwest, do not contain high a high amount of leaf pigment.

For example, one of the most popular shade trees grown locally is a Arizona Ash. The same variety tree growing in other parts of the country will display the same colors we see here. The color may or may not be deeper or more vibrant, dependent upon some of the other influencing factors, but it will still be a shade of yellow. As the amount of daylight hours decrease, so does the occurrence of photosynthesis. Which in turn reduces the amount of chlorophyll (the green color in leaves) produced. Without the bright green color of chlorophyll present, the other pigments in the leaves are unmasked and able to show their colors. Moisture and temperature are the main influences.

Adequate soil moisture, a succession of warm, sunny days and cold, crisp (not freezing) nights seem to bring out the most spectacular display of color. On a bright warm day, a high amount of sugar is produced in the leaves. But, temperatures dropping significantly at night causes a gradual closing of leaf veins, which prevent the sugars from moving out of the leaf. These trapped sugars contain Carotene and xanthophyll which are yellow pigments produced in foliage all year and anthocyanin pigments which are tints of reds, orange and purples. When these pigments are left behind and the green chlorophyl is reduced, the end result is the Colors of Autumn. As you can see, our arid climate with relatively warm fall days and nights are not conducive to Autumn color production. Now, there is a bright side. Occasionally, local weather patterns work in favor of fall color.

The photo above is an Arizona Ash was taken here in the valley a few years ago. In addition, new trees are continually being introduced to the valley that provide fall color and require minimal environmental effect. Some trees that are available locally and provide fall color: Arizona Ash (Fraxinus velutina) Yellow-Gold Chinese Pistache (Pistacia chinensis) Red Chinese Tallow Tree (Sapium sebiferum) Red Modesto Ash (Fraxinus velutina ‘Modesto’) Yellow Raywood Ash (Fraxinus oxycarpa ‘Raywood’) Red Texas Red Oak (Quercus texana) Orange-Red

By Russ Thompson Sunkissed Horticultural Consultants