Creosote Bush

One doesn't have to look far to see a wonder of the plant world in Joshua Tree National Park. Known scientifically as Larrea tridentata, and in common parlance as the creosote bush, it produces small, pretty yellow flowers in spring and summer. But it is the pleasantly pungent smell, which the leaves produce as soon as a summer rain starts, that is most noticeable.

The creosote bush is the signature plant of the southern part of the park and a common, characteristic, and often dominant shrub of the deserts of southwestern North America. Its closest relative lives in the arid regions of Argentina.

Actually, what botanists classify as a single species in the North American deserts is now known to consist of three genetically different shrubs. Creosote bushes of the Mojave Desert have 78 chromosomes, those of the Sonoran Desert (southern Arizona) have 52 chromosomes, while those of west Texas (Chihuahuan Desert) have only 26. Such an increase in the number of chromosomes in plant evolution is not that unusual. Seedless watermelons, for example, were the result of doubling the number of chromosomes of regular watermelons, the lack of seeds being a side effect. In the case of the Mojave creosote, the increase in chromosome number may have been accompanied by an increasing ability to survive on the less summer rainfall in the Mojave.

The genetic and fossil evidence indicate that the Mojave creosote is a relative newcomer to our part of California. Eleven to 12,000 years ago, at the end of the Ice Age, this area would have been dominated by juniper woodland and lots of grass. As the climate became warmer and drier the junipers retreated to the nearby mountains, and a new plant, evolved from the Sonoran Desert form, appeared on the scene: our creosote bush. The newcomer was so successful in the competition for scarce water that it soon became the largest and most conspicuous plant of our desert landscape.

Although creosote bushes produce large numbers of fuzzy seeds at each flowering, few of them are able to germinate. It takes decades for creosote bushes to return to areas that have been cleared of native shrubs. Even a one-foot high plant is probably at least ten years old. As the shrub grows, branches continue to originate around the periphery of the original stem crown. The branches grow upward for about six feet giving the whole shrub the rounded shape of an upside down cone.

As growth continues, the oldest branches gradually die and the stem crown splits into separate crowns. This happens at an age of 30 to 90 years. Eventually, the original stem and early branches die and rot away; the connections between adjoining segments of the stem crown thus disappear. The plant has now become a clone, composed of several independent stem crowns all descended from one seedling. The process continues until the clone spreads across the ground in a circular or elliptical shape. As you travel in the park, see if you can find one or more of these circular creosote clones. Usually, a mound of sand accumulates in the central area.

In a few areas of the Mojave Desert clonal creosote rings have been found that are several yards in diameter. Near Lucerne Valley, “King Clone” has an average diameter of 45 feet! Using radiocarbon dating and known growth rates of creosote, scientists have estimated the age of “King Clone” as 11,700 years. Some of these common residents have been here continuously since the last ice age. They are certainly an integral part of our desert environment and many desert animals depend on the creosote for food and shelter.

The Indians of the Southwest appreciated the creosote bush. The leaves were an important part of their pharmacopoeia. The Apaches prescribed chewing and swallowing a small piece of creosote branch to cure diarrhea. Other tribes made a strong tea from the dried leaves to treat the common cold. The resinous leaf nodes were used to soothe bruises and wounds. And a tea made from the leaves and sweetened with a little honey was said to greatly relieve kidney pain.

Modern herbalists also have found uses for the ancient creosote. An extract is now marketed as a cure for herpes. Another extract is being investigated as an anti-cancer drug. However, large doses of creosote have been shown to cause liver damage.

by Harold DeLisle, Phd.

Last updated: February 28, 2015

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