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The Master Gardener Journal


S O N O R A N   O R I G I N A L S


The Unappreciated Smell of Rain
by Patti Baynham, Master Gardener

How can we live in a desert and not appreciate the smell of rain? Yet we do. Is it better to be hated or not even to be noticed? How does it feel to know that you are a key player, yet you are not recognized as are others who are less hardy than you but greater in stature and more unique in character? Lastly, should we find ways more often to "order" things other than by the alphabet? Those whose names begin with "Z" must get tired of being at the end of every list!

Tucked away at the end of the list of Sonoran Desert plants is the Zygophyllaceae family, caltrop. It is a small family, with only about 250 species, yet it includes one of the most abundant and important plants in North American deserts: Larrea tridentate, the creosote bush. The best-known member of the caltrop family is the dreaded puncture vine, or goat-head (Tribulus terrestris), a weed introduced from Europe and notorious for clinging to rubber tires. A much more desirable caltrop relative of creosote bush is the showy Arizona poppy, Kallstroemia grandiflora.

Larrea tridentate's common name, creosote bush, often makes people think it must smell like the petroleum-based product that is used as a wood preservative. Those who know the Mojave, Sonoran, and/or Chihuahuan Deserts, however, know that this is an unfortunate mis-association. The smell of creosote is quite strong and distinctive, but it is what gives our desert a characteristic refreshing smell after rains.

Creosote is an amazing and beautiful plant, but it is so common that we tend to look past it and never ask the questions everyone asks about saguaros, agaves, and other more imposing plants in North American deserts. We talk about how old saguaros are before they put on their first arms and how old the largest specimens must be, yet creosote bushes may live to be more than 11,000 years old! As the creosote bush matures and individual stems die off, new stems sprout from the outer edge of the root crown. Through the course of centuries this ring of new growth breaks apart into separate bushes, each a clone of the original. Creosote bush clones may be among the world's oldest living organisms. Average longevity at one study site in California was determined to be 1,250 years; at a second site it was 625 years.

This is a surprisingly drought-tolerant plant, having been known to live for 2-3 years with no rain at all. All long-lived desert plants have adapted in some significant way to conserve water. Three characteristics of Larrea tridentate which make it particularly drought tolerant are: (1) a resinous coating on its leaves, which makes them attractively shiny and bright while serving to minimize transpiration, (2) drought-deciduousness to the extent that it will not only drop its leaves, but also its branches, in extremely dry periods in order to conserve moisture for the root crown, and (3) the ability to inhibit germination and growth of other plants around it, to the extent that moisture and soil conditions will limit survival.

Creosote bush is "the single most widely-used and frequently-employed medicinal herb in the Sonoran Desert" Its leaves have been used for centuries to make antiseptics, emetics, and tea. Various peoples have thought that it helps to cure fevers, influenza, sinusitis, colds, upset stomachs, arthritis, anemia, and fungal infections. It may also be useful in treatment of allergies, autoimmunity diseases, and premenstrual syndrome. Today it is being researched as a possible cancer treatment due to its antioxidant characteristics. The same resinous substances that are deposited on its leaves are used to make sealants and glues. American Indians of the Southwest often used creosote extracts to seal pottery and to repair tools.

Other non-medicinal uses include serving as an insecticide, disinfectant, mild sunscreen or massage oil, and prevention of rancidity of vegetable oils.

Natural growing creosote is commonly found on gentle slopes, valley floors, and in desert washes. Site elevation is typically 5,000 feet or less. It has a relatively shallow taproot (30"-35") and several secondary lateral roots that run for about 10' in length 1' below ground level. Creosote is very often found above layers of caliche.

As indicated above, creosote reproduces vegetatively by sprouting from the outer ring of its root crown. It also reproduces sexually, but age distribution studies have shown that many years may pass under natural conditions without any new successful germination and survival in a particular area. Summer rains are required for flowering to occur, however too much rain will result in diseased flowers.

Three to six inches per year seems to result in the highest germination rates (20%-60%). Seed weight and structure reflect reliance upon tumbling as the primary means of distribution. Rodents, who like to burrow beneath creosote bushes, also probably play a significant role in dispersion and "planting" of seeds. White bursage is very often found as a nurse plant for successful seedlings. In his studies Dr. Joe McAuliffe, Director of Research at Phoenix's Desert Botanical Garden, found that 85.5% of all young creosote bushes were beneath white bursage.

As an ornamental plant, Larrea tridentate is not yet recognized as an attractive addition to most landscapes. Like many other desert-adapted plants, when given the favorable and consistent conditions of a maintained yard or garden its size, fullness, blooming frequency and richness of color are all enhanced.

Creosote bush is an evergreen shrub that prefers full sunlight and has a long blooming period. Set against its shiny dark green leaves, 5-petaled yellow flowers that are up to 1" in diameter bloom off and on from spring through fall; they are most abundant in the springtime and after rains. Following the flowers, globe-shaped fuzzy white seed capsules that are up to 1-1/2" in diameter continue to attract attention as they stand out against creosote's dark foliage. With consistent water, although not much more than 7" per year, creosote will grow to be a denser shrub. In its natural setting it tends to be quite open and wispy, with most leaves clustered at the tips of branches. Height can range from 4'-12', depending upon the amount of water and sunlight. Some people think creosote bush has an oriental feel due to its open and sculptural nature.

Many different animals use creosote bush for habitat, including squirrels, kangaroo rats, desert woodrats, and reptiles. Desert tortoises often burrow into areas where creosote roots will hold surrounding soil stable. "Seventy-one percent of desert tortoise burrows studied near San Bernadino California, were associated with creosote bush." Creosote is generally unpalatable to wildlife, although jackrabbits do eat the leaves. Many insect species rely upon the plant. In fact, there are 22 species of bees and other insects such as the creosote katydid and creosote grasshopper, that are specific to it.

There is indeed much we should appreciate about Larrea tridentate. Next time you smell rain in the desert, take time to appreciate the beauty, toughness, and widely diversified value of that plant from which the fresh smell of desert rain emanates.

Resources:

George Macdonald Hocking, Ph.D., A Dictionary of Natural Products: Terms in the Field of Pharmacognosy Relating to Natural Medicinal and Pharmaceutical Materials and the Plants, Animals, and Minerals from Which They Are Derived, 2nd ed. (New Jersey: Plexus Publishing, 1997), p. 431.

www.fs.fed.us/database/feis/plants/shrub/lartri/all.html, Botanical and Ecological Characteristics, Regeneration Processes.

www.fs.fed.us/database/feis/plants/shrub/lartri/all.html, Management Considerations, Importance to Livestock and Wildlife.

Arizona-Sonora Desert Museum, A Natural History of the Sonoran Desert, p. 263.

Photos:

Larrea tridentate in bloom by Tanya Beth Kinsey firefly@fireflyforest.com http://www.fireflyforest.com/flowers/

Larrea tridentate in the landscape by Mountain States Wholesale Nursery.


Maricopa County Master Gardener Volunteer Information
Last Updated November 21, 2004
Author: Lucy K. Bradley, Extension Agent Urban Horticulture, University of Arizona Cooperative Extension, Maricopa County
© 1997 The University of Arizona, College of Agriculture and Life Sciences, Cooperative Extension in Maricopa County
Comments to Maricopa-hort@ag.arizona.edu 4341 E. Broadway Road, Phoenix, AZ 85040,
Voice: (602) 470-8086 ext. 301, Fax (602) 470-8092