The late Dr. Forrest Shreve, for many years Director of the Desert Laboratory of the Carnegie Institution near Tucson, Arizona, defined a desert as “a region of deficient and uncertain rainfall.” He divided the Great American Desert into four major sections: (1) Chihuahuan (chee-WAH-wahn), including the Mexican States of Chihuahua and Coahuila (coa-WHEE-lah), southwestern Texas, and south-central New Mexico; (2) Sonoran, including Baja California, southwestern Arizona, and northwestern Sonora; (3) Mojave (moh-HAH-vee), Colorado, including south-eastern California and extreme southern Nevada; (4) Great Basin, including Nevada, Utah, southwestern Idaho and southeastern Oregon.
Since the steppes and mesas of the Great Basin Desert have generally lower temperatures, higher elevations, and greater precipitation than the other three sections, we are not including its flowers in this work.
Why and When Do Deserts Bloom?
The Great American Desert produces, when conditions are favorable, a gorgeous exhibition of wildflowers. Trees, shrubs, and herbs all contribute to the splendor of the display. Soil composition, slope and exposure, suitable temperatures, and adequate moisture are essential to plant growth and flower production.
Moisture is the uncertain factor, and years may pass without enough rainfall to stimulate plant growth. Rain of less than 0.15 inch is wasted as far as desert plants are concerned, for the moisture evaporates before penetrating the soil. Some annuals produce seeds having water-soluble germination inhibitors in their coverings, hence fail to sprout, even after rain, unless the moisture totals at least half an inch.
When soil moisture from December and January rainfall is enough to support potential plants it dissolves the seed coats, and the desert floor is soon carpeted with eager green seedlings. When winter rains are scant, as is so often the case, the dormant seed population fails to germinate and the spring flower display doesn’t appear. There is no sure way to forecast a spectacular blossom year, for a sudden cold wave or period of drying winds may literally nip in the bud a potential season of brilliant bloom. A great flower year may occur only once in a decade.
Perennials are more dependable than annuals, since some of them, particularly cactuses and other succulents, have water storage tissues in their stems or roots. These perennials may be counted on to blossom each year, but with much less abandon than after winters of above normal precipitation. Many perennials have surprisingly extensive root systems. Fascinating are the ways by which plants manage to thrive under severe conditions of desert heat and drought. As we have seen, most annuals lie dormant as seeds until suitable moisture and temperature occur. Then they grow very rapidly, to bloom and mature seeds while the soil still has moisture. Winter rains produce spring-blooming ephemerals, and summer showers produce summer “quickies.”
Another group of plants, including the ocotillo (oh-koh-TEE-yoh), slows down life processes to become dormant during dry periods, even to dropping all leaves. When rains come they put on new leaves, several times a year if necessary.
Cactuses and other succulents gorge themselves with water when the soil is wet, releasing moisture very sparingly from storage tissues during the “long dry.” Some have discarded or reduced foliage, or have covered leaf surfaces with varnish or wax, to decrease to a minimum the loss of vital moisture through transpiration.