Utah, because of its irregular topography, has six plant zones, ranging upward from the Lower Sonoran, which covers the southern desert lowlands, to the Arctic, which extends above the timberline on the mountain summits. Between these, in ascending order, are the Upper Sonoran, the Transition, the Canadian, and the Hudsonian zones. Accompanying variations in temperature, altitude, and soil have encouraged the growth of nearly every type of plant ordinarily found between Alaska and Mexico. Botanists have recognized more than 4,000 species.
The Lower Sonoran zone in Utah is limited principally to the semi-tropical region of southwestern Utah, covering approximately 500 square miles adjacent to the Virgin River and its tributaries. Because figs, pomegranates, cotton, and similar crops can be successfully grown here, this region has become known as. "Utah's Dixie." The natural flora includes creosote bushes, screwpod mesquite, and several species of cacti and sword plants. The screwpod mesquite is a peculiar shrub that grows in the dry, sandy soil of desert canyons. It bears numerous spirally-twisted pods containing ten to twenty beanlike seeds, from which the Indians formerly made a coarse grade of flour.
In this section also grows the strange Joshua tree, sometimes called Spanish dagger, an arborescent yucca found only in the southwestern part of the United States. The plant has an unusual tufted appearance, entirely different from that of any other tree or shrub in Utah. Its leaves, bristling on large, clumsy branches, are sharply pointed and bayonetlike. The biblical name, Joshua, was bestowed by Mormon pioneers, who likened the upturned limbs to arms lifted in prayer. The tree has an extensive root system that goes deep into the ground for water. It clings tenaciously to life, defying the hardships of storm and drought, and sometimes attains an age of 300 years. Its soft fibrous wood is heavy and full of moisture when green, but very light when dry; doctors find the wood useful in preparing splints and casts for setting broken bones. Indians used the coarse fibers for making baskets and sandals. Growing profusely along the lower streams and washes of southern Utah are dense thickets of tamarix, also called salt cedar, an odd shrubby plant with small scaly leaves and feathery pink flowers. Because of its value in water conservation, quantities of this plant have been transplanted to other sections of the State.
Nearly all of the Great Basin comes within the Upper Sonoran zone, of which the most characteristic plant is sagebrush. An excellent indicator of good soil, sagebrush grows best in fertile, well-drained ground, free from alkali. Much of Utah's prosperous agricultural belt, extending north-south along the foothills of the Wasatch Mountains, occupies land from which sagebrush has been cleared. In the alkaline soil of the lower desert areas, sagebrush gives way to shadscale, greasewood, and forage plants that provide winter range for sheep and cattle. Alfalfa and sugar beets, both alkali tolerants, are the crops most successfully grown in this type of soil. Certain extreme forms of saltbushes grow where alkali is too abundant to permit the growth of anything else. Native trees of Utah nearly all grow in mountainous regions, ranging upward from the Upper Sonoran zone, and only about a score occur in considerable numbers.