Arizona-Sonora Desert Museum Digital Library

Description

Brittlebush is a somewhat woody shrub 3 to 5 feet (1-1.5 m) tall. The dense branching pattern tends to form a hemispherical mound, especially in very arid conditions. The leaves range from nearly hairless bright green through gray-green to white with a dense covering of soft matted hairs. Small yellow flowers are borne on multiply-branched stalks well above the leafy stems; they are usually produced in late winter to mid spring, with occasional bloom at other times.

Range

This distinctive plant is widespread and common in most of the Sonoran Desert and in warmer parts of the Mohave Desert.

Notes

Brittlebush is very drought resistant and often forms nearly pure stands in extremely arid habitats. There is a widespread myth that creosote bush secretes toxins that kill all other plants growing nearby. In fact creosote bush doesn't inhibit many annual plants, but brittlebush does. Rain water dripping off the foliage dissolves chemicals that inhibit the seed germination of many species. For this reason only a few species of annual wildflowers can grow under brittlebushes.

The more arid the conditions of the growing season, the smaller and whiter are the leaves produced. During prolonged drought the leaves are completely lost. It is fairly frost-tender; in Arizona Upland it tends to be restricted to slopes above the cold valley floors.

The stems exude a gum that can be chewed or used for incense (hence the Spanish name incienso). The gum was once exported to Europe by the mission priests and is still used by the Tohono O'odham. The English name was given to it by members of the 1907 Sykes expedition to the Pinacate region; they dubbed it "white brittlebush." (Read the fascinating tale of this pioneering adventure in Camp-Fires on Desert and Lava, by William Hornaday.)

The sunflower family is stunningly successful. It is the largest plant family, or the second largest after orchids, depending on the criteria used, with over 20,000 species occupying almost all of the world's habitats except underwater. The genus Senecio (groundsel) alone has 1000 species. So many variations within a single group make composites the bane of many botanists seeking to identify the species. Because of this and the great preponderance of yellow flowers, unidentified plants are often semi- affectionately dubbed "DYCs"(damn yellow composites). This term has been adopted by some Mexican botanists as "PCAs"(pinchi compuestas amarillas). The family is well represented in the Sonoran Desert, constituting, for example, 16% of the Tucson Mountains' flora (105 species and subspecies).

The flowers, also called florets, are nearly always clustered into heads, with each subtended by a whorl or whorls of modified leaves called bracts (the involucre). There are two general forms of flowers. A disk flower, in its most complete form, has five petals fused into a tube, with a tube of five fused anthers inside the petal tube, and a two-lobed stigma exserted through the anther tube. A ray flower (a "petal" of a daisy) is similar, except that some of the fused petals extend on one side into a flat strap-like ligule. Flowers may be unisexual or sterile, lacking either or both "male" and "female" sexual parts. Each functionally "female" flower, whether ray or disk, has a single inferior ovary with a single ovule. If the ovule is fertilized, it will develop into a single seed in a special fruit called an achene. Each head may have only ray flowers or disk flowers, or both.