Mark A. Dimmitt

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 5 petals fused into a tube, with a tube of 5 fused anthers inside the petal tube, and a 2-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.

Genus Ambrosia

On the surface ambrosias don’t seem to be very interesting. They are wind-pollinated, so they need no animal allies for reproduction. Virtually nothing eats them. And most species are so ordinary in appearance that they are rarely noticed despite their abundance throughout much of our desert. But on closer inspection, these plants reveal themselves to be essential, crucial species of the Sonoran Desert.

Each subdivision of the Sonoran Desert has at least one common species of bursage that contributes subliminally to the visual character of the landscape. The wide range of this group reflects the bursages’ ability to adapt and move into numerous environmental niches. Triangleleaf bursage (Ambrosia deltoidea) grows abundantly among saguaro cacti and palo verde trees where some rain falls in both winter and summer. Canyon ragweed (A. ambrosioides) is a fairly tropical, big-leafed species of somewhat moist habitats that seems to require some summer rain, while white bursage (A. dumosa) rivals creosote bush as the most arid-adapted perennial in North America where rainfall is mostly in winter, and summers are brutally hot. Ambrosia chenopodifolia grows on the mild Pacific coast of Baja California under a winter rainfall regime. Hollyleaf bursage (A. ilicifolia), a bursage with especially attractive foliage, grows in the wetter microhabitats within the hottest, most arid parts of the Sonoran Desert.

Bursages are vitally important to the community as nurse plants. The seedlings of most desert plants cannot survive the extreme environmental conditions in exposed ground; they must start life in the shelter of another (nurse) plant. Bursages are among the few plants that can pioneer exposed sites. Over the following centuries they may be replaced by a series of other species that establish under one another’s canopies. For one possible scenario, imagine a bursage growing today being replaced by another in mid-century, then by a barrel cactus in the 22nd century, a creosote bush in the 23rd, a foothill palo verde in the 26th, and a saguaro in the 27th. Visualize that saguaro producing its first flowers in the year 2670 and you begin to grasp the time-scale on which desert communities function. And here’s hoping you’ll never think of bursages as just drab little allergenic bushes again.

Description

What follows is a general description for the genus Ambrosia. Bursages and ragweeds are mostly inconspicuous shrubs and subshrubs (although some are herbaceous perennials), with tiny to fairly large, usually grayish, leaves. The plants bear inconspicuous staminate (male) and pistillate (female) flower heads on the same plant (monoecious flowering). After pollination by wind, some species (the bursages) develop spiny fruits (burs) which are dispersed when they cling to the fur or feathers of passing animals.

Range

Ambrosia is a mostly American genus of about 24 species, half of which occur in the Sonoran Desert region.

Notes

Despite the abundance of bursages and ragweeds, few vertebrates browse them. Nor do they seem to host many insects; they definitely do not rely on insects for pollination. The copious wind-borne pollen is highly allergenic and a major cause of hay fever.

Ambrosia ambrosioides

English name: canyon ragweed, bursage
Spanish name: chicura

Description

This is a woody shrub to about 6 feet (1.8 m) tall with sparsely-branched, wand-like stems bearing large (to 7 inches, 18 cm long), broad, dark gray-green leaves. It flowers in late winter into spring and occasionally at other times. Though it’s called a ragweed, the fruit is a bur.

Canyon ragweed (Ambrosia ambrosioides)

Range

Canyon ragweed is a subtropical shrub common throughout Sonora, southwestern Arizona, and the southern half of Baja California. There is also a disjunct population in Durango, Mexico. In the drier parts of its range it occurs mainly in drainages and along roadsides where it receives extra water from runoff. In the northern edge of its range it is restricted to the warmer slopes above valley floors.

Notes

O’odham used canyon ragweed in sweat baths to relieve arthritic pain. They spread hot coals on flat ground and covered them with a layer of ragweed leaves, then laid the afflicted person atop them and covered him or her with a blanket.

Ambrosia deltoidea

English names: triangleleaf bursage, burrobush, rabbitbush
Spanish name: estafiate

Description

Triangleleaf bursage is a densely-branched subshrub to about 2 feet (0.6 m) tall with triangular, finely toothed, gray-green leaves. The leaves are lost in very dry periods. Though it can be confused with brittlebush when not in flower; this bursage is a smaller plant with smaller, duller gray leaves. It flowers from late winter into spring, with burs ripening in late spring.

Triangle-leaf bursage (Ambrosia deltoidea)

Range

This species is characteristic of Arizona Upland desert, where it is often the dominant understory plant. It also occurs in the eastern portion of the Lower Colorado River Valley subdivision and in a few localities in Baja California.

Notes

This and other bursages are called burrobush and rabbitbush in pollen reports. Don’t confuse it with the plant more commonly called rabbitbrush, Chrysothamnus nauseosus, which flowers only in the fall and is much less allergenic. Bursage is the least ambiguous of the English vernacular names; the others are also applied to unrelated plants. Triangleleaf bursage lives as long as 50 years.

Among the plants sheltered by this bursage is the local pincushion cactus, Mammillaria grahamii. On some rocky bajadas nearly every bursage has several pincushions growing within its modest canopy.

The Fundamental Importance of Bursage

The Arizona-Sonora Desert Museum has always endeavored to restore the natural areas of its grounds disturbed by construction, our goal being to do it so well that no one could detect that humans had ever even walked on that spot, let alone cleared and trenched through it. Our first attempts were dismal. Although we had restored the land contours, replanted the cacti and trees, and painstakingly replaced the surface rocks right-side-up and partly buried, the site still looked devastated. Upon comparing our restoration to the adjacent undisturbed ground, we recognized that we had replaced less than a quarter of the original vegetation. In our particular type of Sonoran Desertscrub called Arizona Upland, about half the ground’s surface is covered by small shrubs. At the Desert Museum the dominant small-shrub species is triangleleaf bursage. These and other small shrubs greatly contribute to the base color and texture of the terrain. Today we are proud of our undetectable desert restoration efforts, and commercial nurseries propagate tens of thousands of bursage plants for use in desert revegetation projects. Things that we normally overlook can be of fundamental importance.

Ambrosia dumosa

English names: white bursage, burro-weed, burrobush
Spanish names: chicurilla, hierba del burro (burro herb), huizapol

Description

This subshrub to about 2 feet (0.6 m) tall has small, white, deeply-divided leaves. Plants are leafless in dry periods, which is most of the year in its habitat. It flowers in late winter to spring, depending on rainfall.

White bursage (Ambrosia dumosa)

Range

White bursage is common in the driest areas of the Sonoran Desert and southern Mohave Desert. In the parched flats of the Lower Colorado River Valley white bursage and creosote bush are often the only perennial species. In Arizona Upland desert it grows in the fine soils of the valleys; in contrast, triangleleaf bursage grows on rocky or caliche soils.

Notes

Recent research shows that the growth of bursage roots are inhibited by secretions of creosote bush roots. These are the two most common species in the arid valleys of the Lower Colorado River Valley and therefore the most likely competitors for scarce water resources. Perhaps even more interesting is the finding that the roots of different bursage plants seem able to detect one another and respond by growing in different soil volumes. It is likely that many other plants have similar interactions, but we still know very little about what is going on within the soil.

Other Representative Genera in the Composite Family

Baccharis sarothroides

English names: desert broom, broom baccharis
Spanish names: romerillo (rosemary), escoba amarga (bitter broom), hierba del pasmo

Description

Desert broom is a vertical, evergreen, densely-branched shrub usually 3 to 6 feet tall (0.9-1.8 m), occasionally to 10 feet (3 m). The many fine twigs are green; the tiny, linear leaves are deciduous during dry periods. The plants are dioecious (that is, each individual plant bears only “male”or “female” flowers) and blooms in the fall. The wind- dispersed, white-tasseled seeds are produced by the female plants in such abundance that the plants and nearby ground appear to be snow-covered.

Desert broom (Baccharis sarothroides) male plant

Range

Desert broom grows in desert, desert grassland, and chaparral from 1000 to 5000 feet (300-7500 m) elevation in Arizona, California, Sonora, and Baja California.

Notes

Desert broom is a pioneer plant that is efficient at colonizing newly-disturbed soil. Therefore it is often abundant in washes, roadsides, and abandoned plowed fields. Like many pioneer plants, it is rather short-lived, lasting only perhaps a couple of decades. Desert gardeners may dislike desert broom for its aggressively invasive nature, but it can be useful. As the vernacular names suggest, the thin terminal twigs can be bundled together at the end of a pole to make a broom. The flowers attract hordes of butterflies and other insects. Because it flowers in autumn, it’s a good plant for extending the season of a butterfly garden. Grow male plants (offered by some nurseries), if you wish to prevent unwanted volunteers. Unwanted plants have another horticultural use: desert broom branches create ideal shade to protect newly planted cacti and other plants from sunburn. Several layers of tip branches provide 50-75% shade; the twigs slowly disintegrate over several months and the sheltered plant gradually becomes acclimated to full desert sun. Such redeeming value can be found in some of the worst weeds.

The resinous leaves and stems are rarely eaten, except by jackrabbits during droughts when little else is available.

There are several other species of Baccharis. Waterweed (B. sergiloides, escoba amarga) grows in permanently wet soils. Seep-willow (B. salicifolia, formerly B. glutinosa) commonly forms dense thickets along waterways. Other plants called “broom” are in the legume family (Fabaceae or Leguminosae).

Baileya multiradiata

English name: desert marigold, wild marigold, desert baileya
Spanish name: hierba amarilla (yellow herb)

Description

Desert marigold is an annual or short-lived perennial with white-wooly leaves forming a dense mound about 6 to 8 inches (15-20 cm) in diameter. Flower heads perch well above the foliage on 1 foot long (30 cm) stems. The flat 1 inch (2.5 cm) daisies are bright yellow.

Desert marigold (Baileya multinadiata)

Range

Desert marigold grows mostly on bajadas and in valleys throughout the northern Sonoran, southern Mohave, and northern Chihuahuan deserts. It is one of the most common wildflowers, often abundant on disturbed soils such as road shoulders.

Notes

This is perhaps our most consistent showy roadside wildflower; it blooms in any season after a moderate rain. On hard soils such as caliche, desert marigold is replaced by the somewhat similar Bahia absinthifolia (which has no vernacular name except its genus name). The foliage of bahia is more sparsely distributed; its stems arise from perennial underground rhizomes that form colonies. Bahia leaves are less woolly and the flowers slightly smaller than are those of desert marigold. Bahia’s range extends into pine forests.

Bebbia juncea

English names: chuckwalla’s delight, sweetbush
Spanish names: chuparosa (sucked, e.g., by insects for nectar), junco (generic name for rushes)

Chuckwalla's delight (Bebbia juncea)

Description

Bebbia is an intricately-branched shrub to about 4 feet (1.2 m) tall and wide. The threadlike leaves are absent most of the year; the pencil-lead-thin stems are green and photosynthetic. The cylindrical, yellow flower heads are composed only of disk flowers. Flowering is almost year round, most heavily following rains.

Range

Bebbia is widespread in the arid and semiarid habitats of our region.

Notes

The vernacular name “chuckwalla’s delight” was invented by naturalist Edmund Jaeger, who observed that chuckwallas “feed greedily on the flowers.” Many desert plants have yellow flowers, and chuckwallas seek out and eat most of them.

Encelia farinosa

English name: brittlebush
Spanish names: rama blanca (white branch), incienso (incense), hierba de las animas (herb of the souls), hierba del bazo (spleen-herb), palo blanco (white stick), hierba ceniza (gray herb)

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.

Brittlebush (Encelia farinosa)

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.)

Trixis californica

English name: trixis
Spanish names: plumilla (little feather), arnica

Trixis (Trixis californica)

Description

Trixis is an evergreen shrub 1 to 3 feet (30-90 cm) tall that grows compact and upright in sun, but sparse and floppy when in the shade. The 1- to 2-inch (2.5-5 cm) long, dull green leaves emit a rank odor when bruised. The small yellow flower heads have a spicy fragrance. They appear at any time after a rain, with a peak bloom in spring. Range Trixis grows from southern California to west Texas, and south to central Mexico.

Notes

The Seri regard this plant as a panacea, claiming that there is nothing that this medicinal herb is not good for. Among its wide range of uses, trixis is smoked like tobacco and administered as an aid to childbirth. Trixis is successful in many habitats from low desert on the Colorado River to woodlands at 5500 feet (1700 m) elevation. If enough soil moisture is available, it grows anywhere temperatures remain above freezing. Moreover, it belongs to a subgroup of composites (tribe Mutiseae) that is mostly South American. There are many plant and animal groups with similar distribution patterns—a large number of species in their “home” areas, a few of which become very successful, extending their ranges far beyond their places of origin. There is something special about the biology of trixis, but we don’t know what it is.

Mark A. Dimmitt

The bignonia family has about 725 mostly tropical species worldwide, of which only four occur in our region. Well-known members include catalpa, jacaranda, and trumpet vine (Campsis radicans).

Chilopsis linearis

English name: desert willow
Spanish names: mimbre (wicker), jano

Description

Desert willow is a deciduous small tree to 30 feet (9 m) tall that resembles a willow, with its long, narrow leaves and slightly drooping branches. Large flowers shaped like irregular trumpets range from nearly white in the western end of its range to deep purple with yellow nectar guides (visual signals enticing to bees) at the eastern end. Wild trees bloom from mid-spring through midsummer with sufficient rainfall; selected cultivars may bloom into fall with irrigation.

Desert willow (Chilopsis linearis)

Range

Notes

Large bees are the main pollinators, though the flowers are also attractive to other insects and to hummingbirds.

This is one of the few trees in the north half of the Sonoran Desert that is not a legume. Another plant in this family, Tecoma stans (yellow trumpet bush), attains small-tree size in the tropics, but not in the desert. It is a shrub in the north because it periodically freezes to the ground. Tecoma is one of many examples of the Sonoran Desert’s tropical legacy.

Mark A. Dimmitt

The mustards number about 3000 species worldwide, most of which are herbs. Some important vegetables such as radish, broccoli and cauliflower are in this family. Most Sonoran Desert species go unnoticed except by botanists, because they are inconspicuous even though sometimes abundant. Two attractive wild- flowers are bladderpod (Lesquerella gordoni) and silver bells (Streptanthus carinatus).

Bladderpod (Lesquerella gordonii)

The greater story of mustards in the desert is that some alien species are pernicious weeds. The most disturbing is probably Sahara or Moroccan mustard (Brassica tournefortii). First discovered in the United States only in the 1930s, it has by now invaded most of the dry sandy areas of the Sonoran Desert. Unlike most pioneer plants (weeds) it doesn’t require disturbed soil to invade. In wet years it forms extensive, dense stands that crowd out the native annuals. Because most birds and mammals will not eat it, this mustard’s displacement of native species presumably has detrimental effects all the way up the food web. Many potential wildflower displays have also been thwarted by this one weed. Another alien mustard, London rocket (Sisymbrium irio), is a major pest in disturbed soils and gardens.

Mark A. Dimmitt

The New World elephant trees are nearly unknown to the general public, but nearly everyone has heard of their Old World relatives. The aromatic sap of frankincense (Boswellia sacra) and myrrh (Commiphora spp.) was once worth its weight in gold. It’s still quite valuable, and most wild frankincense trees are sacred, hereditary property and are zealously protected. Our native species are also valuable to those who know them. The tropical species south of the desert are used in products as diverse as incense, masks, and asthma treatment.

The torchwood family contains 550 species of shrubs and trees worldwide, many of which have succulent stems and aromatic foliage. There are about 12 species of Bursera in the Sonoran Desert region, about a third of which occur in the desert.

Bursera microphylla

English name: elephant tree
Spanish names: torote blanco, copal, palo colorado (red stick), torote colorado

Description

This large shrub or small tree grows to 25 feet (8 m) tall; it usually has several contorted trunks and reddish branches. The trunks and main branches are swollen with water-storage tissue and covered with whitish sheets of thin, peeling bark. The thin bark transmits sunlight to chlorophyll-bearing tissue in the stems, which can thus conduct photosynthesis when the plant is leafless. The stems and the finely divided, shiny green leaves are highly aromatic. It may leaf out in any month in response to rain, but stem growth is mainly in summer. The flowers, which open around the time of the summer solstice (the hottest, driest time of the year, when almost nothing else is blooming), are tiny and inconspicuous.

Elephant tree (Bursera microphylla)
branch with fruit

Range

This is a true desert species, occurring in western Sonora and almost all of Baja California with a few marginal populations in south-central Arizona (reaching northern limits in some Phoenix-area mountain parks) and extreme southern California. The plants in some Arizona populations look quite different from Mexican plants and may be hybrids or a new species.

Notes

While the aromatic sap of this and other burseras smells pleasant to humans, it tastes foul and functions as an herbivore deterrent. The sap is under pressure; when a leaf is broken, a thin stream of sap spurts out an inch (2.5 cm) or so. This “squirt-in-the-eye” defense is apparently effective, since the elephant tree’s foliage is nearly always intact.

The bitter fruit stimulates salivation and the Seri sometimes chew it to quench thirst. They used the soft wood to make boats. The sap is used to seal cracks in boats and pottery. The aromatic oils are used in a variety of medicines, including treatments for stingray wounds, lice, cuts, and gonorrhea.

The other desert species of Bursera, except for one in Baja California, are scarcely succulent and not easily confused with B. microphylla. But two unrelated plants look almost identical when not in leaf (which is most of the year). Pachycormus discolor (Baja elephant tree, torote blanco, copalquin) often grows side by side with B. microphylla in Baja California, but responds to rainfall in the winter, when burseras are not actively growing. Pachycormus is in the cashew family (Anacardiaceae). Jatropha cordata (sangregado, mata muchachos) grows next to burseras in Sonora. A member of the spurge family (Euphorbiaceae), J. cordata is more closely related to Poinsettia than to Bursera. These three look-alike species are prime examples of convergent evolution.