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       in the Sonoran
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Master Gardener Journal  


M E E T   T H E   N A T I V E S



The Ironwood: Stately Sanctuary in the Sonoran Desert

by Cathy Rymer, Master Gardener, Water Conservation Specialist, Town of Gilbert

BOTANICAL NAME
Olneya tesota

COMMON NAMES
Desert Ironwood, tesota, palo de hierro (iron stick), palo de fierro or palo fierro

Although I've lived in Arizona all my life, I hadn't paid much attention to ironwood trees until one warm day several years ago. It was May, the month that ironwoods bloom. I was traveling north on the Beeline Highway at 60 miles an hour, my attention caught by the trees' blurry pink flower display zooming past on both sides of the road. I decided I had to stop for a closer look.

A pull-off area just north of the Saguaro Lake turnoff provided a perfect place to satisfy my curiosity. I got out and spent time inspecting the trees' ghost-like bark, gray-green leaflets, and lavender-pink flowers. By the time I was ready to resume my trip their stately presence had captured my heart, and I have loved ironwoods ever since.

The ironwood is one of the largest and longest-lived Sonoran Desert plants, growing 45 feet tall and living as long as 1,500 years. They are often referred to as a keystone species; those which "enrich their ecosystem in a unique and significant manner" and whose "effect is disproportionate to their numerical abundance." The state's largest known ironwood can be found in Child's Valley in western Pima County.

CLASSIFICATION & RANGE
The ironwood tree is the sole member of the genus Olneya and is named for the dense, dark heavy wood it produces. Found only in the dry regions of the Sonoran Desert below 2,500 feet, where freezing temperatures are uncommon, the ironwood's habitat is almost an exact match of the Sonoran Desert boundary. (http://www.pima.gov/cmo/sdcp/sdcp2/edu/iw/map.html)

Ironwoods are most common along dry ephemeral washes, where they provide a fertile and sheltered habitat. As the tree matures, the environment beneath it is modified, creating a microhabitat characterized by less direct sunlight, reduced surface temperatures, increased organic matter, more available water, and protection from hungry critters. A member of the Fabaceae family, these legumes indirectly add nitrogen (a critical nutrient often missing in our desert soils) to the earth around them. Their seeds provide a protein-rich resource for doves, quail, coyotes, and many small rodents. Air temperatures may be 15 degrees cooler under the dense canopy of ironwoods than in the open desert sun. The ironwood also shelters frost-sensitive young saguaros, organ pipe cactus, and night-blooming cereus.

Growing taller than most trees in the desert scrub, hawks and owls use its exposed branches as perches and roosts. Its canopy is utilized by nearly 150 bird species; 63 percent more birds than creosote, cactus and bursage alone could support. In addition to the birds, there are 62 reptiles and amphibians, and 64 mammals that use ironwoods for forage, cover, and to raise young. At just one site in the Silverbell Mountains, an ironwood-bursage habitat also shelters some 188 kinds of bees, 25 ant colonies, and 25 other types of insects.

If that weren't enough, more than 230 plant species have been recorded starting their growth within the protective microclimate under the ironwood, giving it the title of "nurse tree of the desert." A propagation site for wildflowers is created under the canopy that, in turn, is foraged by rabbits, bighorn, and other native species. This all adds up to an extraordinary level of biodiversity and the distinction of being a keystone species.

DESCRIPTION
Ironwoods are highly adapted to the hottest environments in the Sonoran desert. The tree's natural growth habit is multi-trunked, with branches forming a broad canopy that touches the ground. The bark is gray and smooth, becoming fissured and shaggy on older limbs and trunks. Painfully sharp, slightly curved paired spines, 1/4 to 1/2 inches long, occur at the base of each leaf. The once-pinnate leaves are up to 2 inches long with 6 to 20 grayish green leaflets. Finely haired leaflets are 1/2 to 3/4 inches long. Foliage is semi-evergreen with leaves dropped in response to long drought or freezing temperatures. Flowers range from pink and pale rose-purplish to white, with a sweet pea-like appearance. After flowering in late spring, the resulting pods mature by early summer, with each 2-inch pod containing 1 to 4 shiny brown seeds that are relished by many Sonoran animals.

PROPAGATION & CARE
In its native setting, the hard coating on the seeds passes through the digestive system of deer or cattle. While scarification is thought necessary for seed germination, one source reports success by simply soaking seeds for 24 hours and planting in soil or medium that is thoroughly warm. Seeds should be planted at a depth of about twice the seed diameter. Most germination should occur within one week. Irrigation should keep soil moist but never soggy. Transplant into the ground from May to September. Some growers are using vegetative propagation (cloning) to ensure consistency of both desirable physical qualities (branching habits, leaf color, leaf canopy, and flower color) and sound horticultural characteristics (rooting, cold hardiness and growth rate).

LANDSCAPE USES
Ironwood's dramatic character can create a focal point in entries, or as a signature tree in high-visibility areas. On commercial properties, trees salvaged from desert construction sites are transplanted and incorporated into landscapes. These natural forms are excellent in plantings that transition landscape back to undisturbed desert or when used as a security planting. While not considered a fast grower, in xeric landscape situations they will compete with trees like the blue palo verde, growing about two-thirds as fast. If you are considering creating an urban wildlife habitat, the ironwood is a perfect choice. Because they drop a relatively small amount of litter, they are ideal for poolside landscapes. As their popularity has increased, nursery-grown trees are more readily available in a variety of sizes. Note that ironwoods are not recommended for lawn areas because they could be over-watered.

HISTORY, FOLKLORE & USES
Ancient Hohokam made digging sticks of durable ironwood to retrieve tuberous roots. Native Americans have used the seeds of Olneya tesota for food for centuries. Fresh, uncooked seeds are said to have a taste similar to soybeans or peanuts. The Seri Indians of Sonora, Mexico, cooked the seeds in water, rinsing twice, and ate them whole, or ground and salted. Roasted seeds have been used as a substitute for coffee. The wood is so dense that it will not float, and so hard that it has been used for bearings. Dead trees can remain standing for a millennium. One cubic foot of ironwood can weigh up to 66 pounds.

STATUS
The ironwood is widespread within its historical range in Arizona. However, populations in Sonora, Mexico are suffering from the conversion of desert scrub to agricultural use. Woodcutting for fuel and charcoal has further contributed to ironwood losses. The Seri and other artisans depend on the wood for crafting stylized figurines to sell to tourists. Due to the disappearance of ironwood, they now find it difficult to sustain their livelihoods. Arizona's native plant laws prohibit transportation of ironwood, but do not require its preservation in place. In Pima County, the ironwood has been identified as a habitat associate for the endangered cactus ferruginous pygmy-owl. Urbanization in northwest Tucson is resulting in the loss of these trees. As a result, Pima County's Native Plant Preservation Ordinance requires that a minimum of 80 percent of "specimen" ironwood trees must either be preserved or transplanted during development.

IRONWOOD NATIONAL FOREST
To protect one of the richest stands of ironwood trees in the Sonoran Desert, the Ironwood Forest National Monument was established in June of 2000 by presidential proclamation. It is located 25 miles northwest of Tucson and covers 129,000 acres of cultural and historical sites. (http://azwww.az.blm.gov/ironwood/ironwood.htm)

You can preserve a piece of the Sonoran Desert by adding an ironwood to your landscape. I have never regretted purchasing a 36-inch boxed tree when landscaping my back yard nearly four years ago. It has doubled in size in that time and become home to a pair of verdin that raised three broods of young. This past spring an Inca dove decided to build her nest on top of the verdins' - a sort of penthouse arrangement. They must know they're safe from our family cat in its thorny branches. Yes, you might say I'm infatuated, something that started with that trip up the Beeline years ago.

"By keeping ancient ironwoods alive, we maintain the oldest medicine show, native wildlife menagerie and migratory pollinator bed-and-breakfast in town. These hardy old trees provide ideal habitat for everything from night-blooming cacti to tree lizards, desert bighorn and cactus owls. The list of residents living under a 45-foot ironwood reads like the Who's Who of the Sonoran Desert."
- Gary Nabhan

Photograhy: Candice Sherrill


Maricopa County Master Gardener Volunteer Information
Last Updated July 28, 2003
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