Olneya tesota, the Desert Ironwood, is the signature tree of the Sonoran Desert, and the perfect tree around which to create a desert landscape. The Ironwood will grow at a slow to moderate rate to 25 feet wide and 25 feet tall, and eventually taller. Just now, in mid-May, mature Ironwoods produce a mass of tiny lavender-and-white sweet-pea-shaped flowers (above); they're not much at a distance, but up close the flowers are beautiful. The medium gray-green foliage is evergreen and fine-textured, casting a filtered shade; to my eye the overall look is reminiscent of an olive tree. The Ironwood can take full to reflected sun and very low water.
The Ironwood has a reputation for slow growth that isn't justified. In the wilds of the desert it grows painfully slow, but with a small amount of supplemental irrigation in the domestic landscape, it's not markedly slower than any other desert tree. As with all desert trees, the key is deep but infrequent irrigation.
Olneya tesota does have small thorns on the young branches, but these are subsumed in the bark on older branches and the trunk; you'd really have to work at it to hurt yourself with an Ironwood.
The Ironwood is the largest tree in the Sonoran Desert and acts as a "nurse plant" to many other species, providing shade for young Saguaros and other cacti to get their start in life. It's common name comes from its extremely dense and heavy wood, which is used for carvings. The botanical name Olneya is in honor of a 19th Century American botanist named Stephen T. Olney; tesota is an indigenous name for the tree.
Ironwood should only be pruned in the cooler months and then only lightly. It has a distinct habit of forming clusters of long upright branches when the crown in pruned or thinned with any but the lightest touch, and not just where the cuts have been made, but all over the tree. These upright branches wreck the form and shape of the tree. Left to its own devices it will grow into a graceful and spreading shape. It looks best when allowed to grow as a low-breaking or multi-trunked tree, and most pruning should be done gradually over a period of years to shape and direct the growth, or to raise the canopy.
Use Olneya tesota as a signature tree, or as the foundation plant for a low-water but lush-looking desert landscape; it provides the perfect canopy of filtered shade for many species of desert-adapted plants, and it's fine-textured gray-green leaves make a perfect backdrop for some of the bolder and darker-green agave and cactus. An Ironwood may cost just slightly more than your average nursery plant, but with a life expectancy of up to 1500 years, on a pro-rated basis it's practically free.