Ironwood Trees in the Landscape

Ironwood Trees in the Landscape

Rick Gibson

Cooperative Extension Agent, Agriculture

Pinal County, Arizona

Let me give you a common scenario that I see way too often. 

A homeowner wants to lower his or her monthly water bill and decides to do it by cutting back on outdoor landscaping.  In a moment of decision, the outdoor irrigation system is shut off and, just like that, the water bill begins to drop.  Unfortunately, a few weeks or months later, the plants begin to turn yellow and die. 

As the plants begin to shed leaves, the amount of shade provided by the plants begins to diminish and sunlight begins to break through onto graveled yard surfaces and structural walls.  The increased sunlight raises the temperature of these hard surfaces.  With increasing heat radiating back into the environment, the home owner begins to notice an increase in the power bill as the air conditioning unit works overtime to keep the home cool.  Seeing the electricity bill, the homeowner recognizes the problem, taps the head and says, “What was I thinking!”

Trade offs between water and power bills are not uncommon as we struggle to balance between our need to conserve precious resources and the rigors of every day life.  However, we do not have to settle for a bare, unattractive yard just to save money.  By using plants that get by with a minimum of water, we can lower water and electricity bills and still have pleasing outdoor living areas.

Because of their size, trees use a large percentage of the water in a landscape so one area where we can cut back on water is to choose  water stingy trees for landscape use.  One of the stingiest of low water use trees is the local desert ironwood, Olneya tesota.  It is a hardy, well-adapted, handsome plant that works exceptionally well in a low water use landscape.  It takes full sun, has many landscape uses, and gets by on very little water.

A member of the legume family, it has the typical compound leaves of the mesquite and palo verde family.  A compound leaf is one that has a central stalk, called a rachis, and along the stalk are little leaflets that together form the leaf.  These leaflets are a striking gray-green color and are typically larger than the mesquite leaflets.  The dense canopy gives the ironwood top billing in my book as a screening and shade tree in the landscape.  Be careful, though.  At the base of each leaf, not leaflet, there are two sharp-pointed spines that can easily prick a finger.

The tree flowers in the late spring in a showy display of pinkish-lavender sweet pea-shaped flowers, each about one-half inch long.  I think the tree is absolutely gorgeous in full bloom.  Just before flowering, the tree will give you heart failure as it sheds most of its leaflets.  You would think that the tree is dying just to look at it, but not to worry, it is a normal part of the growth pattern for the tree.

The flowers turn into dark brown, fuzzy, two inch-long pods that split open at maturity and let the seeds fall to the ground.  The seeds are highly prized by small mammals in the desert for food.  They have also in times past been collected and used for food by humans.

Ironwood trees, depending upon soil and water conditions, will grow to between fifteen and thirty feet tall and the same dimensions wide, so in landscape situations it is important to give them enough room to grow.  The spines, like many desert trees, can make them formidable so do not plant them next to walkways or other high traffic areas. 

The wood in the center of the branch or trunk, as aptly described by its common name, is extremely hard.  The wood has been used in the past for arrow heads and other tools.  The cured wood is also highly prized as a cooking fuel.  It burns hot and Dutch oven enthusiasts appreciate the hot, long lasting coals.  The wood is also used by artisans for carving and other uses.  The Seri Indians of Mexico have developed quite an art form for their carvings.  The wood is heavy, weighing up to 66 pounds per cubic foot.  I know of an ironwood dining room table, over eighty years old, that is so heavy that it took a crew of men to move it into place. It has never been moved since!

As a landscape tree, the ironwood has many uses.  Because it can be either a single or multi-trunk plant, it can be developed to fit into that special spot made difficult because of angles or corners. It is possible to plant a whole forest of ironwood trees and no two would be the same.  It may take some careful pruning, especially early on, to develop and maintain a desired form, but it will be worth the effort later on.  Don’t worry if the branches tend to grow up while the tree is young, they will begin to spread as the tree matures.

In the wild, ironwood trees grow near washes or dry stream beds where deep water is usually available.  This means that it will require some water to do well, a deep irrigation once or twice a year at maturity will probably be enough, but the naturally slow growing tree will hasten its growth with more frequent irrigations, especially during the summer.

Beside its trunk shape, the ironwood also has other characteristics that make it valuable as a landscape plant.  Its dense canopy of leaves makes it ideal for screening, background, and shade uses.  Its flowers provide spring color.  It is excellent as a thorny barrier to reduce traffic flows or for security, and its striking appearance makes it valuable as a specimen plant.

The tree requires full to reflected sunlight so it should be placed in a location where it will not be shaded by other plants.  It performs well under harsh growing conditions but it is frost and freeze sensitive so it should be planted in regions where winters are warm enough for citrus.  Since it prefers a soil with good drainage, it is not a good choice when you are dealing with a clay or caliche soil.  

If you intend to plant your ironwood tree from seed, it will be important to scarify the seed either by running them through a rock tumbler filled with sand or by using a file to cut through the hard seed coat.  Insects usually are not a problem but mistletoe infestations can distort and weaken branches.  Mistletoe seeds are usually spread from tree to tree on the feet of birds so most infestations occur most frequently on larger trees that are apparently more attractive to the animals.  Watch for signs of mistletoe plants and cut them out early.

The ironwood tree offers many benefits as a low water use landscape tree and can provide color, texture, shade, and screening to any landscape.

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Rick Gibson
University of Arizona Cooperative Extension
820 E. Cottonwood Lane, Building C
Casa Grande, AZ 85222
(520) 836-5221 ext. 227
(520) 836-1750 - Fax
gibsonrd@ag.arizona.edu