Desert Ironwood

Olneya tesota

About the Plant

Desert ironwood is one of the largest and longest-lived plants in the Sonoran Desert. The boundaries of its natural distribution match almost exactly those of the Desert, where it grows at the warmer, lower elevations. In the Tucson area it is found in the western mountains and is especially abundant in Ironwood Forest National Monument and Saguaro National Park-West.

Desert ironwood is a slow growing tree that can eventually reach to 30 feet tall and wide. It can tolerate long periods of drought but irrigation in summer can increase its growth rate. This tree should not be planted at higher elevations or in frost pockets, as prolonged freezing can kill it. Branches may be damaged at about 20°F. Attractive purple and white flowers appear on larger trees in May followed by rapidly maturing fruit. Desert ironwood can function as a shade tree. Prune lower branches only in the cool months to avoid sunburn on newly-exposed branches.

Wildlife value:

Desert ironwood is a habitat-modifying keystone species, influencing distribution and abundance of over 500 associated species. It nurtures a wide array of plants under its canopy, provides roosts and nesting sites for birds, and protein-rich leaves and seeds for animals. For a fuller discussion of its important in the Sonoran Desert see Biological Survey of Ironwood Forest National Monument and Ironwood from Sonoran Desert Conservation Plan website.

More Information

Weekly Plant on desert ironwood

Horticultural information from ASU

Map of natural distribution

Technical botanical description from SEINet

In books:

Native Plants for Southwestern Landscapes by Judy Mielke, page 202

ID Characteristics

This plant is in the Fabaceae - the legume (pea and bean) family.
Desert ironwood is a large, multistemmed tree that can grow 30 feet tall and spread 30 feet wide. Young plants are dense with many stems. Older trees can be pruned to remove lower branches and excess stems. Upper branches may droop and can be pruned back to maintain clearance height if needed. The trunk is grey and becomes fissured with age.  Desert ironwood can function well as a shade tree and produces relatively little litter.
There are two curved stipular spines at the base of each leaf. They are quite sharp and can catch on clothing. They can make pruning awkward as there is no safe place to grab onto a shoot. Small, shrubby forms of desert ironwood work well as a barrier plant.
The leaves are alternate, though they may be clustered together. The pinnately-compound leaves may be up to 4 inches long and 1 inch wide, but are often smaller. Their grey-green color gives an overall grey appearance to the tree. (Note: the small white object on a stalk in the lower center of the photo is a lacewing egg. Lacewings are beneficial insects that help control insect pests.)
The flowers are held in loose clusters on short shoots along the branches. The flower is the typical pea flower, white mixed with shades of purple. Flowering usually occurs in May in the Phoenix and Tucson area and lasts for 10-18 days.
The pods are about 2 inches long, slightly constricted between seeds. They mature 4-8 weeks after pollination and seeds are released during the summer rainy season. The seeds are protein-rich and relished by larger birds and mammals.