Olneya spp.

desert ironwood

Fabaceae Sonoran Desert of North America

HardinessZones 9–11

Overview

Olneya is a monotypic genus in the family Fabaceae, represented by the single species O. tesota, the desert ironwood, native to the Sonoran Desert of the southwestern United States and northwestern Mexico. It is a slow-growing evergreen to semi-evergreen tree reaching 5-10 m (16-33 ft) tall and as wide, with a short trunk, grey bark, and a spreading, often multi-stemmed crown. Paired spines sit at the base of the bluish-grey, pinnately compound leaves, which the tree may shed in severe drought or hard frost. In late spring, usually May to June, clusters of pea-shaped flowers in pale lavender to pink-purple cover the branches, followed by constricted seed pods holding shiny brown seeds. The wood is dense enough to sink in water, and dead trunks persist for centuries in the desert. As a long-lived nurse plant, the tree shelters seedlings of cacti and other desert species beneath its canopy and supports many desert animals. The genus is restricted to frost-tender desert habitats below about 1,000 m (3,300 ft) and is sensitive to prolonged cold.

Native Range

Olneya tesota is native to the Sonoran Desert of Arizona, southeastern California, Sonora, and Baja California, growing along desert washes, bajadas, and valley floors below about 1,000 m (3,300 ft).

Suggested Uses

Olneya tesota is used as a shade and specimen tree in desert and xeriscape gardens, as a nurse plant in habitat restoration, and along desert streets where its low water needs suit the climate. Its dense wood has been used for carving and tool-making.

How to Identify

A spreading desert tree 5-10 m (16-33 ft) tall with grey bark, paired spines at the leaf bases, and bluish-grey pinnately compound leaves. In late spring it carries clusters of pale lavender to pink-purple pea flowers, followed by constricted pods; the very hard, dense wood and persistent dead limbs also help separate it from associated desert trees.

Appearance

Size & Dimensions

Height16' - 33'

Width/Spread16' - 33'

Colors

Flower Colors

Foliage Colors

Fall Foliage Colors

Bloom Information

Bloom Period

Spring

Flowering occurs in late spring, mostly May into June, when clusters of pale lavender to pink-purple pea flowers open across the crown. Flowering can be reduced or skipped in drought years, and pollinated flowers form constricted pods that ripen to brown by early summer.

Detailed Descriptions

Flower Description

pale lavender to pink-purple

Foliage Description

bluish-grey-green

Growing Conditions

Sun Requirements

SunFull Sun

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range7.0 - 8.5(Alkaline)

357912

Soil Types

sand rocky loam

Drainagewell drained

Water & Climate

Water Needs

Very Low

Frost Tolerance

tender

Time to Maturity

slow; several decades to full size

Drought Tolerance

Drought tolerant when established

Care & Maintenance

Care Guide

Olneya tesota grows in full sun in deep, free-draining desert soils, including sandy, gravelly, and rocky ground, with a neutral to alkaline pH around 7.0-8.5. It is highly drought-tolerant once established, drawing on a deep root system, and needs little to no irrigation in suitable climates. The tree is frost-sensitive, with young plants damaged below about -4 C (25 F) and limited to USDA zones 9 to 11. Growth is slow, and plants may take many years to reach flowering size. Excess water and poor drainage shorten its life.

Pruning

Pruning is done in the warm season to remove dead, crossing, or low branches and to lift the canopy on young trees. The spines make close work awkward, and heavy cutting is limited because the slow growth replaces removed wood gradually.