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For The Garden

Landscaping With Legumes

Author: Cathy Babcock
Issue: May, 2017, Page 116
Photo by Garrett Cook

Texas mountain laurel
Familiar desert plants are also natural food sources

Many think of legumes as plants that produce edible beans, pods and seeds, including peas and lentils. You may not be as familiar with the leguminous plants found in your own neighborhood. While their pods might not produce foods you’ll want to consume, legume trees and shrubs are prized for their shade and colorful flower contributions to the Southwestern plant palette. 

Plants in the legume (Fabaceae) family are prevalent in the Sonoran Desert and, in fact, most of our common desert trees north of the Mexican border belong to this family. Legumes that hail from the arid areas of South Africa, South America and Australia also do well here as they are all drought-tolerant.

So why should you include legumes in your landscape? Many have the ability to fix nitrogen in the soil, making fertilizers unnecessary. Because they originated in arid tropical regions and can tolerate our hot summer temperatures, once established, they need little irrigation.

There is a plethora of leguminous plants used in Arizona landscapes, and one would be hard-pressed to find a garden that didn’t include at least one. Mesquites, ironwoods and palo verdes are the most common trees and provide the backbone of many garden designs.

Several species of mesquites are available, all of which provide filtered shade under which succulents or shade-loving plants can thrive. The velvet mesquite (Prosopis velutina) also has culinary attributes, producing pods that can be ground into flour for baking. Mesquite flour is high in protein, fiber and minerals and low in fat, serving as an excellent gluten-free flour replacement. Pods should be harvested when ripe but before they fall to the ground. 

Palo verde trees (Parkinsonia spp.), known for their lime green bark, make for attractive specimens, especially the palo brea (P. praecox). The foothills palo verde (P. microphylla) has culinary uses. The seeds of this tree can be ground into flour, while the green pods can be cooked and eaten. The flowers are also edible. 

Conversely, the Texas mountain laurel’s (Dermatophyllum secundiflorum) brilliant red seeds are highly poisonous. However, as a specimen planting it can’t be beat with its large clusters of purple flowers in the spring that smell enticingly like grape soda. Green- and silver-foliaged options are also available. 

Ironwood (Olneya tesota) makes a stunning landscape tree with light purple flowers and gray leaves that give it a smoky look from a distance. When watered regularly in its early years, it is actually a fairly moderate grower.

For an alternative to the somewhat ubiquitous mesquites and palo verdes, three underutilized but no less attractive legumes that should be considered for your landscape are the blackbrush acacia (Vachellia rigidula), Texas redbud (Cercis canadensis v. texensis), and kidneywood (Eysenhardtia orthocarpa). 

Blackbrush acacia is an attractive tree because of its deep green leaves and fragrant pale yellow spikes of flowers. It is ideal for residential gardens because it only reaches a height of about 20 feet. This thorny evergreen tree wants to be multitrunked but can be trained to a single trunk.

he Texas redbud tree greets you in the spring with a profusion of dark pink flowers before it leafs out. The flowers attract hummingbirds, and its round, wavy-edged, glossy green leaves make it a pleasant sight long after flowers are spent. These trees are fairly small, reaching a height of up to 20 feet, and are deciduous and thornless.

Kidneywoods are shrubs or small trees 12 to 18 feet in height. They are fast growers and produce fragrant white flowers in the spring and bloom sporadically through the fall. This thornless plant makes a nice small patio tree because of its upright form and lacy canopy.

Other common legumes you will encounter are fairy dusters, various acacias, Texas ebony, bird of paradise shrubs, orchid trees and senna species. Although there are thornless varieties of some of the above trees, there are, as yet, no podless cultivars. To see mature specimens, visit the state’s several botanical gardens and arboretums.

The Desert Legume Program (DELEP), which is jointly sponsored by the Boyce Thompson Arboretum and the University of Arizona College of Agriculture and Life Sciences, distributes seeds from its seed bank to people throughout the U.S. and around the world who are working with these plants on a variety of potential uses, including identifying potential landscape prospects. According to the program’s director Matt Johnson, “There’s a legume for virtually any landscape application; their diversity is astounding.” DELEP is directly responsible for several plant introductions here in the Valley, including the kidneywood. Learn more at cals.arizona.edu/desertlegumeprogram.

Cathy Babcock is the director of horticulture for Boyce Thompson Arboretum, located in Superior.
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