The Whitethorn Acacia

Acacia constricta

Whitethorn Acacia shrub near a small arroyo.

Whitethorn acacia shrub near a small arroyo, spreading multi-stemmed branches.

Most often, the whitethorn acacia – a legume, or bean-producing, plant – takes the form of an upright, woody, semi-evergreen shrub with multiple trunks. Typically, it grows from a few feet to 15 or 16 feet in height, often forming dense thickets.

The individual plant may look almost frothy with its small, greenish to grayish lace-like leaves. It often loses its leaves during frigid weather or a prolonged drought. It blossoms in the spring and sometimes again in late summer, bearing miniature yellow puff-ball blooms and producing a sweet nighttime desert fragrance. There are nearly 800 species worldwide.

Distinctive Features

Trunks, branches and twigs: Multi-stemmed and branched with slightly zigzag-shaped twigs, with newer growth bearing a somewhat reddish color and older growth, a grayish brown.

Bark: Fairly smooth on younger branches and twigs and somewhat coarse and grooved or furrowed on older trunks and branches.

Roots: Wide-spreading lateral roots and long penetrating tap root, reaching for both shallow and deeper waters (according to Volney M. Spalding, Distribution and Movement of Desert Plants). Roots harbor bacteria that can take nitrogen from the air and convert it into soil-enriching compounds.

Thorns: Roughly 1/2 inch in length, needle sharp, paired and white, giving the “whitethorn acacia” its common name. More obvious on younger branches and younger plants.

Leaves: Several inches in length, “bipinnately compound,” or “twice compound,” which means that a leaf comprises several pairs of major leaflets, each of which, in turn, comprise several pairs of minor leaflets.

Bloom: 1/4th to 1/3rd of an inch in diameter, bright yellow, dense sphere of stamen, which offer insects minimal pollen and no nectar. As a result, blooms draw relatively few pollinators.

Bean pods: Slender, reddish brown, two- to five-inch-long capsules with distinctive constrictions between the beans, giving the Acacia constricta its scientific name.

Whitethorn acacia's bean pods

Whitethorn Acacia's Feathery Leaves and Ripe Beans

Native Distribution and Habitat

The whitethorn acacia grows naturally in southern Arizona, southern New Mexico, western Texas and northern to central Mexico, inhabiting streamsides, floodplains, intermittent drainages, rocky hillsides and mesas at elevations ranging from 2,000 to 6,000 feet.

Regeneration

The whitethorn acacia sprouts from seeds, but it can sprout from its root crown should its trunks and branches be damaged or killed, for example, by wildfire.

Pollination and Seed Production: The plant receives pollination by bees and other insects, which often visit the short-lived blooms only on the first day they open. Absent sufficient insect interest, the plant can self-pollinate, but its yield of beans will likely be significantly reduced.

Seed Dispersal and Germination: The whitethorn acacia’s seeds are distributed primarily by birds, particularly quail, and by mammals, particularly rodents. Seeds which have had their coatings removed in passing through birds’ or mammals’ digestive tractsgerminate more readily. Some seeds are cached by the rodents below the soil surface, protected from wildfire.

Seed Establishment: Seeds buried a half inch to an inch below the soil surface have the highest likelihood of becoming established seedlings, especially as they germinate during warm temperatures with some moisture. Seeds on the surface may germinate, but they seldom make viable seedlings.

Maturation: With its growth regulated by the availability of moisture, the whitethorn acacia typically matures slowly in its desert environment. Once established, it may live for seven decades or more.

Interesting Facts

The whitethorn acacia belongs to one of the largest families of shrubs and trees - Fabaceae, the pea family. It contains about 400 genera and 10,000 species.

In a classic example of nature’s mutually beneficial relationships, acacias in Central America and South Africa provide ants with seeds that bear a nutritious fatty body called elaisome or aril. The ants carry the seeds to their nests, eating the elaisome and discarding the unharmed seed, but effectively planting it for the acacia, contributing to its spread. Both the acacia and the ant benefit.

Some acacia species produce a wood so hard and durable that it has been used as shipbuilding nails, wagon axletrees, and fence posts. Reports say that acacia fence posts sometimes lasted for up to a century.

Native American Uses

Prehistoric Americans ground the whitethorn acacia beans into a meal for use in food preparation. They used the beans, leaves and roots to brew a medicinal tea with which they treated stomach and respiratory disorders. They used powdered beans and leaves to make a poultice for treating skin rashes. They used the flowers and leaves to make a medicinal tea for relieving hangovers.

Modern desert inhabitants plant the whitethorn acacia in gardens, knowing the plant requires relatively little water, attracts wildlife (for instance, quail, dove, pyrrhuloxias and other birds), and enriches the surrounding soil.

By Jay W. Sharp

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