Published April 2, 2022, McAllen Monitor.
Story and photos by Anita Westervelt, Texas Master Naturalist, South Texas Border Chapter
Blackbrush (Acacia rigidula), overlooked much of the year and often mistaken for a short version of Texas Ebony, in spring, dominates the thronscrub in all its blooming glory.
A prolific bloomer in spring, clusters of fragrant three-inch long blossom spikes contrast starkly with the dark bark of the branches. Blooms appear before the leaves are fully out. The creamy white to golden-yellow flowers are a source of honey. Bees, butterflies, moths and other nectar-insects benefit from the flowers.
Considered a large shrub, blackbrush is multi-stemmed, thorny, with stiff paired spines that can reach to two inches long on rigid, zigzag branches. The shrub can reach heights of 10 to 20 feet and spread nearly as wide.
The leaves are dark green, heavily ribbed, and twice compound, which means they are separated into two or more distinct leaflets, similar to leaf groupings of brasil and Texas ebony; it is a typical growth pattern of legume family plants, to which blackbrush belongs.
The bark of the trunks is smooth and tight and a mottled light and dark ashy gray; it turns darker as the tree ages and nearly black when wet; hence the name blackbrush.
The tree is adaptable to many soils from sandy, to loam, clay and caliche, to dry sand and limestone as long as the soil is well-drained. It prefers full sun and can tolerate partial shade; this is an excellent specimen for xeriscapes and rock gardens. It is slow-growing and long-living.
Once the blossoms are spent, seedpods appear. They are a legume about two to four inches long, narrow, curved and flat; reddish brown when ripe. The seed pods help to distinguish blackbrush from Texas ebony when flowers are not present. Texas ebony produces thick, dark brown woody pods six to eight inches long.
Blackbrush is a multi-purpose addition to forest and garden alike for wildlife; many birds and small mammals rely on it for food, cover and protection. Chachalacas and northern bobwhite quail and other granivorous birds eat the seeds. Long- and curved-billed thrashers, cactus wrens and scissor-tailed flycatchers have been known to nest in the dense branches of blackbrush.
It is a larval food plant for some butterflies and moths in its range: Mexican yellow, mimosa yellow and Reakirt’s blue butterflies and the black witch moth, merry melipotis, mesquite stinger flannel and the orangeworm moths.
Chaparral and thrornforest are blackbrush’s traditional habitat in the middle and lower Rio Grande Valley and in west Texas to the lower Pecos Valley in New Mexico to southeastern Arizona and into northern and central Mexico.
Vegetation cover, climate, soils and geology help determine ecoregions: Thornscrub is a combination of diverse drought-tolerant vegetation that includes small-leaved, thorny and woody species, ranging in height from about three to 20 feet, interspersed with savannas; chaparrals are dense impenetrable thickets of shrubs or dwarf trees adapted to dry summers and moist winters; savannas are grassy flatlands without many trees, in tropical or subtropical regions.
From a distance, the white blooming blackbrush is easily identified in the landscape.
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