Shrubs or trees, erect, to 4 m; bark light gray to brown, smooth or shallowly furrowed. Stems slightly flexuous, glabrous or sparsely puberulent, not glu­tinous; short shoots present. Leaves 2–9 mm; stipular spines terete, straight, 10–50(–80) × 0.6–2.1 mm near base; petiole 2–5(–9) mm, densely puberulent; petiolar gland 1, mostly medial on petiole, sessile, circular, 0.3–0.6(–1.1) mm diam., apex depressed; rachis 0 mm; pinnae 1 pair, 5–20 mm; leaflets (2 or)3–5 pairs per pinna, blades flat, oblong to elliptic, (4–)6–13(–16) × 2–6(–8) mm, base oblique and obtuse, apex obtuse, surfaces usually glabrous, rarely sparsely pubescent abaxially. Peduncles 0–3 mm. Inflorescences cylindrical spikes, loosely flowered, 10–35 mm, solitary or clusters of 2–8 on short shoots; involucre at base of peduncle. Flowers white to cream; calyx 0.4–0.8 mm, puberulent; corolla 1.2–1.7 mm, usually glabrous; filaments white, 2.7–4 mm. Legumes linear, flattened, 40–100 × 3–6 mm, constricted between seeds. Seeds uniseriate. 2n = 26.

Phenology: Flowering Feb–Jul.
Habitat: Disturbed sites, rocky slopes, bluffs, fencerows, arroyos, in extensive thickets.
Elevation: 0–1400 m.

Distribution

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Tex., Mexico (Coahuila, Jalisco, Michoacán, Nuevo León, Querétaro, San Luis Potosí, Tamaulipas, Veracruz).

Discussion

Vachellia rigidula has been documented in southern Texas from Brewster County in the west, southeast to Calhoun County, and south to Mexico.

The name Acacia amentacea de Candolle has sometimes been proposed for this taxon. The type of A. amentacea is a painting, represented by plate no. 208 of de Candolle. The species represented by this drawing is probably what is now called Vachellia bilimekii (J. F. Macbride) Seigler & Ebinger, a species known from southern Mexico. Without a specimen, no accurate determination can be made (Y. S. Lee et al. 1989). Plants of V. rigidula occasionally harbor ants in the spines (Lee et al.). This does not appear to represent a highly co-adaptive interaction such as exists in other, better known ant acacias, for the ant species is found on a number of unrelated plants (Lee et al.).

Vachellia rigidula hybridizes with V. bravoensis (V. × ruthvenii); it is an uncommon hybrid found only at the Chaparral Wildlife Management Area in Dimmit County, Texas.

Selected References

None.

Lower Taxa

None.