Acacia crassifrugis Tindale & Maconochie

Acacia pachycarpa is a small, single-stemmed tree or shrub with several stems, 3-4 (6 max.) m tall; pendulous branches and, on more favourable sites, a large crown; bark on the trunk is furrowed, hard and dark grey, becoming smooth and light grey on upper branches. Phyllodes are elliptical to linear, long and narrow, 15-23 cm in length, 6-20 mm wide, lax, more or less curved, with 1-5 more prominent longitudinal nerves. Creamy white flowers in spikes 3-3.5 cm long on a short peduncle, 4-20 mm long. Pods are swollen when fresh, narrow oblong, 7-15 cm long, 1-2 cm wide, woody, yellowish-brown, more or less shiny, straight or twisted, convex over the seeds but not strongly constricted between them, often breaking between the seeds; seeds are transverse in the pod, smooth, shiny, somewhat variable in colour from pale brown to black, 6-9 mm long, 5-7 mm wide, 3-4 mm thick, with a small, white, terminal aril. The generic name ‘acacia’ comes from the Greek word ‘akis’, meaning a point or a barb. The specific name is from the Greek words ‘pachys’, meaning thick or stout, and ‘carpus’, meaning from fruit, referring to the appearance of the pod. A. pachycarpa is related to A. cuthbertsonii; the latter has hairy branchlets, smaller phyllodes and flower heads and a gland on the phyllode margin up to 3 mm from the base.

A. pachycarpa is hermaphroditic. Flowering is variable, and in Australia mature pods have been observed from March to October.

Grows on alluvial and deep red siliceous sands, alluvial loams and heavy black and red cracking clays.

A. pachycarpa occurs in hot, arid climate zones. It is usually found as single shrub communities on heavier soils along drainage lines. It is confined to the Western Plateau physiographic division in Australia, where it is found in the plains along drainage lines, in clay depressions or on clayey flats. The tree is frost intolerant, and this prevents its occurrence in cold, arid areas. In the Great Sandy Desert in Australia, it forms thickets in or overtopping hummock grasses, and with increasing rainfall, it becomes taller and denser, attaining heights of 8 m in the wettest climates and 3 m in the driest climates. It is also found on the coast of Australia. Although reportedly tolerant of some degree of salt in the soil in its natural habitat, it ranked amongst the less tolerant species of a range of acacias screened for salt tolerance in a glasshouse trial. It has been recorded growing with Acacia stenophyla, A. victoriae, Atalya hemiglauca, Eremophila bignoniiflora and Senna oligophylla. In the wetter parts of the range, it is associated with eucalypts, especially E. dichromopholia, E. setosa and E. victrix.

A. pachycarpa has been cultivated in trial plots in Australia and Kenya, with disappointing survival rates. As only a few provenances have been available for trial, it is possible that the best material has not yet been tested.

A. pachycarpa grows relatively fast in its natural habitat. It produces large seed crops and regenerates readily from seed. Trees produce basal shoots and root suckers after fire. Seeds require presowing treatment with boiling water or mechanical scarification to break seed coat dormancy. Germination rate averages 74%.

Plant communities where A. pachycarpa occurs are regularly burned so that the composition of the stands varies from year to year. Burning does not kill the Eucalyptus species but almost completely removes acacias that regenerate from seed.

There are about 5000-7000 viable seeds/kg.

Shade or shelter: A. pachycarpa can serve as a windbreak. Nitrogen fixing: A. pachycarpa is nodulating and is able to fix nitrogen. Ornamental: The pendulous habit of A. pachycarpa makes it a potential ornamental plant for arid areas, but its phyllodes, branches and flowers produce an unpleasant odour, so that planting it close to human habitation is undesirable.

Food: The seeds are nutritious, containing 22% crude protein, 8% fat and 57% carbohydrate. The Aborigines of Australia reportedly used them as a staple food. Fodder: In its natural habitat, A. pachycarpa is heavily grazed by animals and has potential as a good fodder species. Fuel: The tree has not been used for fuel but has the potential to produce small-sized firewood.