Harrisia gracilis grows shrubby, is richly branched and forms groups with upright to spreading shoots. The green, cylindrical shoots are greatly elongated. They have diameters of 2.5 to 4 centimeters and reach lengths of 3 to 5 meters. There are nine to eleven ribs. The seven to 16 thorns are spread out, straight, strong and needle-like. They are grayish ocher or gray, almost completely covering the shoots and are up to 2.5 centimeters long.
The flowers reach a length of 15 to 20 centimeters and a diameter of 10 to 12 centimeters. Its flower tube is covered with soft, white hairs. The yellow, smooth fruits are covered with a few hair-like thorns. They have a diameter of 3 to 6.2 centimeters and reach a length of 3 to 4 centimeters.[2]
It was first described as Cereus gracilis in 1768 by Philip Miller.[4] The specific epithet gracilis means 'delicate, slim, thin'. Nathaniel Lord Britton placed the species in the genus Harrisia in 1909.[5] Other nomenclature synonyms are Cactus gracilis (Mill.) Weston (1770) and Echinopsis gracilis (Mill.) Molinari & Mayta (2015).
^Miller, Philip; Miller, J. (1768). The gardeners dictionary : containing the best and newest methods of cultivating and improving the kitchen, fruit, flower garden, and nursery, as also for performing the practical parts of agriculture, including the management of vineyards, with the methods of making and preserving wine, according to the present practice of the most skilful vignerons in the several wine countries in Europe, together with directions for propagating and improving, from real practice and experience, all sorts of timber trees. London: Printed for the author and sold by John and Francis Rivington ... [and 23 others]. doi:10.5962/bhl.title.541.
