Name and proposed parentage
Plants of the World Online accepts Stenocactus × irregularis Gonz.-Zam., D.Aquino & Dan.Sánchez as a natural hybrid. The publication proposes the formula S. coptonogonus × S. pentacanthus. In the current POWO treatment, S. pentacanthus is a synonym of S. obvallatus. The × sign indicates hybrid origin, and the plants described come from natural sites. [2-WWW]
González-Zamora, Aquino and Sánchez compared 132 adult plants, 53 S. coptonogonus, 56 S. pentacanthus and 23 intermediate individuals. Parental material came from three populations per species, and the hybrid from a population in San Diego de la Unión. The first discriminant-analysis axis separated groups primarily by rib number. Origin from two species is an inference from morphometrics and co-occurrence. [González-Zamora et al. 2023, p. 90–97, tabele I–V]
Habit and incomplete ribs
The description covers solitary plants with an enlarged, turnip-shaped root and much of the stem below ground. Stems are 5–11 cm tall and usually 7.5–10 cm in diameter. There are 15–28 complete ribs. Additional short ribs occur in the upper or lower part of the stem and may bear areoles or end without them. In the sample of 23 adult plants, the mean number of complete ribs was 19 ± 3.6, expressed as mean ± standard deviation. [González-Zamora et al. 2023, p. 95, 97–99, tabela V, ryc. 4–5]
The flower, fruit and seeds
Flowers are 21–28 mm long. Inner tepals are white with a purple central band, and the innermost may be shorter than the others. Fruits are dry, with papery scales and a retained perianth. They open by splitting of the wall or at its junction with the perianth. Seeds are 0.9–1.1 mm long. [González-Zamora et al. 2023, p. 97–99]
A total of 85 seeds from three intermediate plants were sown, and 78% germinated. The result confirms the germination capacity of some seeds obtained. The complete course of germination, subsequent offspring survival and their genetic relatedness were not subjects of the analysis presented. [González-Zamora et al. 2023, p. 97]
Sites and seasonal observations
The type population grew at 2285 m in San Diego de la Unión, on the Guanajuato–San Luis Potosí border, in dry scrub on flat or gently sloping ground. The list also cited a specimen from Santa María del Río in San Luis Potosí. Observed flowering occurred in January and February, sometimes into March, and fruiting in April and May. [González-Zamora et al. 2023, pp. 98–99]