Stem, ribs and spines
The plant usually has a solitary stem. Young stems are flattened or hemispherical, while older ones may take the form of a short column. The species description most often gives up to 15 cm in diameter and 25 cm tall, with larger specimens reaching about 20 and 45 cm respectively. Surface colour ranges from grey-green to bluish. Damage to the apex can trigger branching and the formation of several heads. [Baker 2022]
The stem usually has eight ribs, within a broader range of seven to eleven. They are broad and rounded, running vertically or twisting slightly around the plant’s axis. Areoles form distinct rows on their crests. Typical armature includes about three central and five radial spines. The spines may be flattened, have transverse annular thickenings and become paler with age, often retaining a pinkish tint. [Baker 2022]
Spine number, the length and curvature of the lowest central spine, rib height and stem proportions vary among populations. Subspecies identification rests on this set of characters and plant provenance. A photograph of a single specimen shows only part of the variation that formed the basis of field studies. [Baker 2022]
Flowers and fruits
Flowers are pink to intensely carmine, reach about 7 cm in diameter and arise near the apex. Long areolar hairs accompany the developing organs. Flowering usually occurs from April to July and is linked to rainfall conditions. Additional flowering periods may occur in wetter years. The mature fruit is pink or red, nearly globular to ovoid, and reaches about 3 cm long. [Baker 2022]
Seeds are dark, black-grey, up to about 3 mm long. Compared with E. platyacanthus, they differ in features including seed-coat sculpturing, with convex cell surfaces. Flower colour, the small rib number and seed characters form a coherent set distinguishing the two species of the genus. [Baker 2022] [Vargas-Luna et al. 2018]
Range and substrate at natural sites
The species occurs from southern Arizona and New Mexico through western and southern Texas to central Mexico. Many sites lie in the Chihuahuan Desert. There, plants occupy stony surfaces on sedimentary and volcanic substrates, particularly often limestone or gypsum. Baker describes elevations of about 750–1675 m above sea level as typical for this part of the range. [Baker 2022] [2-WWW]
The western, spatially separated populations in Arizona and Sonora belong to the Sonoran Desert region. In Arizona, they grow, among other places, on limestone ridges and alluvial terraces. Farther south, on the Mexican Plateau, they occur in grasslands and low scrub, on shallow soils containing gravel and silt. This range of habitats helps explain interest in regional variation in habit. [Baker 2022]
Subspecies and naming history
Baker analysed field material from 2007–2021. He compared 17 measured stem characters and three derived characters, including populations from different parts of the range. The comparison group was Homalocephala texensis (Echinocactus texensis in the original). Multivariate analyses revealed regional character clusters associated with the Chihuahuan Desert, Sonoran Desert and central Mexican Plateau. [Baker 2022]
In 2022, E. horizonthalonius subsp. australis was described. Plants in this southern group have, on average, lower and more flattened stems, lower ribs, and shorter, less curved central spines. Character values partly overlap among groups, so the description emphasises proportions and combinations of traits. The study compared plants in their habitats, so the observed pattern combines population variation with the influence of growing conditions. [Baker 2022]
POWO currently accepts two subspecies, australis and the nominotypical horizonthalonius. Baker additionally retained subsp. nicholii, historically also described at varietal rank. In the current POWO list, the names subsp. nicholii and var. nicholii are synonyms of the nominotypical subspecies. Material from Arizona and Sonora discussed under these names in older papers remains part of the documentation of species variation. [2-WWW] [Baker 2022] [2-WWW]
Relationships
The name Meyerocactus horizonthalonius reflects an earlier proposal to establish a separate genus. The revision from 2018, combining morphology with DNA sequences, supported retaining the species together with E. platyacanthus in Echinocactus. Rounded cotyledon tips are among the shared characters of this lineage. Differences in adult stem size and appearance therefore occur within a close relationship. [Vargas-Luna et al. 2018] [2-WWW]
Uprawa pojemnikowa
The starting point is general RHS guidance for desert cacti. Strong light, ventilation and a freely draining substrate are needed. During growth, water after the substrate surface has dried, ensuring excess water can drain. The rest period should be cooler, with very sparing watering. Seed sowings require a bright location with diffused light and more evenly available moisture. [1-WWW]
Related articles
Materials from the author's archive
The species also appears in Konrad Szychowski's private material archive. [Cactáceas y Suculentas Mexicanas 1959, file p. 18, 24]
Synonyms and names in the literature
Meyerocactus horizonthalonius · Echinocactus horizonthalonius var. nicholii · Echinocactus horizonthalonius subsp. nicholii · Echinocactus horizonthalonius subsp. australis [2-WWW]