Echinocactus horizonthalonius (a plant from Las Tablas)
This is the smallest of the Echinocactus, the only one that will bloom
as a small container plant (starting at about 5 cm in diameter).
 

Description: Echinocactus horizonthalonius is a relatively small barrel cactus that normally grows unbranched.
Stems: Pale, grey-green to bright glaucous grey-blue, flat-topped or hemispheric, and deep-seated in substrate, spherical with age or stoutly short cylindrical, stem slowly growing up to 30 cm tall, 15-20 cm in diameter or more.  The apical meristem is protected by whitish-yellow wool, which shades developing flower buds and fruits.
Ribs: 5 to 13 ribs (mostly 8), vertical to helically curving around stem, rib crests broadly rounded, uninterrupted or slightly constricted between areoles.
Areoles: Round, full of white wool when young.
Spines: 5 to 10 (more frequently 8) per areole, loosely projecting or strongly decurved, pink, grey, tan, or brown, strongly annulate-ridged, subulate, ± flattened, glabrous, generally not hiding stem surface
Radial spines: 5(-8) per areole, similar to central spines;
Central spines: 1(-3) per areole, 18-43 mm, longest spine usually descending, straight or decurved throughout its length.
Flowers: Pinkish-red , 5 to 7 (9) cm in diameter; stigma lobes pinkish to olive.
Blooming season: From the end of March to late May (occasionally on September).
Fruit: Red or pink, oblong, fleshy or semi dry and ephemeral, containing black seeds, indehiscent or weakly dehiscent through basal abscission pore. Once fruits are open, the rough black seeds (2 mm in diameter) lie on the wool atop plants, with seeds rolling off between the ribs.
 

Because of the slow growth rate of plants, it appears that Echinocactus horizonthalonius may have a life-span of 75-100 years.

The Sonoran Desert populations of Echinocactus horizonthalonius have been segregated as var. nicholii, but are relatively similar to plants in New Mexico and the westernmost part of Texas.
 

E. horizonthalonius El Pilar, GTO, Mexico

Young spines are bright red.
 

Cultivation: This species is not the easiest to cultivate, requiring the maximum amount of sun and care with watering, as it can easily rot.  Keep this plant dry in winter at a minimum temperature of 0°C (but it is hardy as low as -10° C).  It prefers a very draining mineral compost, and does better with some limestone in the soil mixture.  The plant tolerates extremely bright situations,  but enjoy  some shade during the hottest part of the day in summer.

Propagation: Seeds (it usually doesn't produce offsets), grafting.

Seed Collecting: Permit fruits to ripen:  Fruit must be significantly overripe before harvesting seed.   Clean and dry seeds

Scientific name: Echinocactus horizonthalonius Lemaire,
Published in: Cact. Gen. Sp. Nov. 19. 1839.

Common Name:  Devil's-Head Cactus, Blue barrel cactus

Origin:  Mexico (Nuevo Leon, San Luis Potosi), USA (Arizona, New Mexico, Texas) Echinocactus horizonthalonius is a species which covers a large habitat range, from the Big Bend National Park in Texas almost as far south as Mexico City.

Habitat: Rocky slopes, at 600-1700(-2500) m. This Echinocactus comes from very arid areas receiving only 20-30 cm of water a year, with harsh heat and sun.  Most populations come from limestone soils.

Conservation status: Listed in CITES appendix 2.

• Meyerocactus horizonthalonius

Photo & © copyright by Gianluca Scibilia (Italy)
 

The body of this plant is a chalky blue colour, and some forms have magnificent spines.