Echinocactus horizonthalonius
Light: Full sun to light shade; in Phoenix, provide afternoon shade or filtered sun during extreme summer heat. Acclimate gradually to direct sunlight.
Water: Very low water needs. Water sparingly during warm weather, allowing soil to dry completely between waterings. Keep nearly dry in winter; avoid watering already-moist soil after monsoon rain.
Soil: Exceptionally well-drained, mineral-rich soil with gravel, pumice, or coarse grit. Tolerates alkaline, limestone-based soils; avoid moisture-retentive mixes and poorly drained clay.
Mature size: Typically 6–12 inches tall and 6–10 inches wide; old specimens may grow taller.
Bloom: Flowers intermittently from late spring through summer, often following warm-season rain once mature.
Evergreen / deciduous: Evergreen cactus with persistent stems and no true foliage.
Growth rate: Very slow.