Echinocereus rigidissimus
| Light | Bright light with several hours of direct sun; happiest in full sun with good airflow |
|---|---|
| Water | Sparingly in the growing season; keep bone-dry and cool through winter |
| Soil | Very gritty, fast-draining mineral mix (see Soil and potting mix) |
| Temperature | Frost-tender in cultivation; USDA zones 9a–11, though tougher when kept dry |
| Propagation | Seed (primary); offsets on clustering plants |
| Toxicity | Non-toxic to cats and dogs |
Echinocereus rigidissimus, the rainbow cactus or Arizona rainbow cactus, is a small barrel-shaped hedgehog cactus from the southwestern United States and northern Mexico, prized for the horizontal bands of colour in its densely combed spines. Its short, stiff spines press flat against the body in tidy comb-like rows, and older plants build up alternating zones of pink, cream and rusty spination that give the species its name. In late spring it crowns itself with large, glossy magenta flowers, and the pink-spined Mexican form rubispinus is especially sought after by collectors.
Description
Echinocereus rigidissimus forms a solitary (occasionally few-headed) cylindrical stem, typically reaching 10–30 cm tall and around 5–10 cm across, with numerous close-set ribs. The spines are the plant's defining feature: short, rigid and radial, they lie almost flat against the body and interlock like a fine comb, largely obscuring the green skin beneath. As the plant grows it lays down seasonal bands of differently coloured spines, so that mature specimens show horizontal rings of pink, white, tan and reddish-brown stacked up the stem.
The flowers are large for the size of the plant — funnel-shaped, up to around 6–8 cm across, in vivid magenta-pink with a paler, often whitish throat. They open near the top of the stem in late spring and early summer, lasting several days. The rubispinus form is a smaller, more slender plant clothed almost entirely in bright pink to rose spines, and is the version most often seen in collections.
Distribution and habitat
The species grows in the sky-island and desert-grassland country of southern Arizona and New Mexico in the United States, and south into the Mexican states of Sonora and Chihuahua. It favours rocky slopes, gravelly flats and open oak or grassland at moderate elevations, often rooted in mineral, well-drained ground among rocks that shed water quickly. The rubispinus form is associated with parts of northern Mexico.
Like all cacti, the species is listed under CITES Appendix II, so international trade in wild-collected plants is regulated; nursery-propagated plants are widely and legally available.
Cultivation
The rainbow cactus is a rewarding grower that rewards restraint with water and generosity with light. Plant it in a very free-draining, mostly mineral mix in a pot only a little larger than the plant, and give it as much sun as you can — strong light brings out the richest spine colour and keeps the body compact. Water thoroughly during warm active growth once the soil has dried, then let it dry out completely before the next drink.
The most important rule is a hard, dry, cool winter rest: kept bone-dry and cool from late autumn, the plant sets flower buds reliably and shrugs off cold far better than it would if damp. Wet, cold roots are the quickest way to lose it. See Watering and Repotting for general technique, and handle the plant with folded newspaper or foam strips, as the combed spines catch on skin.
Propagation
Seed is the usual and most reliable method. Sown on a warm, gritty surface and kept humid until they sprout, the seedlings grow steadily and eventually take on their banded spination. Plants that cluster can be increased by separating offsets once they have a few of their own roots, though many individuals stay solitary and never oblige. See Propagation — seed for a full walkthrough.
Cultivars
The most celebrated selection is the rubispinus form (sometimes traded as Echinocereus rigidissimus subsp. rubispinus), grown for its dense, all-over pink spination. Growers also select ordinary rigidissimus for the boldest colour banding. Named horticultural cultivars beyond these colour forms are few, and most collection plants are simply seed-grown selections chosen for spine colour.
Common problems
- Rot — nearly always from overwatering, a slow-draining mix, or wet roots in cold weather; the base softens and browns.
- Etiolation — too little light makes the stem elongate and pale, and dulls the spine colour that makes the species worth growing.
- Pests — red spider mites (fine webbing, bronzed tissue) can hide among the dense spines, and mealybugs (white fluff) lodge at the base and in the areoles. See Pests and diseases.
See also
- Echinocereus — the genus overview
- Soil and potting mix · Watering · Repotting · Propagation — seed · Propagation — offsets