Echinocereus rigidissimus is a solitary, growing cactus, belonging to Cactaceae (Cactus) family. It rarely branches or offsets with age.  Each year’s growth is differentiated by differently coloured band of spines, hence the common name “Rainbow Cactus“. Arizona Rainbow and Sonorian Rainbow Cactus are other names.

It is naturally found in USA (south-eastern Arizona, south-western New Mexico) and Mexico (northern Sonora, north-western Chihuahua) at the attitude between 1200-2000 meters above sea level. Gravely hills, steep canyon sides, rocky benches in semidesert grasslands and oak woodlands are their natural sides. Plant grows to a
height of up to 40 cm, and a width of 10 cm when mature, with pectinate radial spines curved slightly towards the stem. The new spines are initially reddish to magenta and fades to a yellow or light pink colour when they mature. Each stem has 15-26 low and slightly tuberculate ribs.

Echinocereus rigidissimus, flowers in flushes at the sides of the stems throughout the spring season, with multiple buds borne atop the plant from younger areoles.  6-7 cm long flowers are bright pink in colour with a white coloured throat. They have numerous stamens, shorter than the style. 3 months after the flowering, circular shaped, greenish to dark purplish fruit, with white flesh and black seeds appear.

Rainbow Cactus needs an airy location in bright sun, prefers a neutral to slightly acidic compost with plenty of grit. Summertime it needs water when soil is dry.
To achieve the best spine colours, plants should have received lots of sun. In the winter plant should be kept at dry condition. Supposed to be cold resistant up to -12° C for a short period if it is kept dry. Abstain from watering in winter helps encourage flowering in the following spring season.  Since it rarely offsets, Echinocereus rigidissimus is propagated by seed.

Apart from the nominate form (Echinocereus rigidissimus var. rigidissimus)  two subspecies are recognized; Echinocereus rigidissimus subs. rubispinus and Echinocereus rigidissimus subs. rubispinus f. albiflorus.

Echinocereus rigidissimus subs. rubispinus (Rubispinus meaning ruby ‘red’-spined): has erect, rigid, short cylindric stems to only 6 cm in diameter and less than 30 cm tall. It occurs only in Chihuahua, Mexico. It has 11-13 ribs, forming tubercles and 30-35 radial spines. Mature plants with a good light will generally have red to deep pink spines that band the stems. The spines are also more ruby pinkish coloured compared to the ‘rigidissimus’ subspecies. Echinocereus pectinatus var. rubispinus is used as synonym.

Echinocereus rigidissimus subs. rubispinus f. albiflorus: is a culture variety, has white flowers and paler creamy white spines, all the other characteristics namely size and shape of spines, stems, fruits, roots etc. it is comparable to the standard from.

Use in Landscape: Rainbow Cactus is one of the showiest in the cactus kingdom. In cold and wet climate, it should be kept in a container to bring it indoor at winter. In mild climate, it may grow well when planted freely outside in well-drained soil.