Opuntia macrocentra
| Light | Full sun; the more light, the deeper the purple |
|---|---|
| Water | Sparingly; drench then dry out completely, keep dry in winter |
| Soil | Very free-draining mineral mix (see Soil and potting mix) |
| Temperature | Cold-hardy; tolerates hard frost, roughly USDA zones 8–10 (and into zone 7 where winters are dry) |
| Propagation | Pad cuttings (easiest) or seed |
| Toxicity | Not chemically toxic, but glochids and spines cause injury |
Opuntia macrocentra is a low-growing prickly pear of the Chihuahuan Desert, easily recognised by its blue-green to lavender pads that flush a vivid purple under stress, and by the long, dark spines gathered mostly toward the top edge of each pad. These traits earn it the common names purple prickly pear and black-spined prickly pear. It is one of the most ornamental of the hardy opuntias and a favourite for colour in cold-climate cactus gardens.
Description
Opuntia macrocentra forms a shrubby, sprawling clump usually less than a metre tall, built from flattened, roughly circular pads (cladodes) around 10–20 cm across. The pads are blue-green in good conditions but develop rich purple to reddish tones — strongest in cold weather, drought or strong sun, and often concentrated around the areoles and pad margins.
The species is named for its long spines (macrocentra meaning "large-spined"): dark brown to nearly black, sometimes reddish at the base and paler at the tip, and typically one to a few per areole. Distinctively, these long spines are usually confined to the areoles near the upper edge of each pad, leaving the lower areoles nearly spineless — though, as always with prickly pears, every areole carries a tuft of fine, barbed glochids that are the real hazard to handle.
Large yellow flowers with red centres open in spring, followed by fleshy reddish-purple fruits (tunas).
Distribution and habitat
The species is native to the Chihuahuan Desert of the southwestern United States and northern Mexico, ranging across parts of Texas, New Mexico and Arizona and south into the Mexican states of Chihuahua and beyond. It grows on open desert flats, rocky slopes and gravelly grassland, in full sun and sharply drained soils, where it endures both intense summer heat and hard winter frost.
Cultivation
Opuntia macrocentra is an easy and very cold-hardy plant, making it popular in outdoor rock gardens well beyond the frost-free zones where most cacti are grown. Give it full sun and a gritty, very free-draining mix; the purple colouring is far stronger in bright light and cool, lean conditions than in a rich, shaded, well-fed situation.
Water generously during the warm growing season once the soil has dried, then withhold water through winter — a dry plant tolerates cold vastly better than a wet one, which is prone to rot. In regions with cold, wet winters, the main risk is not frost itself but sodden roots. See Watering and Repotting for general technique. Handle with folded newspaper or thick gloves, and beware the glochids, which detach at the lightest touch.
Propagation
The simplest method is by pad cuttings: detach a healthy pad at the joint, let the cut surface callus over for several days to a week, then set it upright in dry, gritty mix and water only lightly until roots form. This is fast and reliable. The species also grows readily from seed, though seedlings are slower to reach flowering size. See Propagation — cuttings and Propagation — seed.
Common problems
- Rot — the main killer, caused by wet soil in winter or persistent overwatering; pads soften, discolour and collapse.
- Cochineal scale — white cottony masses on the pads that yield red dye when crushed; wipe or wash off and treat persistent infestations.
- Glochids — not a plant problem but a grower one; the fine barbed bristles lodge painfully in skin, so always handle with care. See Pests and diseases.
See also
- Opuntia — the genus overview
- Propagation — cuttings · Propagation — seed · Soil and potting mix · Watering · Pests and diseases