Opuntia robusta
| Light | Full sun; happiest with as much direct light as you can give it |
|---|---|
| Water | Sparingly; let the soil dry fully between waterings, dry through winter |
| Soil | Gritty, fast-draining mineral mix (see Soil and potting mix) |
| Temperature | Frost-hardy when dry to roughly −9 °C; USDA zones 8b–11 |
| Propagation | Pads (very easy) or seed |
| Toxicity | Not considered toxic, but the spines and glochids are a serious hazard |
Opuntia robusta is a large, shrubby to tree-like Mexican prickly pear grown for its striking, near-circular blue-gray pads and its edible fruit. Big, robust and eventually forming a massive clump or short trunk, it is one of the most recognisable of the Opuntia and is widely cultivated ornamentally as well as for its fruit and pads, earning common names such as wheel cactus and camuesa.
Description
Opuntia robusta builds up from a series of flattened stem segments — the "pads", or cladodes — that are unusually large and rounded, often nearly circular and up to 30 cm or more across. The pads are thick, firm and coated in a heavy waxy bloom that gives them their characteristic pale blue-gray to glaucous colouring. With age the plant can grow into a sprawling shrub or a small tree several metres tall, with older basal segments becoming woody and trunk-like.
Areoles are spaced across the pad surface and typically bear a few stout spines together with tufts of fine, barbed glochids. Spine density varies a great deal between plants — some clones are heavily armed while others are nearly spineless — but the glochids are always present and are the feature to respect most (see below). Bright yellow flowers open along the pad margins in late spring, followed by squat, rounded fruit (the "tunas") that ripen to a deep red-purple and are edible.
Distribution and habitat
The species is native to central and northern Mexico, where it grows across arid and semi-arid highlands, scrub and grassland, often on rocky slopes and exposed plateaus. In these habitats it endures strong sun, seasonal drought and cold nights, which is why well-established plants tolerate more cold than many people expect from a cactus.
Like other prickly pears, O. robusta spreads readily from dropped or detached pads, and it has naturalised or become weedy in parts of the world with similar climates — in some regions the "wheel cactus" is regarded as an invasive pest. Growers outside its native range should be mindful of this and avoid letting stray pads escape into the wild.
Cultivation
This is an easy, vigorous plant for a hot, sunny spot. Grow it in a gritty, free-draining mix and give it as much sun as possible — full exposure keeps the pads compact, well-coloured and firm. Water generously during the warm growing season once the soil has dried, then taper off; keep the plant dry through winter, when good drainage and dryness let it shrug off surprising cold. Waterlogged, cold soil is the one thing it will not forgive.
O. robusta can get very large, so give it room or be prepared to prune. Handle it with thick gloves and folded newspaper or tongs rather than bare hands — the glochids detach at the lightest touch. See Watering and Repotting for general technique, and wear eye protection when working closely with any spiny Opuntia.
Propagation
Propagation from pads is almost effortless and is the usual method. Detach a healthy pad at the joint, let the cut surface callus over for several days to a couple of weeks in a dry, shaded spot, then set it upright in barely moist gritty mix; it will root and begin growing with little fuss. Seed is also viable but slower, and seedlings vary in spine and pad form. See Propagation — cuttings and Propagation — seed for full walkthroughs.
Common problems
- Glochids — the tiny barbed bristles are the number-one hazard; they lodge in skin at a touch and are miserable to remove. Always handle with gloves and tools, and keep the plant away from paths and pets.
- Rot — from wet, cold soil or poor drainage; pads soften, discolour and collapse. Keep it dry in winter.
- Pests — cochineal scale (a white, waxy fluff on the pads that yields red dye when crushed) and mealybugs can establish in the areoles; see Pests and diseases.
- Weediness — dropped pads root wherever they land, so clean up fallen segments to stop unwanted spread.
See also
- Opuntia — the genus overview
- Propagation — cuttings · Propagation — seed · Soil and potting mix · Watering · Repotting · Pests and diseases