Acanthocereus tetragonus
| Light | Bright light to full sun once established; part shade indoors |
|---|---|
| Water | Moderate in growth, sparingly in winter; let the mix dry between waterings |
| Soil | Fast-draining, gritty cactus mix (see Soil and potting mix) |
| Temperature | Frost-tender; keep above about 4 °C, USDA zones 9b–11 |
| Propagation | Stem cuttings (very easy); also seed |
| Toxicity | Non-toxic to cats and dogs; spines are the main hazard |
Acanthocereus tetragonus is a sprawling, night-blooming cactus native to the Americas, from Florida and Texas through Mexico, Central America and the Caribbean into northern South America. In the wild it grows into a scrambling, shrubby thicket of slender, sharply angled green stems armed with stout spines — habits that have earned it common names such as triangle cactus, barbed-wire cactus and sword pear. It is best known to hobbyists, however, through a compact columnar mutant sold worldwide as the Fairy Castle cactus.
Description
Acanthocereus tetragonus is a many-stemmed cactus whose green stems are strongly angled — usually three- to five-sided in cross-section, with the young growth often crisply triangular. The stems are slender and can clamber, arch or lean on surrounding vegetation, rooting where they touch the ground and building into dense, spreading colonies that may reach several metres across in habitat. Grey to brownish spines are borne in clusters along the ribs.
Large, funnel-shaped white flowers open at night and are fragrant, closing by the following morning. They are followed by rounded red fruits. As with most nocturnal cacti, flowering is a treat that comes only on well-grown, mature plants.
The plant most collectors actually own is the Fairy Castle form: a slow, densely branching, upright mutant that piles up short, five-ribbed green columns into a miniature "castle" of turrets. It is a stable ornamental form of this species rather than a distinct plant, and it rarely if ever flowers.
Distribution and habitat
The species has a very wide natural range, occurring from the southern United States (Florida, Texas) through Mexico, Central America and the Caribbean islands and into northern South America. It favours coastal thickets, hammocks, dunes and dry scrub, where its scrambling stems weave through shrubs and low trees. Across much of this range it is a common, weedy survivor rather than a rarity.
Cultivation
Acanthocereus tetragonus is an easy, forgiving cactus. Grow it in a free-draining, gritty mix and give it bright light — the wild plant takes full sun, though the Fairy Castle form kept indoors appreciates a bright windowsill and will stretch and pale in deep shade. Water moderately through the warm growing season, letting the mix dry out between drinks, and keep it much drier and cooler over winter to avoid rot. See Watering and Repotting for general technique.
The wild species is vigorous and can become a large, sprawling plant, so it is more often grown for hedging or novelty than as a tidy pot specimen. The Fairy Castle form stays small and slow, making it a popular, low-maintenance houseplant. Be cautious of the spines when handling either form, and pot on into a slightly larger container only when the plant has clearly filled its pot.
A word of warning for the Fairy Castle cactus: it is frequently sold topped with a brightly coloured dried straw flower glued to the crown. These are fakes — the glue and the wound beneath can invite rot, so gently remove them.
Propagation
Propagation is very easy from stem cuttings. Sever a segment or branch, let the cut end callus over for several days in a dry, shaded spot, then set it in barely moist, gritty mix; roots form readily in warmth. This is the usual way to increase the Fairy Castle form, which comes true from cuttings. The wild species also sets viable seed in its fleshy red fruits and can be raised from seed, though cuttings are quicker. See Propagation — cuttings and Propagation — seed.
Common problems
- Rot — the commonest cause of loss, almost always from overwatering, a slow-draining mix, or moisture sitting in a wound (including under a glued-on fake flower). Affected stems soften and discolour.
- Etiolation — too little light makes stems thin, pale and elongated, and the Fairy Castle form loses its compact, turreted shape.
- Pests — mealybugs (white fluff in the angles of the ribs) and, in dry indoor air, spider mites are the usual offenders. See Pests and diseases.
See also
- Acanthocereus — the genus overview
- Propagation — cuttings · Soil and potting mix · Watering · Repotting · Pests and diseases