Pilosocereus leucocephalus
| Light | Bright light to full sun; acclimatise gradually to prevent scorch |
|---|---|
| Water | Moderate in warm growth; allow to dry between waterings; keep dry in winter |
| Soil | Fast-draining mineral-rich mix (see Soil and potting mix) |
| Temperature | Keep above about 5 °C; frost-tender, USDA zones 9b–11 |
| Propagation | Seed and stem cuttings |
| Toxicity | Non-toxic to cats and dogs |
Pilosocereus leucocephalus is a blue-green columnar cactus from Mexico and Central America that, at flowering size, develops a conspicuous woolly lateral pseudocephalium of long white hairs and bristles near the top of each stem. Its slender, branching, tree-like habit and cool glaucous skin have made it a favourite in cultivation, where it is sometimes sold under the common names woolly torch or old man of Mexico. It is one of the more familiar members of the genus Pilosocereus.
Description
Pilosocereus leucocephalus is a shrubby to small tree-like cactus, branching from near the base or above to form clusters of erect stems that can reach several metres tall in habitat. The stems are relatively slender, cylindrical and distinctly blue-green to grey-green with a waxy, glaucous bloom, and are divided into a modest number of low ribs. The areoles carry short, spreading spines together with tufts of pale wool, and long white hairs are produced along the ribs — most heavily near the growing tips.
The species' most striking feature appears only on mature stems: a dense, one-sided (lateral) pseudocephalium of long, silky white hairs and bristles that develops on the flowering side of the stem near the apex. From this woolly zone emerge the flowers — nocturnal, funnel-shaped, and whitish to pale pink — which open in the evening and are pollinated by bats in the wild. Flowering is followed by rounded, fleshy fruit.
Distribution and habitat
The species is native to Mexico, ranging southward into Central America, where it grows on rocky slopes, in thornscrub and in dry tropical woodland, often on limestone. Across its range it favours warm, seasonally dry sites with sharp drainage and plenty of light. As with the whole cactus family, Pilosocereus is listed under CITES Appendix II, but nursery-grown plants are common, affordable and entirely legal to own and trade.
Cultivation
Pilosocereus leucocephalus is one of the easier and faster-growing columnar cacti, which is part of its appeal. Grow it in a very free-draining, mostly mineral mix and give it as much bright light as you can — strong sun encourages the bluest skin and the heaviest wool, though plants moved suddenly into fierce sun can scorch, so increase exposure gradually. Water moderately through the warm growing season, always letting the mix dry out before watering again, and keep the plant nearly dry and above about 5 °C through winter to avoid cold, damp rot.
Because it is frost-tender, this is a container plant in most temperate climates, brought into a bright, frost-free spot for winter. Tall stems can become top-heavy, so a snug, stable pot and occasional repotting into fresh gritty mix will keep it growing steadily. See Watering for general technique.
Propagation
Both seed and cuttings work well. Seed is sown on a warm, moist mineral surface and is the usual way to raise numbers of plants; see Propagation — seed. Stems root readily from cuttings, which is a quick way to make new plants from an existing specimen — allow the cut end to callus for several days before setting it in a dry, gritty mix and watering only sparingly until roots form. See Propagation — cuttings.
Common problems
- Rot — the usual cause of loss, from overwatering, a slow-draining mix, or cold damp conditions in winter; stems soften and discolour.
- Scorch — pale, bleached or scarred patches where a shaded plant was moved abruptly into strong sun without acclimatising.
- Etiolation — too little light produces thin, pale, weakly-ribbed growth and sparse wool.
- Pests — mealybugs (white fluff in the areoles and wool) and spider mites are the most common; the dense hairs can hide mealybugs, so inspect regularly. See Pests and diseases.
See also
- Pilosocereus — the genus overview
- Soil and potting mix · Watering · Propagation — seed · Propagation — cuttings · Repotting · Pests and diseases