Cephalocereus
Cephalocereus is a genus of tall, columnar cacti native to Mexico, best known for Cephalocereus senilis, the beloved "old man cactus" whose shaggy coat of long white hairs makes it one of the most recognisable and widely grown cacti in the world. The genus name comes from the Greek for "head" and refers to the woolly flowering structure, or cephalium, that mature plants develop near the top of the stem.
Description
Members of Cephalocereus are slow-growing, erect columnar cacti that are solitary or branch sparingly, in time reaching several metres in habitat. The ribbed stems carry areoles that, in many species, produce dense white or grey hairs, wool or bristles in addition to true spines — an adaptation thought to shade and insulate the growing tip under fierce sun. This hairiness is most extreme in the old man cactus but appears to varying degrees across the genus.
As plants approach flowering size they form a cephalium, a patch of thickened wool and bristles from which the flowers emerge. The nocturnal, funnel-shaped flowers are typically pale — cream, pink or greenish — and are followed by fleshy fruit. Because the cephalium only develops on old, tall specimens, most cultivated plants are enjoyed for their juvenile form and hair rather than their flowers.
Distribution
The genus is endemic to Mexico, where the species grow on arid limestone hills, canyon walls and thornscrub, often in exposed, well-drained sites at low to moderate elevations. Several species have restricted natural ranges. The old man cactus, though long collected from the wild for the horticultural trade, remains common in habitat, and nursery-grown plants are abundant and inexpensive.
The circumscription of Cephalocereus has shifted over the years as botanists have moved species between it and related columnar genera such as Pilosocereus and Neobuxbaumia. As a result the exact species count varies between sources.
Notable species
- Cephalocereus senilis — the old man cactus, clothed in long white woolly hairs; the signature species of the genus and a hobby staple.
- Cephalocereus columna-trajani — a tall, grey-green column named for its likeness to a classical pillar, from arid southern Mexico.
- Cephalocereus apicicephalium — a species that forms its cephalium prominently at the stem apex.
- Cephalocereus totolapensis — a robust columnar species from the Oaxaca region.
Cultivation
Cephalocereus are undemanding but slow, and the usual advice for desert columnar cacti applies. Grow them in a very free-draining, largely mineral mix and give as much bright, direct light as you can — strong light keeps the hairs of the old man cactus dense and white, whereas shade produces sparse, greenish growth. Water thoroughly through the warm months once the soil has dried, then keep plants dry and cool over winter; they are frost-tender and rot easily if left wet. See Watering and Repotting for general technique.
The long hairs of the old man cactus tend to trap dust and can mat or discolour over time; many growers gently rinse or comb them, and take care because sharp spines are hidden beneath the wool.
Propagation
Most Cephalocereus are raised from seed, which germinates readily in warmth on a mineral surface, though seedlings are slow to size up. Species that branch can sometimes be increased from stem cuttings taken in warm weather and left to callus before rooting. Slow-growing seedlings are occasionally grafted onto a vigorous rootstock to hasten early growth.
See also
- Cephalocereus senilis — the old man cactus
- Pilosocereus — a closely related genus of columnar cacti
- Soil and potting mix · Watering · Propagation — seed · Grafting · Pests and diseases