Cleistocactus strausii
| Light | Bright light to full sun; the more sun, the whiter the spines |
|---|---|
| Water | Moderate in growth; allow to dry between waterings, dry rest in winter |
| Soil | Fast-draining mineral mix (see Soil and potting mix) |
| Temperature | Keep above freezing; hardy to roughly USDA zone 9 if kept dry |
| Propagation | Seed, cuttings, and offsets |
| Toxicity | Non-toxic to cats and dogs |
Cleistocactus strausii, widely known as the silver torch or silver torch cactus, is a slender, densely white-spined columnar cactus from the mountains of Bolivia. Its narrow, upright columns are sheathed so thickly in fine white spines that the whole plant takes on a soft, silvery-woolly look, and mature stems produce distinctive deep red, nearly tubular flowers that barely open. Easy-going and fast for a column cactus, it is one of the most popular and widely grown members of the genus Cleistocactus.
Description
Cleistocactus strausii forms erect, cylindrical stems roughly 4–8 cm thick that lengthen steadily with age, eventually reaching a metre or more and branching from the base to build a clump of parallel silver columns. Each stem carries many low ribs closely set with areoles, and it is the spines — numerous, fine, hair-like and white — that give the species its character, veiling the green body almost completely.
The flowers are unmistakable and typical of the genus: slim, tubular and deep carmine to purplish-red, held nearly horizontal from the upper flanks of established stems. Like other Cleistocactus, they scarcely open, the petals staying tightly furled so that only the tip parts — the trait behind the genus name, from the Greek for "closed." They are borne freely in summer on plants of flowering size and are attractive to hummingbirds in habitat.
Distribution and habitat
The silver torch is native to the high valleys of the eastern Andes of Bolivia. It grows on rocky mountain slopes at considerable elevation, where days are bright and intense but nights are cool and the dry season is long. This montane origin explains its tolerance of cold: kept dry, established plants shrug off light frost far better than most columnar cacti, though they should never be exposed to hard freezes.
Cultivation
Cleistocactus strausii is one of the easier and faster column cacti to grow, which accounts for its popularity. Give it the brightest position you can — full sun encourages dense, brilliant-white spination, while too little light produces thin, greenish, etiolated growth. Plant it in a free-draining, mostly mineral mix in a pot deep enough to anchor the growing column, and water generously through the warm months whenever the soil has dried, easing off entirely as autumn cools.
Through winter keep the plant cool and completely dry; this winter rest both prevents rot and improves flowering the following season. Tall stems can become top-heavy as they lengthen, so a stable, weighty pot is worthwhile. See Watering and Repotting for general technique.
Propagation
The species is easily raised from seed, which germinates readily on a warm, humid mineral surface (see Propagation — seed). Because it clumps freely from the base, it is also simple to propagate vegetatively: basal offsets or sections of stem can be removed, left to callus for several days, and then rooted in a dry, gritty mix. See Propagation — offsets and Propagation — cuttings for details.
Common problems
- Rot — the usual cause of loss, from overwatering, a slow mix, or moisture during the cold winter rest; stems soften and discolour, often from the base.
- Etiolation — insufficient light makes new growth thin, pale and greenish, with sparse spines instead of the dense white sheath.
- Pests — mealybugs (white fluff tucked among the spines and at the base) and red spider mites are the common offenders; the thick spination can make early infestations easy to overlook (see Pests and diseases).
See also
- Cleistocactus — the genus overview
- Soil and potting mix · Watering · Propagation — seed · Propagation — offsets · Propagation — cuttings · Repotting