Cleistocactus strausii

Cleistocactus strausii, the silver torch or wooly torch, is a perennial cactus of the family Cactaceae. It is native to mountainous regions of Department Tarija, Bolivia between 1,500 m (4,921 ft) and 3,000 m (9,843 ft).

Cleistocactus strausii
Scientific classification
Kingdom:
Plantae
(unranked):
Angiosperms
(unranked):
Eudicots
(unranked):
Core eudicots
Order:
Caryophyllales
Family:
Cactaceae
Genus:
Cleistocactus
Species:
C. strausii
Binomial name
Cleistocactus strausii
(Hesse) Backeb.

Its slender, erect, grey-green columns can reach a height of 3 m (9.8 ft), but are only about 6 cm (2.5 in) across. The columns are formed from around 25 ribs and are densely covered with areoles, supporting four yellow-brown spines up to 4 cm (1.5 in) long and 20 shorter white radials.

The cactus prefers free draining soils, strong sunlight, but not high temperatures — in fact it can withstand hard frosts down to -10 °C. In its natural habitat it receives plenty of water during the summer, but almost none over the winter. In cultivation, watering too much in winter often leads to root rot.

Older cactuses, over 45 cm (17.5 in) tall, produce deep red, burgundy, flowers in late summer. The 6 cm (2.5 in) long cylindrical flowers protrude horizontally from the columns. In common with other cacti in the genus Cleistocactus, the flowers hardly open, with only the style and stamens protruding. Cultivated plants often flower freely. In the United Kingdom, this plant is usually grown under glass, and has gained the Royal Horticultural Society's Award of Garden Merit.[1]

Etymology

The silver torch cactus (Cleistocactus strausii) derives its name from the Greek word "kleistos" which means "closed." This is due to its deep red/burgundy flowers rarely opening.

Description

The cactus has gray-green columns that grow up to 3 meters tall and several centimetres in diameter which are covered in white spines. Only older plants will blossom deep red/burgundy cylindrical flowers. These flowers emerge horizontally from the stem of the cactus. They are deep red to burgundy and grow up to 10cm long.

Habitat

The silver torch cactus, which has many names (snow pole, wooly torch), is a perennial cactus endemic to Department Tarija of Bolivia. As such, this cactus prefers mountainous regions that are dry and semi-arid. Like other cacti and succulents, it thrives in porous soil and full sun. While partial sunlight is the minimum requirement for survival, full sunlight for several hours a day is required for the silver torch cactus to bloom flowers.

Silver torch cacti can thrive in low-nitrogen soil without facing the consequences of low nitrogen. Too much water will make the plants weak and lead to root rot.

Conservation Status

This cactus is locally abundant in its native land and has few local threats to its survival, so it is rated Least Concern.

Propagation

This cactus can be propagated by cuttings or seed.

Cuttings should be taken near the base of the main stem, similar to cuttings for aloe vera. Rooting of this new cutting usually occurs within 3-8 weeks, therefore it is usually better to propagate silver torch cactus via seed.

As a houseplant, the silver torch cactus is an easy to take care of so long as over-watering is avoided. To naturally simulate the porous effects of the sand in its native habitat, it is important to ensure that excess water will not pool in the soil, causing the roots to become water logged.

Winter survival

This plant’s native habitat does not have winter, but when temperatures drop to the low twenties, it can exhibit signs of tip frost.

Pests

Silver torch cactus is most susceptible to mealybugs and spider mite.

Mealybugs are one of the most common pests of cacti and succulents. Mealybugs can be identified by their white, cottony masses on the plant. These are signs that the bugs are reproducing. These pests are especially problematic to plants such as the silver torch cactus because they suck out plant sap, depleting the strength of the plant. These plant-sucking bugs can also cause sooty mold along with their fluffy white wax, detracting from the appearance of these plants. Another form of mealybug attacks the root system of plants, which is harder to detect.

Mites thrive in the same hot, dry conditions that the silver torch cactus lives in. Spider mites cause damage by sucking out vital nutrients from the plant. Large populations of mites can cause irreversible damage on the plant, eventually killing the plant.

However, both pests can be hosed off the cactus with water.

  • Flowers by silver torch cactus
  • Young cacti columns
  • C. strausii in a greenhouse
  • Exemplar in Tallinn Botanic Garden

References
  1. "RHS Plant Selector Cleistocactus strausii AGM / RHS Gardening". Apps.rhs.org.uk. Retrieved 2020-04-17.


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