An erect, branching cactus with thick spines and an attractive, grayish-green color, silver torch cactus (Cleistocactus strausii) is an easygoing plant that requires little attention once its basic growing requirements are met. Mature silver torch plants, which reach heights of up to 10 feet, display deep red, tube-shaped flowers in late summer. Native to Argentina and Bolivia, silver torch cactus grows in U.S. Department of Agriculture plant hardiness zones 9 through 11.
Prepare the soil in a sunny area of your garden. If you live in a hot desert climate, the plant benefits from light shade during the afternoon.
Plant silver torch cactus in soil amended with at least 50 percent sand. If the soil is clay-based, add even more sand to improve drainage. Alternatively, use 25 percent sand, 15 percent pea gravel and 10 percent peat moss or compost. Without drainage, the cactus will rot.
Water the plant regularly in the absence of rainfall during the summer months. Frequency of watering depends on factors such as soil type and temperature. Keep an eye on the cactus, and it will indicate when it needs water. A healthy cactus is plump, but a thirsty cactus looks shriveled and wilted. Water the plant deeply to a depth of at least 6 inches, then let the soil dry before watering again. Never water if the top of the soil feels damp. Waterlogged soil leads to rot fairly quickly.
Withhold water beginning in late August or early September and don't water during the winter months. Silver torch cactus requires a period of dormancy and is likely to rot in moist soil. Water the plant very lightly if it looks shriveled.
Feed silver torch cactus lightly in late spring and again in mid-summer. Use a dilute solution of a balanced, water-soluble fertilizer. Mix a solution of no more than 1 teaspoon of fertilizer in 1 gallon of water. Never over-fertilize because the cactus may become weak, floppy and more prone to disease, pests and winter damage.
Spray the cactus with insecticidal soap spray if the plant is bothered by spider mites or mealy bugs. Mix the spray at a ratio of 1 teaspoon of commercial, concentrated soap spray to 1 pint of water. Reapply every four-to-seven days, as needed.