Cereus forbesii aka Spiral Cactus

Cereus forbesii is a branching cactus in the Cactaceae family. Kew places its native range from Bolivia into north-central Argentina. LLIFLE also reports it from Paraguay. It grows in dry shrubland, on arid plains, and in forests. It also occurs on hills and beside Chaco salt flats. Its naming remains disputed. LLIFLE links it with Cereus hankeanus, but notes that some authors separate them.

This succulent forms a shrub or a small tree. Its usual stated height is about 6.6 feet [2 metres]. Some plants reach 23 feet [7 metres] or more. Large plants may develop a clear trunk. This can reach about 1.3 feet [0.4 metres] across. The many upright branches give the plant its columnar shape. Young stems have a blue, waxy surface. Older growth turns grey-green or pale olive-green. Branches measure about 0.16–0.33 feet [0.05–0.10 metres] across.

Each stem has four to seven ribs, sometimes eight. These ribs become less marked on older growth. Small, round areoles carry sparse pale wool. This wool darkens with age. Each areole bears several short, stout outer spines. These start yellowish or pale brown, then turn grey. The central spine is darker and thicker. Old trunk spines can become much longer than those on young stems.

Large flowers appear from spring into early summer. They grow singly from the sides of the stems. Each flower has a funnel shape and a pale green tube. The inner parts are white or tinged with pink. The outer parts are reddish, with green midribs. Flowers reach about 0.54–0.66 feet [0.165–0.20 metres] long. Their width reaches about 0.26–0.39 feet [0.08–0.12 metres]. The fleshy fruits turn red beneath a bluish surface bloom. Inside, lilac pulp surrounds small black seeds. The supplied sources do not establish edibility or toxicity.

LLIFLE describes this cactus as easy to grow, but sensitive to cold. It gives a lower limit near 28°F [−2°C], while warning against freezing. Use a soil mix that drains well. Water during summer, then let the mix dry fully. Keep it mostly dry through winter. Growth is fast, and roots need room to expand. Repot every other year, or when the pot becomes too small. Grow new plants from seed or spring cuttings. Let cut ends form a callus before planting. The evidence names no specific pests or diseases.