Identifying a Fast-Growing Columnar Cactus: Cereus repandus

A reader named Manny wrote in with a puzzling cactus that had shot up over 30 inches in just four months, reaching 54 inches tall. The plant was columnar, starting with six ribs at the base and transitioning to four ribs halfway up. Manny suspected it might be a San Pedro cactus, known for rapid growth, but noted his plant lacked the typical rounded shape.

Expert Greg identified the cactus as Cereus repandus (syn. Cereus peruvianus), a common South American species. The change from six to four ribs, along with the thinning growth, was diagnosed as etiolation caused by insufficient light. Most cacti require about 15 hours of bright outdoor light daily to maintain compact, healthy growth.

Greg cautioned against suddenly moving the plant into direct sun, as it would sunburn after growing in lower light. Instead, he recommended gradually acclimating it to brighter conditions. Despite the stretching, the plant appeared otherwise healthy.

Key Takeaways

• Rapid, elongated growth with fewer ribs signals inadequate light.
• Cereus repandus is a common, fast-growing columnar cactus.
• Acclimate slowly to brighter light to avoid sunburn.