Red Moon Cactus Fading & Blackening: Sunburn Diagnosis

Classic Sunburn

A year-old red moon cactus (grafted Gymnocalycium) fades to pale, then blackens on the window side. Cause: direct or reflected sun on a greenhouse-grown plant.

Why It Happens

• Moon cacti are raised in shaded greenhouses.
• They lack the thick epidermis of sun-acclimated cacti.
• Even bright indoor light can burn if concentrated through glass.

Prognosis

Severe blackening = likely fatal. The scion (red top) has no chlorophyll; it cannot recover photosynthetic capacity. The rootstock may survive but will revert to green growth.

Prevention for Next Time

• Bright indirect light only (north/east window, sheer curtain).
• Rotate weekly for even exposure.
• No direct sun ever.

Consider hardy, non-grafted cacti for sunny windows.