Black Spots on Indoor Cactus: Rot from Overwatering and Low Light

Kim's cactus, kept under a lamp with no south/west windows, developed black areas. She suspected overwatering and asked about proper watering frequency and lamp use.

Diagnosis

Black tissue usually indicates rot from excessive moisture. Indoor cacti typically need water only once a month (or less in winter). The lamp helps, but only if it's a full-spectrum grow light run 10+ hours daily in summer, 8 in winter.

Corrected Care Routine

• Water only when soil is bone-dry—check with a chopstick or moisture meter.
• Use a fast-draining cactus mix with added perlite or pumice.
• Keep temperatures above 60°F; avoid cold drafts.
• If the plant is already rotting, cut away all blackened parts, let the wound callous 1–2 weeks, then reroot in fresh dry soil.

Light and water must be balanced: more light allows more frequent watering; low light demands extreme drought.