Reviving a Neglected Fairy Castle Cactus: Repotting, Fertilizer, and Rot Check

Repotting Assessment

Your actions were correct: larger pot, cactus mix + sand, drainage rocks, light watering. Pot-bound 10-year plants often yellow from nutrient depletion.

Discoloration Diagnosis

• Yellow + firm: Nitrogen deficiency — apply Superthrive or ¼-strength balanced fertilizer.
• Yellow + soft/mushy: Rot — cut above affected area, callus, re-root.
• Washed out + firm: Light deficiency — increase light gradually.

Ongoing Care

• Bright indirect light (acclimate to morning sun).
• Water when dry; reduce winter.
• Fertilize 2×/year (spring, mid-summer) at low strength.
• Monitor for mealybugs in stem joints.