Root Rot on cactaceae
What's Happening
Cactus root rot develops when the shallow, fibrous root system remains in waterlogged soil for 7+ days, creating anaerobic conditions below 2mg/L dissolved oxygen. Unlike desert-adapted CAM photosynthesis that minimizes water loss, saturated soil triggers bacterial and fungal pathogen proliferation (Phytophthora, Pythium, Fusarium) that break down root tissue. The cactus's water-storing stems mask decline until structural failure occurs—mushy black tissue at the base with distinctive sour/foul odor.
How to Fix It
- 1
Inspect immediately: Unpot plant and rinse all soil to expose root system—healthy roots are white/firm; rotting roots are black/mushy/foul-smelling
- 2
Surgical removal: Cut ALL rotted tissue with sterilized knife until only firm, green stem remains—rot spreads like chain reaction
- 3
Callus period: Lay beheaded cutting on dry paper towel in shade for 48-72 hours until cut end forms dry scab
- 4
Repot in dry gritty mix: Use fresh cactus soil (60-70% drainage material); plant with growth ring at soil surface
- 5
Withhold water: Do not water for 2-3 weeks until roots develop resistance when gently tugged
- 6
Resume deep-soak protocol: Fully saturate soil, then allow complete dryness before next watering
How to Prevent It
Use terracotta pots with 3-5 drainage holes and gritty soil mix (50% perlite/pumice + 30% coarse sand + 20% organic matter). Water only when soil is bone-dry 2-3 inches deep—typically every 2-4 weeks indoors, extending to 6+ weeks in winter dormancy. Bottom-water only when necessary to control saturation levels.