How to grow old lady cactus
Overview
Old lady cactus (Mammillaria hahniana) is a small, clumping cactus from central Mexico, grown for its dense white spines and pink flowers that form a crown. It needs bright, indirect light and very dry conditions in winter to avoid rot.
Soil Preparation
Use a very gritty, free-draining mix: 1 part commercial cactus mix to 1 part coarse sand or pumice. Aim for pH 6.0–7.0. Add a handful of crushed charcoal to prevent souring. Never use garden soil or heavy loam.
Growing Guide
Position in part shade, such as morning sun with afternoon protection, in a gritty, free-draining mix like 50% coarse sand and 50% pumice. Space plants 15–20 cm apart in a shallow pot or dish. Water sparingly from spring to autumn, then keep almost dry through winter. Repot every 2–3 years to refresh the mix and prevent compaction.
Sowing
Sow seed in spring (Sep–Nov) on the surface of a fine, sterile cactus mix; barely cover with sand. Germination takes 2–4 weeks at 20–25°C. Keep moist but not wet, and transplant after 6 months. Seedlings are slow; most growers buy small plants.
Care
Feed with a low-nitrogen cactus fertiliser (e.g., 2-7-7) once in early spring and again in late summer. No pruning needed; remove spent flowers and dead spines for neatness. Repot every 2–3 years in a gritty mix to refresh soil and prevent compaction.
Companion Planting
Pair with other small cacti like Mammillaria elongata or Echinopsis chamaecereus for a low-care collection. Avoid planting near moisture-loving ferns or leafy perennials that would keep the soil damp.
Common Pests & Risks
Mealybugs appear as white cottony clusters on spines and stems; dab with isopropyl alcohol or use a systemic insecticide. Rot starts as soft, brown patches at the base from overwatering—cut back affected tissue and dry out the plant immediately.
Troubleshooting
Yellowing lower spines: usually from too much water or poor drainage—reduce watering and check mix. Soft, brown base: rot from overwatering or cold—cut away rotted tissue and dry out. No flowers: needs more light or a cool, dry winter rest—move to brighter spot. Mealybugs on stems: treat with alcohol or systemic insecticide. White, fuzzy patches on crown: normal for this species (areoles), not a pest.