A tall tree of the spiny forest in the Anosy region of southern Madagascar. Described by Drake in 1903, Alluaudia ascendens is the largest member of the genus and reaches 12–15 m at maturity. Compared with its better-known relative A. procera, it grows taller and stays more strongly columnar with less branching when young, sending up a straight trunk crowded with thick, densely set spines and short, fleshy leaves — a quietly imposing form within the genus. Growth is slower than for procera and the species is also less cold-tolerant, putting it a notch above procera in cultivation difficulty.
Native climate
Rainfall is spread fairly evenly across the year. Overall a warm climate.
A broad-scale picture of the native range. Real growing spots — rock crevices, fog belts — can be milder.
Sources: climate & elevation WorldClim 2.1 (1970–2000) · occurrences GBIF · native range POWO · current weather Open-Meteo
Care
Light & Placement
In the open upper canopy of southern Madagascar's spiny forest it grows under intense direct sun, so give it as much full light as possible during active growth. Strong sun keeps the trunk tight and the spines well-coloured. Through Japan's midsummer, light shading at 20–30% with the pot raised on a bench for airflow is the safer setting. Overwinter on a bright sunny window kept above 8°C and dry. Respond early to autumn cooling.
Watering
In active growth, water thoroughly once the topsoil has dried fully, then dry the surface back quickly with good airflow. Excess moisture goes straight to root rot. Taper as autumn cools and go fully dry through winter.
Substrate
Drainage first, inorganic-led. Akadama : Kanuma : pumice = 4:3:3. The first few years produce a strong taproot, so a deeper pot supports a stable root system.
Fertilizer & Supplements
A monthly dilute liquid feed in active growth, or a small pinch of slow-release at repotting. Excess nitrogen elongates new growth and weakens the spines — keep doses modest.
Temperature & Overwintering
Optimal 22–35°C, minimum 8°C. Below 5°C twig tips blacken and leaf drop accelerates. Damp soil + cold is the main failure mode — disciplined dry overwintering is the single most important factor.
Starting from Seed
Where to source seeds
Pre-sowing treatment
Soak seeds for about half a day (overnight) in a mix of a registered seed-treatment fungicide (Benlate or Daconil) and a plant tonic (Menedael; outside Japan, SUPERthrive or a chelated iron / seaweed extract works similarly), each diluted per label. The ones that don't sink are usually past their prime; stock reaches the trade in smaller quantities than procera and often loses freshness in the pipeline.
Substrate
Use a separate seedling mix that's fine-grained and near-sterile: fine Akadama, fine Kanuma, and vermiculite in 1:1:1 parts. Sterilize beforehand with boiling water or a microwave pass.
Sowing method
Sow with no covering, or only the thinnest dusting of substrate so the seeds remain partly visible. Space the seeds at least 1 cm apart so they don't clump or overlap on the surface.
Light & temperature
Bright shade, 25–30°C. Germination spans roughly 10–30 days; hold the heat steady on a heat mat and stay patient with stragglers.
Watering
Bottom-water with the level 1–2 cm up the pot. For the first 2–3 weeks prioritize not letting things dry out, then drop the level gradually once seedlings are stable.
Fertilizer
No feeding right after germination. Once true leaves emerge, give diluted liquid fertilizer at double dilution or weaker, once or twice a month.
From Germination to Repotting
Germination through true leaves
Continue bottom watering, in bright shade.
Weaning off bottom watering
Phase it out gradually over 1–2 months.
First repotting
In the first or second year, once roots have filled the pot.
Common Pitfalls
Mold & damping-off
- Cause: excess moisture, contamination, poor air flow
- Prevention: sterilize the substrate, refresh the bottom-water, and use a circulation fan
Etiolation
- Cause: insufficient light
- Prevention: move LEDs closer right after germination, or shift the tray to bright shade outdoors
Seeds fail to germinate
- Cause: stale seed, insufficient warmth
- Prevention: source from reliable sellers and hold 25–30°C steady on a heat mat
Notes
The spines are thicker and more densely set than those of procera — wear thick gloves and long sleeves when handling. The species is rated Vulnerable (VU) on the IUCN Red List, and its Anosy spiny forest is being lost to firewood, charcoal harvest, and land conversion. The Didiereaceae are listed on CITES Appendix II — source from reputable, properly documented sellers.
