The flagship species of the Madagascar-endemic genus Uncarina (Pedaliaceae), native to the spiny forests of the south and southwest. Mature plants reach 2–4 m, with a chunky grey caudex, velvety heart-shaped leaves, and large bright-yellow summer flowers carrying a dark purple-black throat reminiscent of a hibiscus. Its most distinctive feature is the hard fruit armed with four sharp hooks — a velcro-like dispersal strategy that latches onto animal fur and human clothing, giving rise to the common name "mousetrap tree". The epithet honours the 19th-century French Madagascar explorer Alfred Grandidier. A robust species and a good entry point into Madagascan summer-growing caudex from seed.
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
A summer-grower from spiny forest with annual rainfall of 400–600 mm in the Toliara region and Andohahela. Through the growing season, give it as much full direct sun outdoors as possible — strong light keeps the trunk tight, the leaves expanded, and pulls flower buds in late summer. Light shading at 20–30% helps through Japan's worst midsummer weeks; raise pots off the ground for airflow. Overwinter on a bright sunny window kept above 8°C and dry, sheltered from rain.
Watering
In active growth, water deeply once the substrate has dried, then dry it out fully again — this species is less drought-shy than other genera but is still vulnerable to overwatering. Taper water as autumn cools, then go nearly dry once leaves drop, with at most a single light misting per month through winter.
Substrate
Drainage first, inorganic-led. Akadama : Kanuma : pumice = 4:3:3 is a reliable baseline. A small pinch of slow-release at potting helps the caudex thicken faster. A deeper pot suits the downward-running taproot.
Fertilizer & Supplements
A diluted liquid fertilizer monthly through active growth, or a small pinch of slow-release at repotting. The species responds steadily to feeding, but pushing the dose elongates branches and softens the trunk silhouette. Keep it modest and let the caudex thicken on its own time.
Temperature & Overwintering
Optimal 22–35°C with an 8°C minimum. As a Madagascan species it is cold-sensitive — branch tips suffer below 5°C, and damp soil with cold is fatal. As autumn cools and leaves drop, taper water early and overwinter on a bright indoor window, holding the dry-warm rhythm until spring.
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 still floating are likely no longer viable. The seed coat is hard, typical of Pedaliaceae — light scarification (gentle abrasion with sandpaper) improves water uptake.
Substrate
Use a separate seedling mix that's fine-grained and near-sterile: fine Akadama, fine Kanuma, and vermiculite in 1:1:1 parts. Sterilizing beforehand with boiling water or a microwave pass meaningfully reduces damping-off.
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.
Light & temperature
Bright shade, 25–30°C steady. Germination spreads over 10–30 days; a heat mat keeps the temperature stable and helps even out the spread.
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 water level gradually once seedlings are stable.
Fertilizer
No feeding right after germination. Once true leaves emerge, give diluted liquid fertilizer at double dilution, once or twice a month, to anchor early growth.
From Germination to Repotting
Germination through true leaves
Continue bottom watering, keep strong light off them.
Weaning off bottom watering
Phase out gradually over 1–2 months.
First repotting
In year one or two, 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, hard seed coat blocking water uptake
- Prevention: source fresh seed, lightly scarify before soaking, hold 25–30°C steady on a heat mat
Notes
Mature fruits carry four sharp hooks that grip clothing and fingers tenaciously. Use leather gloves when pruning or harvesting seed, and keep harvested pods away from pets and small children.
