Pachypodium horombense is a succulent shrublet about 1.5 m high with a plump, bottle-shaped, main stem surmounted by a number of short, spiny fat arms each bearing a rosette of leaves and a single long-peduncled inflorescence that carries 3-10, broadly cup-shaped, horizontal, gold flowers during the warm months of the year. The flower has an inflated corolla that forms five large sacs into which the pollinator can crawl. Unless in flower, it’s difficult to distinguish it from the cactus pachypodium (Pachypodium rosulatum).
Habit: P. horombense is a succulent shrub or dwarf tree up to 60 cm high similar to Pachypodium rosulatum but with stiffer, more erect stems branching nearer to the base.
Stem: The cactus-like trunk and branches are covered with short paired spines. A plump tuberous base often called a caudex 50-70 cm in diameter, is also present, it is a swollen trunk-root transitional area that stores water. Trunk, 20-30 cm in diameter; bark grey-green, shiny, smooth, medium green, wood cream. Branches up to 10 cm in diameter; branchlets 20-40 cm long and 6-20 mm in diameter, not linear but variously twisted, bent and gnarled, covered with paired straight spines, 3-10 mm long, 1-3 mm in diameter at the base, basal part conical.
Leaves: Thick, stiff, subsessile in a whorled tuft on branch tips elliptic to narrowly oblong 25-80 mm long 5-20 mm broad rounded at the apex, wedge shaped at the base. Petiole 1-3 mm long, pubescent. Deep green, glossy, with a pale green or yellowish midvein and secondary veins above and felted, white undersides with again paler midrib and dark green reticulate venation. Papery when dried. Completely deciduous in winter As temperatures cool and soil dries out in the tropical winter, the foliage drops away.
Inflorescence: 4-11-flowered an an upright pubescent, peduncle, 10-60 cm tall, 3-8 mm wide. Pedicels 5-45 mm long, pubescent. Bracts pale green, oblong to obovate, 8-25 mm long 2-6 mm wide, acuminate at the apex sometimes hidden by the hairs.
Flowers: Chrome-yellow. Sepals pale green, fused at the base for about 0.5 mm, ovate 6-15 mm long 2-4 mm broad, acuminate at the apex, pubescent outside, glabrous inside. Tube broadly cup-shaped, 17-23 mm in diameter, 5-lobed in cross-section with dark red longitudinal lines in bud, pale green in open flowers. Corolla bright yellow, 2.5-5.5 cm long in the mature bud and forming a comparatively wide ovoid head. Basal part almost cylindrical, abruptly widened where the stamens are inserted into the upper part which is urceolate, 18-40 mm long, 15-26 mm wide at the mouth, with 5 again widened parts below the lobes and therefore 5-winged inside; lobes obliquely and broadly obovate.
Blooming season: During the warmth and increased moisture of late spring and into summer, thin flower stalks tower above the leafy branch tips.
Fruits: 14 cm x 7 mm.
A wild hybrid has been recorded with Pachypodium densiforum.
Subspecies, varieties, forms and cultivars of plants belonging to the Pachypodium horombense group
Origin and Habitat: Horombe plateau in southern Madagascar.
Type locality: Madagascar, Fianarantsoa, Ambatolahy-Kelivondraka Road,
Altitude range: 500-1000 metres above sea level.
Habitat: It is found in granite outcrops on hilltops, which are also home to some of Madagascar’s rarest species, for example Pachypodium brevicaule, Euphorbia quartziticola, Aloe calcairophylla, Aloe isalensis and Schizolaena tampoketsana.