In the dormant state, without leaves and flowers, this species is difficult to distinguish from P. densiflorum and P. rosulatum. They share the same habit: low, multi-branched shrubs up to 1.5 m in diameter and up to 1 m tall.
P. horombense is named after the Horombé plateau in the southern highlands of Madagascar, where it often occurs in great numbers on granite rocks.
The inflorescence has an erect peduncle of no less than 60 cm tall, with broadly cup-shaped flowers 1.7-2.3 cm in diameter.
According to Werner Rauh in his great book on the succulents of Madagascar, this is:
“the most beautiful Pachypodium species in cultivation”.
As a young plant, this species looks rather uninteresting, but that changes dramatically over time. Unfortunately not many readers will have the time and the space needed to grow a specimen to perfection, but in north and west Madagascar, where it is endemic, it is often grown as an ornamental tree (see first picture). It is also a characteristic component of deciduous forests and palm savannas.
These pachycaul trees may become 3-6 m tall, with a swollen stem-base 50-70 cm in diameter and deciduous leaves 16 cm long and 4.5 cm wide.
The inflorescences appear at the branch tips and bear numerous white flowers with
lobes 2-4 cm long and about 2 cm wide.
For a little prologue to this post, see the preceding one: Kalanchoe integrifolia).
Although Mt. Ibity is perhaps the most accessible place, P. brevicaule also occurs in a couple of other habitats in Madagascars central highlands.
Plants can be found from Ambositra to Antananarivo in quartz rock at altitudes between 1400 and 1600 m.
In some spots where P. brevicaule grows together with P. densiflorum or P. rosulatum, one can come across hybrids. Judging from the long flower stalks, I suppose the plants shown in pictures # 3 and 4 belong to either of these hybrids.
The first two pictures show views from where P. brevicaule grows on Mt. Ibity. On # 2 you can see the quarry belonging to the massive cement factory nearby.