Four of the six species belonging to the section Columnares of Crassula are more or less well known (barklyi, congesta, pyramidalis and -of course- columnaris).
One other (C. multiceps) I have never even seen and the subject of this post is not widely known either.
It is a small, more or less erect plant, 8-25 cm tall when in flower, sometimes with several short branches at the base. The green to brown leaves are normally all about the same length (usually 5-8 mm but sometimes to 1.5 cm). They often covered with sand particles.
In September-November, the main stem bears many small rounded inflorescences on the upper part of the flower stalk.
The plants occur from Vanrhynsdorp to Calvinia, Worcester and Montagu on sandy or gravelly slopes (often facing south).
This species occurs in the western Karoo from Matjiesfontein to near Calvinia and is closely related to E. loricata and E. multifolia. (Peter Bruyns in “Plants of the Greater Cape Floristic Region”, published in 2013, even considers it a synonym of E. loricata).
The plants form rounded cushions to about 15 cm tall and to 30 cm or more in diameter, with many branches, which usually completely hide the main stem.
The 2-5 cm long white spines, which are in fact modified peduncles, are very obvious in the dry season; in winter and spring they are partly hidden by the up to 4 cm long leaves.
Plants belonging to this subspecies occur fairly widespread in northeast Uganda and northwest Kenya in sandy rocky soils, usually in the open at altitudes between 1075 and 1850 m.
They have a thick fleshy root with densely tufted, more or less round branches, which are 5-8 mm thick and erect to 15 cm tall or creeping to 50 (-100) cm long.
The branches bear tubercles to 2 cm apart in 4 series, with triangular spine shields 6 x 1.5 mm in size and spines 5-15 mm long; they differ considerably in colouring, from bright green often with darker longitudinal stripes to greyish-green with purplish stripes.
This beautiful species occurs widespread from DR Congo and Tanzania to Sudan, Eritrea and Somalia on rocky slopes in wooded areas and cultivated lands at altitudes between 1200 and 2400 m.
It is also often cultivated as an ornamental as well as medicinal plant.
It has upright stems (often creeping at the base) 0.5-1.3 m. or more tall, with leaves up to 25 cm long and about 13 cm wide which are often marbled with brown to purple markings on both sides.
The inflorescences are 30 cm or more tall and the flowers are white (rarely cream), sometimes flushed with pale pink. The flower have long tubes, usually between 4.5 and 12 cm long.
There is only one place in the world where this species is known to occur: Tanjona Vohimena, the southernmost tip of Madagascar*.
On this stark, wind-swept limestone terrace about 100 m above sea level, the species occupies an area of less than a km².
Old plants possess a large turnip-shaped root to 30 cm long and to 10 cm wide, topped by a densely branched crown to 30 cm in diameter. In habitat the branches creep along the ground as a result of the constant wind; in cultivation they are more or less erect.
The leaves form rosettes at the tips of the branches, they are to 25 x 8 mm in size and green to red-brown.
* The old name for this is Cap Sainte Marie, which explains the specific epithet.
Plant in cultivation; scanned slide
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”.