Origin and Habitat: Euphorbia horombensis is endemic to Madagascar, located in the areas of Horombe, Ihosy and Isalo (Province of Fianarantsoa). This species has a small extent of occurrence of (2,947.07 km²) and a small area of occupancy (371.954 km²). Recorded from five localities and comprises one to four subpopulations.
Altitude range: Around 750 metres above sea level. Pachypodium (Apocynaceae)
Habitat and Ecology: E. horombensis occurs on rock outcrops (Mostly on bare gneiss rocks in full sun) of the inselbergs and is locally common throughout its range, when suitable rocks are available. (Mean Annual Temperature = 20.5°C, Mean Annual Precipitation 840 mm). This species is accompanied by other succulents such as Pachypodium lamereiSN|10823]]SN|10823]], Pachypodium horombense, Aloe betsileensis, Euphorbia milli, Alluaudia dumosa, Stapelianthus sp. and Tetradenia sp.. It is threatened by habitat degradation, fire, and collection for the horticultural trade.

• Euphorbia horombensis Ursch & Leandri

Description: Euphorbia horombensisSN|23360]]SN|33880]] is a laxly and irregularly branching, xerophytic shrub with spiny succulent stalk of the Euphorbia miliiSN|23360]]SN|23360]] group, 1 to 1.5 m high. It is similar in growth habit to Euphorbia miliiSN|33880]]SN|23360]] var. breoni but differs from it in the shape of the spines, which are flattened base, in the densesly branched inflorescences and in the smaller, reddish brown cyathophylls (Specialized leaves that surround the cyathium and give an overall flower-like appearance to the whole).
Derivation of specific name: 'horombensis' is a geographical epithet that refers to its occurrence in the Horombe Mts.
Stems: Branches up to 6 cm in diameter at the base, 2 to 3 cm towards the apex, re-branching above, with brownish grey bark.
Spines: Stipular spines in 8 vertical series, grey, acute, flat and longitudinally broadened at the base, up to 1.5(-3) cm long, 5 mm apart, solitary or with 1 to 2 small prickles.
Leaves: Persistent during growth period, in a terminal rosette at branches tips, 6 to 8 cm long, 2.5 to 3 cm broad, greyish green, margins red, petioles short (± 2 mm).
Inflorescences (cymes): clustered at branches tips on peduncles to 5 cm long, 5- to 6-forked, rays to 1 cm long, with up to 40 cyathia, cyathophylls 6 x 6 mm, apex rounded, apiculate, crimson to brownish red.
Flowers (Cyathia): 4 mm in diameter. Nectar-gland 5, these ovate, obtusely lobed, separate, greenish or orange red. Pedicels 2 mm long.

Bibliography:
Major references and further lectures
1) Urs Eggli “Illustrated Handbook of Succulent Plants: Dicotyledons” Springer Science & Business Media, 2002
2) Haevermans, T. 2004. Euphorbia horombensis. The IUCN Red List of Threatened Species 2004: e.T44363A10886737. http://dx.doi.org/10.2305/IUCN.UK.2004.RLTS.T44363A10886737.en. Downloaded on 14 March 2017.
3) Buddensiek, V. “Sukkulente Euphorbien”. 1–176.1998.
4) Schill, R. “Cytotaxonomische untersuchungen an sukkulenten vertretern der gattung Euphorbia L. aus Madagaskar.” Ber. Deutsch. Bot. Ges. 84(1/2): 71–78. 1971.
5) Ursch, E. & J. Leandri. “Les euphorbes malgaches epineuses et charnues du Jardin Botanique de Tsimbazaza.” Mém. Inst. Sci. Madagascar, Sér. B, Biol. Vég. 5: 109–185.1954.
6) Walter Minuth, Rikus van Veldhuisen & Volker Buddensiek “Euphorbias from A – Z” Euphorbia world Vol. 7 n. 1 - April 2011 web: https://www.euphorbia-international.org/journal/pdf_files/EW7-1-sample.pdf
7) S. H. J. V. Rapanarivo “Pachypodium (Apocynaceae)” CRC Press, 01 June 1999
8) Werner Rauh “The Wonderful World of Succulents: Cultivation and Description of Selected Succulent Plants Other Than Cacti” Smithsonian Institution Press, 1984
9) Urs Eggli, Leonard E. Newton “Etymological Dictionary of Succulent Plant Names” Springer Science & Business Media, 29 June 2013
10) MARGARET EVANS, XAVIER AUBRIOT, DAVID HEARN, MAXIME LANCIAUX, SEBASTIEN LAVERGNE, CORINNE CRUAUD, PORTER P. LOWRY II and THOMAS HAEVERMANS “Insights on the Evolution of Plant Succulence from a Remarkable Radiation in Madagascar (Euphorbia)” Systematic Biology Advance Access published June 27, 2014

Cultivation and Propagation: Euphorbia horombensis is a particular favourite houseplants. Cultivation of this plant is the same as that for the other shrubby varieties of Euphorbia from tropical areas (Madagascar, and central Africa). It flowers in spring.
Exposure: Full sun to lit shade appears to be the optimum range, but can tolerate the most shade, and a plant that has been growing in shade should be slowly hardened off before placing it in full sun as the plant will be severely scorched if moved too suddenly from shade into sun.
Wind and salt tolerance: This species is particularly tolerant of seaside conditions and salt spray, making it an excellent choice for planting along the coasts.
Waterings: Relatively flexible in its watering requirements. They can be watered regularly as long as the medium is open and well drained. As with any normal plant when watering, it is best to do so thoroughly, until a little water comes out through the drain holes. Allow the medium to dry out somewhat between waterings. During the resting time occasionally moisten the substrate when needed, don’t let plants go completely dry during winter. Space plants apart to allow air movement between branches and leaves. This will help with evaporation of extra water droplets collected during watering.
Soil: The ideal potting-medium is loose peat with a little gneiss, pH 4.5.
Fertilization: Regular fertilizing with low nitrogen and high phosphorus and potassium ratios are preferred. Feed during spring and summer to mid autumn and withhold feeding during winter.
Maintenance: Tall plant will benefit from being staked, with bamboo or other suitable stakes. In the absence of staking, the stems of the plants may snap under the weight.
Temperatures: Very tender, protect from frost. Temperature from spring to autumn: nocturnal 15°C and diurnal 35°C. Wintering: nocturnal 10°C and diurnal 18°C.
Pest and diseases: Mildew can occur in high humidity.
Propagation: Propagation by seeds and cuttings. It is recommend taking Euphorbia cuttings in Spring/Summer when the plant is growing so that they have a better chance of success. They key is heat & good air circulation. These cuttings should be dipped in Hormone powder (but it is not needed) and left for a period of 3-4 weeks to callous. Then pot the cutting and don't water ( or kept slightly moist) until rooted. These will root just fine, if you can put the pot outside in the summer, or put pot on a heating pad.
Warning: The milky sap produced by this shrub has been known to cause burning and blistering of the skin, and even temporary blindness, and death may result if any of its parts are eaten. All of these problems can be avoided if care is taken when handling the plant.