Senna is a large genus of approximately 300 species in the the Fabaceae (peas and beans family). Most of the species in this genus are woody shrubs and of little interest to the succulent collector, but this pachycaul species is very attractive and an excellent bonsai subject.
Growth Habit: Even when small, the fat, gnarled stems and small pinnate leaves make this species look like a miniature tree. A very slow grower, it will eventually attain a height of 2-3m. The flowers are a bright yellow and similar to others in the family.
Scientific name: Senna meridionalis (R. Vig.) Du Puy
Common names: None in English. In Malagasy, Taraby, Andapary (Mahafaly), and Tainjazamena.
Etymology: From the Arabic 'sanaa' ("thorny bush") and Latin 'meridionalis' ("midday"). The latter may refer to this species' habit of closing up its leaves at night.
Taxonomy: First described as Cassia viguierella var. meridionalis by Jean H.P.A. Ghesquière (Ghesq.), an invalid name. Described as C. meridionalis by René Viguier (R. Vig.) in 1948, then renamed as S. meridionalis by David J. Du Puy (Du Puy) in 1995.
Distribution: Endemic to (only found in) Madagascar, with limited distribution. Only about 400 plants remain in the wild.
Habitat: Varied, usually on limestone plateaux.
Cultivation: An easy grower, not particularly fussy about either light, watering or soil. It may be best to give it as much sun as possible to maintain its compact shape, but in my experience over a few years, this isn't really necessary. Watering is slightly problematic. In summer, it will take quite a bit of water (assuming well-draining soil), but will also remain in leaf during the winter if you continue to water it. I've found it best to cut back drastically on water during the winter, to allow the leaves to drop and give it a dormant period. My plant got mealy bugs, but responded well to a combination of horticultural oil as a spray and the systemic imidacloprid, and has been mealy-free for about a year.
Minimum temperature: I haven't been able to find any temperature records specific to this species' native habitat (southern Madagascar), but it is probably safest to not let it go below 50 degrees.
Conservation Status: Not subject to CITES restrictions, but as of 2006, considered "Vulnerable." In March 2010, proposed to be included in CITES Appendix II ("not necessarily now threatened with extinction but that may become so unless trade is closely controlled"), see http://www.cites.org/eng/cop/15/prop/E-15-Prop-30.pdf
Observations: A poster in another forum mentioned that their plant closes up its leaves in the heat of the midday sun. Mine doesn't, but that's a much better explanation for the specific (species) name.