Euphorbia knuthii

Origin and Habitat, Southern
Description: Euphorbia knuthii is a dwarf spiny succulent shrublet with a tuberous main root, which continues into a serpentine caudex, freely branched from the top. It is sometimes accompanied by a number of underground rhizomes. It is closely allied to Euphorbia squarrosa, and Euphorbia Schinzii. It blooms in mid to late summer and the seed capsules ripe about one month later.
Cultivation and Propagation: Euphorbia knuthii is an easy ornamental plant to grow, and is a particular favorite of caudiciform plant enthusiasts. Cultivation of this plant is the same as that for the other tuberous varieties of Euphorbia, (e.g. Euphorbia stellata, Euphorbia decidua, etc.), but it is the easiest of all caudiciform Euphorbias to propagate from cutting. It tolerates any conditions (it is practically indestructible), surviving extended neglect and relatively low light.
Stem pruning: Every year or two in very early spring, trim off all of the stems, cutting them back to the soil line (or to the roots if you grow your plants with the roots partially exposed). After this pruning, allow the plant to heal at least a week to ten days before watering it. The heavy pruning encourages the growth of many additional branches and keeps the length of each branch comparatively short, and uniform.
Exposure: full sun to light shade exposures,
Watering: Water regularly during the active growing season. No water should ever be allowed to stand around the roots. Keep almost completely dry in winter. Even though it is extremely drought tolerant, during the active growing season it is best to keep its soil reasonably moist, watering it about once every week. In summer it will also tolerate lots of water
Hardiness: tolerate temperatures down to –6ºC and even a little snow. (USDA Zones 9-11).

Some of the information in this description has been found at desert-tropicals.com, llifle.com and cactus-art.biz