Crown-of-thorns (Euphorbia milii 'Ruby Red') is a slow-growing plant with a rambling growth habit. Individual plants can grow to 3 feet by 3 feet on this Madagascar native that may be extinct in the wild. Its 5- to 6-sided fleshy, thorny stems are adorned with short, oblong leaves. The flowers—yellow cyathia enclosed by red bracts—require full sun, moderate temperatures, and low humidity. It's a perfect addition to the desert garden.

The name of this plant is from the Latin euphorbea for Euphorbus, a Greek physician in 1 C.E. who used the sap medicinally. The milky latex sap of this Madagascar native "bleeds" when stems are cut; its bleeding can be stopped by immersing the stems in warm water. The Euphorbiaceae is a very large family, containing more than 2,000 species, including the popular poinsettia.