Crown-of-thorns is a slow-growing tropical shrub with a rambling growth habit. Plants can grow to 3 feet by 3 feet on this Madagascar native that may be extinct in the wild. The inconspicuous flowers, called “cyathia” are surrounded by colorful bracts. 'Pet Mong Koi' is a cultivar from Thailand, with bright pink flowers. Its 5- to 6-sided fleshy, thorny stems are adorned with long leaves with purple undersides. At the Chicago Botanic Garden, crown-of-thorns bloom all 12 months of the year in the greenhouses. Plants require full sun, moderate temperatures, and low humidity.

The milky latex sap of this Madagascar native "bleeds" when stems are cut; the 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.