Answer: Crown of Thorns is Euphorbia milii, a woody, spiny, climbing shrub with stems to 4' long in its native Madagascar. It's often grown as a houseplant and is commonly called Crown of Thorns or Christ Plant.
Euphorbia is from the Latin euphorbea for Euphorbius, a Greek physician in 1AD who used the sap medicinally; native to Madagascar; milky latex sap "bleeds" when stems are cut, and can be stopped by immersing in warm water; sap may cause skin irritation or blindness; from a very large genus of over 2000 species including the popular poinsettia; supposedly the plant used for Christ's crown of thorns
Crown of Thorns is an excellent houseplant for a sunny windowsill. It has a natural period of dormancy, and leaves may drop at this time, but new leaf buds will appear in a month or two. Provide your plant with average household warmth (minimum 55F in winter), and as much light as possible, but shade from direct summer sun. Water moderately from spring to autumn, and sparingly in winter, allowing the soil surface to dry between waterings.
To propagate, take stem cuttings in the spring or summer and allow the the milky sap to dry before placing in potting soil.
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