Native to Madagascar, Crown of thorns, (Euphorbia milii), is a tropical, succulent plant with thick, grayish-green leaves and showy, yellow, red or pink blooms that grow on a prickly, cactus-like stem. If you live in a climate with frost-free winters, crown of thorns can be grown outdoors, but for most gardeners, crown of thorns is an interesting, low-maintenance houseplant.
Plant crown of thorns in a sturdy container with at least one drainage hole in the bottom. Put a 1-inch layer of gravel in the bottom of the container, then fill the container with commercial potting mixture with a handful of coarse sand to improve drainage. Put a thin layer of gravel on top of the potting mixture.
Water crown of thorns deeply with lukewarm water, and allow the water to run through the drainage hole. Pour out water that remains in the saucer, and never allow the bottom of the container to sit in water, as the plant will rot if the roots are too wet. Allow the top of the soil to dry out between waterings. Avoid getting water on the stems and foliage.
Place the crown of thorns plant in full sunlight. If desired, move the container outdoors on warm, summer days.
Feed crown of thorns with a water-soluble fertilizer for indoor plants in April, and again in July. Crown of thorns is a slow-growing plant and doesn't require frequent fertilizing.
Keep crown of thorns in a warm room. Daytime temperatures should be between 60 and 85 degrees Fahrenheit, with nighttime temperatures about 10 degrees lower.