Grown as a house plant since Victorian times, crown of thorns (Euphorbia milii) is experiencing a recent explosion in hybridization and cultivar development, resulting in a greater choice of plant sizes and flower colors for both landscaping and container use. Hardy in U.S. Department of Agriculture plant hardiness zones 9b through 11, this African native produces long-lasting red, pink, yellow or blended-color flowers. Thick gray stems have sharp thorns and succulent leaves on new branches.
Soil and Water
Categorized as a succulent plant, crown of thorns needs excellent drainage and doesn't tolerate overwatering. In the landscape, place it as a specimen in rock gardens, cactus and other xeriscape gardens, or use it as a hedge, massed planting or border. Give plants a soil with excellent drainage, amending soil with up to one-third its volume with pumice or perlite, and using a raised bed if needed. Once established, it needs little supplemental water. For container plants, use a cactus soil mix, a container with holes about 2 inches larger than the root mass, and water only when the top two inches of soil becomes dry. Some of the more recent hybrids have higher water needs.
Light and Heat
Give crown of thorns full sun in areas with cooler summers and partial shade during the afternoon where summers are hot. They can tolerate more shade but the growth becomes leggier, petal color dims and flowers are less abundant. The colored flower-like structure is composed of bracts, with the true small flowers tucked into the center of the apparent flower. Crown of thorns may alarm you by losing all its leaves in response to heat and dryness, but it will grow a new set once favorable conditions return. It normally loses the leaves on older growth, leaving a bare lower stem.
Fertilizer and Pruning
Not a fast-growing plant, crown of thorns doesn't need a lot of fertilizer. For landscaping plants, use a balanced fertilizer such as 10-10-10 monthly in spring and summer, usually from March through August. Apply 4 tablespoons of granules per 10 square feet to landscaping plants. For container plants, feed with a solution of water-soluble 24-8-16 fertilizer, or 1/2 teaspoon per gallon of water. Use monthly to replace a usual watering. Prune crown of thorns to keep it to the shape and size you want, but avoid contact with the milky white sap, which is toxic. It can cause skin irritation, temporary blindness if it contacts the eye and is poisonous if eaten. Wear gloves and long sleeves when working with the plant, and take care not to touch your face. Clean pruners by dipping them in rubbing alcohol before use, and dip them in water during and after use to remove the sap.
Pests and Problems
Crown of thorns is generally pest- and disease-free. Indoor plants can sometimes suffer from common houseplant pests such as mealy bugs, spider mites and scales. Physically remove them with a cotton swab dipped in soapy water or wash them away under running water. Root rot can occur if the plants are overwatered or have poorly draining soil.