Blooming prolifically throughout the year, euphorbia (Euphorbia millii) or "crown of thorns," makes a colorful addition to the landscape. It is slow growing and relatively maintenance free, whether grown indoors as a potted plant or outdoors in the ground in frost-free areas. Legend states that its originally white flowers turned red with shame after it was used to make the "crown of thorns" worn by Jesus, according to Texas A&M University Extension. Modern plant breeders have developed varieties with yellow, gold and orange flowers.
Yellow euphorbia is a cactus-like plant with thick, succulent, five-sided stems. What is commonly thought of as its flowers are actually bracts surrounding small, insignificant yellow flowers that are similar to those of the poinsettia, to which it is related. It grows to a height of approximately three feet with a spread of about two feet. Euphorbia naturally has light-green leaves, which is often mistakenly interpreted as a need for fertilizing.
Native to tropical Madagascar, crown of thorns is only winter hardy in USDA Zone 10 or warmer. However, it makes an excellent potted plant for summering outdoors in a hot, sunny and dry location. Planted in the ground in warmer areas, it prefers a spot in full sun and dry, well-drained soil. Exposing your euphorbia to temperatures below 65 degrees F. will cause it to go dormant and stop producing flowers.
Cultivars with yellow, gold and orange bracts have recently been introduced, according to the University of Florida Extension. Other varieties are now available with smaller flowers, fewer thorns and more prominent leaves. A strain of larger varieties with 12-inch long leaves has been introduced from breeders in Thailand.
Care and Culture
Euphorbia should be treated like any cactus or succulent and allowed to dry out completely between waterings, particularly when grown as a potted plant. Placed in full sun, the plants will produce flowers all year round, as long as temperatures remain above 65 degrees F. Feed your crown of thorns sparingly, as excess fertilizer can cause it to stop producing flowers. Use a water-soluble fertilizer formulated for blooming plants, and feed the euphorbia a half-strength solution every three weeks during active growth periods.
Pests and Diseases
Euphorbias are relatively pest and disease free, although young cuttings can be susceptible to fungal diseases. Insects common to many potted plants, such as whitefly, scale or mealybug, can infect crown of thorns. Treat infestations with a horticultural oil spray or insecticide.