Crown of thorns

Identification and origin

Euphorbia milii belongs to the spurge family, Euphorbiaceae, alongside poinsettia, castor bean and cassava. It is native to Madagascar. The epithet milii honors Baron Milius, governor of the island of Bourbon, who introduced it to France in 1821. The common name refers to a legend that its stems were woven into the crown of thorns worn by Jesus, which is unlikely.

It is a woody, succulent, heavily branched shrub. The gray-brown stems have five to seven sides, store water and bristle with sharp spines 0–1 in long. The leaves are bright green, smooth, wider toward the tip and about 2–2 in long; they grow only on young shoots and drop as the stem ages, so an older plant has bare stems with tufts of leaves at the ends.

What look like flowers are the typical spurge inflorescences called cyathia: tiny greenish flowers framed by a pair of colored bracts, bright red in the species and yellow in some forms. Hybrids come in white, cream, yellow, pink and red, sometimes double. Indoors it blooms from late winter well into fall, and almost year-round in tropical climates.

In a pot the plant stays around 12 in – 2 ft; the species reaches 3 ft, and some larger forms grow to 5–6 ft. Among the many selections, the so-called giant California hybrids such as ‘Rosalie’ and ‘Vulcanus’ have thick stems and large bracts; the Thai hybrids carry very full clusters; ‘Mini-Bell’ is a dwarf with small red flowers, and ‘Peppermint Candy’ has white-variegated leaves.

Light

Light is the key to flowering. Give it the sunniest spot in the house, with six to eight hours of direct sun a day, and turn the pot weekly for even growth. In part shade it grows but blooms much less.

It can spend summer outdoors: acclimate it gradually to outdoor sun to avoid sunburn and, in very hot climates, give it some shade during the harshest hours. At night it needs darkness, since long evening exposure to artificial light can keep it from setting flowers.

Watering

Water thoroughly until the whole mix is moist, let the excess run out of the drainage hole, then empty the saucer. Wait until the soil is just dry before watering again. A mix that stays wet can kill the plant, yet roots left dry for a long time make the leaves drop, so aim for balance. It blooms best kept on the dry side.

Modern hybrids, which are leafier, often need somewhat more frequent watering than the species. After flowering and during the winter rest, reduce water slightly without letting the root ball stay dry for long. The dry air of heated homes does not bother it.

Potting mix

The mix should be rich and, above all, very free-draining. Combine one part perlite with two parts organic potting soil with no added fertilizer, or use a cactus mix amended with perlite, pumice or coarse sand and a little compost. In a mix that is too dense, the plant thrives at first and later suffers root rot.

In spring and summer, feed lightly with a balanced fertilizer once or twice a month until flowering ends. Avoid high-nitrogen fertilizers, which give lots of soft stems and few flowers, and products containing boron, to which it is sensitive.

Temperature

It likes warm household conditions and grows and blooms well at 64–81 °F. Keep it above 61 °F at all times, away from cold air currents: as soon as temperatures dip toward 50–59 °F, leaves fall early. A slight drop in fall and winter, while staying above that threshold, helps it rest.

Outdoors it survives down to about 36 °F but does not tolerate cold, wet soil, so bring it in well before the first frost. In the ground it is grown in USDA zones 9a to 11b.

Repotting

Repot in spring, about every two years or when the plant outgrows its container, moving up only one pot size: slightly cramped roots favor flowering. Wear thick gloves, protect your eyes and hold the plant with a pot holder to avoid spines and sap. Once the plant is mature, simply replace the top layer of mix each year.

Pruning

Pruning in spring keeps the plant compact and encourages branching. Use a clean knife rather than shears, which the sap gums up, and spray the cuts with cool water to stop the flow. Remove damaged or spindly branches first, then shorten the others just above a healthy bud or a branch junction. Keep at least half of the foliage, because a plant stripped too bare recovers poorly.

Propagation

Tip cuttings are easy. Water the plant the day before, then in spring or summer take the top 3–6 in of a young, leafy stem no thicker than your little finger. Rinse the cut in cold water or dip it in powdered charcoal, then let the cutting dry for two or three days out of direct sun so it calluses. Keep only three or four leaves.

Insert it about 2 in deep in a lightly moist mix of potting soil and perlite, after dipping it in rooting hormone if you like. Place the pot in bright light without direct sun and water only enough to keep the mix barely damp. Roots form in five to eight weeks in warm conditions. Seed is rarely used, as plants seldom set seed without hand pollination.

Common problems

• Leaf drop: normal aging of the stems, but also a mix left too dry, cold, or too little light.
• No flowers: not enough light, too much fertilizer, light at night, or a weakened plant.
• Parts turning brown and rotting: too much water or a mix that holds too much moisture; cut out affected parts right away to stop the spread.
• Leaf spots, gray mold: foliage that stays wet; remove dead leaves and spent flowers.
• Pests: mostly mealybugs, less often spider mites, scale and thrips.

A single symptom can have several causes: look at light, watering, temperature and roots together.

Toxicity

The whole plant contains a white latex rich in diterpene esters, toxic to people, though usually of low severity, and to animals. On skin it causes irritation that can be severe; in the eyes it can cause temporary blindness and damage the cornea. Swallowed, it irritates the mouth and blisters the lips. In dogs and cats it causes drooling, loss of appetite, vomiting or diarrhea. Wear gloves and eye protection when handling it, and keep it out of reach of children and pets.