Crown of Thorns

How to care for Crown of Thorns

Light

Crown of Thorns performs best in a south- or west-facing window where it receives several hours of direct sunlight each day. Insufficient light is the most common reason for reduced flowering; plants kept in shade will produce elongated, weakly spined stems and very few blooms. A minimum of three to four hours of direct sun is recommended, and outdoor placement in summer is beneficial if temperatures remain above 15 C.

Watering

Water thoroughly when the top half of the compost has dried out, then allow excess to drain freely. Overwatering is the leading cause of plant loss, as the roots are prone to rot in persistently moist soil. Reduce watering significantly in winter, giving just enough to prevent the stems from shrivelling. Always use room-temperature water and avoid wetting the stems or leaving the pot standing in water.

Soil and potting

A coarse, fast-draining mix is essential. A proprietary cactus and succulent compost is suitable, or a standard peat-free potting compost amended with around 30 to 50 percent horticultural grit or perlite. Good drainage prevents root rot and replicates the rocky, well-aerated soils of the plant's native habitat.

Feeding

Apply a balanced liquid fertiliser at half strength once a month from spring through to early autumn. A fertiliser with a relatively low nitrogen content, such as a tomato feed or a specialist cactus feed, encourages flowering rather than excessive vegetative growth. Do not feed during winter when the plant's growth slows.

Humidity and temperature

Crown of Thorns tolerates the dry air typical of centrally heated homes without difficulty and does not require misting or humidifiers. It prefers temperatures between 15 and 30 C and should not be exposed to temperatures below 10 C for prolonged periods. Keep it away from cold draughts, unheated windowsills in winter, and sudden temperature fluctuations, all of which can cause leaf drop.

Repotting

Repot every two to three years in spring, moving up only one pot size at a time. The plant blooms most freely when slightly pot-bound, so avoid overpotting. Use gloves when handling to protect against the spines and the caustic latex sap. Terracotta pots are preferable as they allow the compost to dry more evenly.

Pruning

Pruning is not strictly necessary but can be carried out in spring to maintain a compact shape or to encourage branching, which leads to more flowering stems. Cut back any leggy stems by up to one-third using clean, sharp scissors or secateurs. Wear gloves and eye protection, as cutting the stems releases white latex sap that is irritating to skin and eyes. Allow cut surfaces to callous by placing the plant in a dry, bright spot for a day or two before resuming normal watering.

Is Crown of Thorns toxic?

Common problems

How to propagate

Stem cuttings

Take a 7 to 10 cm cutting from a healthy stem tip in spring or summer. Dip the cut end in water briefly to stop the flow of latex, then allow the cutting to dry and callous for 24 to 48 hours. Insert into barely moist, gritty compost and place in a warm, bright spot, avoiding direct sun until roots form in four to eight weeks.

Seed

Sow fresh seed on the surface of moist, fine cactus compost at around 21 to 24 C in spring. Germination typically occurs within two to four weeks. Seedlings are slow to develop and variable in flower colour, so this method is more common in breeding than in general cultivation.

Where Crown of Thorns comes from

Native to Madagascar.

Story and origins

The genus name Euphorbia honours Euphorbus, the Greek physician to King Juba II of Numidia and Mauritania, who reportedly used the latex of a related plant medicinally. The specific epithet milii commemorates Baron Milius, who was Governor of Bourbon (now Reunion Island) and is credited with introducing the plant to France in 1821. The common name Crown of Thorns refers to the legendary association of the plant's long, sharp spines with the crown of thorns placed on Jesus Christ at his crucifixion.

Euphorbia milii is native to Madagascar, where it grows on rocky hillsides in seasonally dry habitats. It was introduced to Europe in the early nineteenth century, with records of its cultivation in France dating to around 1821 following its introduction by Baron Milius. The plant subsequently spread throughout the Mediterranean, the Middle East, and tropical regions worldwide, becoming naturalised in parts of South and South-east Asia. In Thailand, an extensive breeding programme beginning in the latter decades of the twentieth century produced the so-called Thai Hybrids, which transformed the range of available flower colours and bract sizes and greatly increased the plant's commercial popularity as a houseplant. Its association with the Passion of Christ has contributed to its symbolic use in churches and religious households in many parts of the world.

Varieties to know

Euphorbia milii var. splendens

The most commonly cultivated variety, with vivid red cyathia; tends to be more vigorous and larger in habit than the species.

Thai Hybrids

A large group of cultivars developed in Thailand with notably larger, often double or ruffled bracts in a wide range of colours including pink, yellow, white, and bicolour.

'Lutea'

A cultivar producing pale yellow to cream-coloured bracts, providing a softer alternative to the typical red forms.

'Short and Sweet'

A compact, dwarf cultivar well suited to smaller pots and windowsills, retaining the same flowering habit as the species.

Fun facts

• Despite being native to Madagascar, Euphorbia milii has been cultivated for so long in the Middle East that it is thought by many to be a native plant of the region, and is commonly associated with the crown of thorns worn by Jesus Christ.
• The plant is capable of flowering almost year-round under optimal conditions, making it one of the longest-blooming houseplants in common cultivation.
• What appear to be the flowers are not petals at all; the colourful structures are modified leaves called bracts (cyathophylls), and the true flowers are tiny structures at the centre called cyathia.
• Euphorbia milii has been used in some tropical countries as a component of living fences, as its dense spines make it an effective deterrent.
• The latex of several Euphorbia species, including E. milii, has been investigated for potential molluscicidal properties, meaning it may be capable of killing freshwater snails that act as intermediate hosts for schistosomiasis.

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