Crown of thorns

Crown of thorns: how do you recognise it?

Habit

Branching shrub, upright to somewhat sprawling, 30–80 cm tall in a pot and up to 50 cm wide.

Stems

Thick, grey-brown and densely covered with sharp spines up to about 2 cm long.

Leaves

Oval, bright green, 3–5 cm, only at the tips; they drop in drought or cold and grow back later.

Flower

Small true flowers, each surrounded by two round, coloured bracts, in clusters on long stalks.

Colour

Usually red, also pink, salmon, yellow, cream or white.

Sap

White and sticky; it oozes from every wound and is toxic and irritant.

Flowering: March–October · Plants: March–April · Pruning: March–April

Why grow it?

• It flowers almost all year, with clusters of brightly coloured bracts.
• It tolerates dry room air, full sun and a forgotten watering.
• It grows slowly and stays compact in the same pot for years.
• A tough plant for a sunny south-facing windowsill, where many houseplants scorch.
• In summer it can go outdoors in a warm, sheltered spot.

Crown of thorns: where does it grow best?

• As sunny as possible: a south- or west-facing window. The more light, the more flowers.
• Warm, 18–25 °C, and never for long below about 10 °C.
• Free-draining cactus compost, or potting compost mixed with one third coarse sand.
• Dry: the compost may dry out completely between waterings.
• From June to September it can go outdoors in a warm, sheltered spot, first for a few days in partial shade.

Light

Sunny

Potting soil

Cactus compost

Acidity

Neutral

Watering

Sparingly

Size

0.8 m tall, 0.5 m wide

Temperature

Minimum 10 °C

Goes well with

• Aloe: Another sun-lover that needs little water, with thick, toothed leaves.
• Cacti: The same cactus compost, the same sunny spot and the same sparing care.
• Snake plant: Upright, striped leaves as a calm contrast to the colourful flowers.
• Echeveria: Low rosettes in a bowl beside the crown of thorns.

Crown of thorns: when and how to plant it?

Plant in March–April.

1. Repot in March–April, every two to three years or when the pot is full of roots.
2. Choose a pot only slightly larger, with a drainage hole; terracotta is ideal, as it is heavy and dries out well.
3. Use cactus compost, or ordinary potting compost mixed with one third coarse sand or fine gravel.
4. Wear thick gloves and hold the plant with a rolled-up newspaper, to avoid the spines and the sap.
5. Set it at the same depth as before and give it only a little water after a week.

Care

Water

From March to October, water when the compost is dry and let the excess drain away. In winter, very little.

Feeding

From April to September, a half-strength cactus feed once a month.

Light

Full sun all year round; the more light, the more freely it flowers.

Leaf drop

Cold, draughts or a soggy root ball make it shed its leaves. They grow back once it is warm and bright again.

Repotting

Every two to three years in spring, wearing gloves.

Tip: Give it a permanent, sunny spot and do not move it too often: it reacts to change by dropping its leaves.

Crown of thorns: when to prune it?

Pruning months: March–April.

When: In March–April, as growth starts. Pruning is not necessary, only to keep it in shape or rejuvenate it.

How:

• Cut overlong or bare stems back to just above a branch with sharp scissors.
• Dab or mist the cut with lukewarm water so the sap stops flowing sooner.
• Use the removed tips as cuttings.
• Wear gloves, glasses and long sleeves, and put newspaper under the plant.

Caution: If you get sap in your eyes, rinse them straight away with plenty of water and seek medical help if they stay irritated.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Cuttings

In spring or summer: cut off tips 8–10 cm long, rinse off the sap with lukewarm water and let the cuttings dry for a few days. Set them in slightly moist cactus compost in a warm, bright place; they root within a few weeks.

Crown of thorns: how does it get through the winter?

• Houseplant: keep it at no less than 10–12 °C all year, and bring it back indoors in September if it was outside.
• Give it as much light as possible, right by the window; in a warm, bright spot it keeps flowering through winter.
• In winter, only water when the compost is completely dry, and then only a little; do not feed.
• Some leaf drop in a cool spot is normal; it will grow again in spring.

Diseases and pests

• Leaf drop: Caused by cold, draughts, too much water or a sudden move. Keep it warm and bright and water less.
• Root and stem rot: A soft, black stem base caused by wet compost, especially in winter. Let the compost dry out completely between waterings; take healthy tips as cuttings.
• Mealybug: White, woolly tufts between the spines. Wipe them off with a cotton bud dipped in methylated spirit.
• Few flowers: Too little light. Move it to the sunniest spot in the house or outdoors in summer.

Through the year

• January: Bright and warm. Very little water.
• March: Prune and repot. Flowering begins. Water a little more.
• April: Prune if needed. Feed. Take cuttings.
• May: Flowering. Water when the compost is dry.
• June: Move outdoors to a warm, sheltered spot. Take cuttings.
• July: Full flower. Feed monthly.
• August: Flowering. Watch for mealybug.
• September: Bring indoors. Last feed.
• October: Less water. Give it as much light as possible.
• December: Rest. Only water when the compost is completely dry.

Edible and useful

Not edible. Spurges grown as houseplants, such as crown of thorns and African milk tree (which looks like a cactus), contain a white, milky sap in all their parts that is poisonous and caustic: on the skin it causes redness and itching, in the eyes severe pain and possibly damage to the cornea. Cats, dogs and small children that chew them can get a burning mouth, drool and vomit. Keep the pot out of their reach, wear gloves and safety glasses when pruning, taking cuttings and repotting, and wash your hands and tools afterwards. If sap gets in your eyes, rinse them at once under running lukewarm water for at least a quarter of an hour and see a doctor. If any is swallowed, call a poison centre (in Belgium 070 245 245) or your doctor at once.

Did you know?

• The colourful ‘petals’ are bracts, as in the poinsettia, which belongs to the same genus.
• The species is named after Baron Milius, governor of the island of Réunion, who had the plant sent to France in 1821.
• In Dutch the genus Euphorbia is called wolfsmelk, ‘wolf’s milk’, after the white sap the species have in common.