How to Grow the Crown of Thorns Plant 

Crown of Thorns, or Euphorbia milii, is an ideal houseplant. Able to adapt to room temperature it also thrives in dry conditions and copes well if you occasionally forget to water or feed it.

Originating in Madagascar, this is an attractive and reliable succulent. Its name refers to the belief that the crown of thorns worn by Jesus Christ at the crucifixion was made from this plant. It is also known as the Corona de Cristo or Christ Thorn.

One of the most distinctive houseplants, Euphorbia milii is pleasingly easy to care for. 

During the late spring and early summer months, the crown of thorns produces flowers in shades of pink, red and yellow. In certain conditions Euphorbia milii continues to  flower throughout the year.

While typically grown as a houseplant, crown of thorns is perennial in USDA Zones 9 to 11.

Warning: Crown of thorns is considered poisonous. When cut or damaged the foliage and stems emit a milky latex sap. This can cause the skin to blister. It can also cause open wounds to ache or become sore. If consumed the sap can also irritate the mouth as well as causing gastro-intestinal problems or hemorrhaging. These issues can occur in humans and animals.

Different Varieties of Crown of Thorns

Most modern crown of thorn varieties are hybrids. This means that you will be able to find a range of different varieties to grow. Nurseries in particular are a great place to find attractive or unusual hybrids.

Creme Supreme produces attractive creamy white flowers while Short and Sweet is prized for its red flowers. As the name suggests this is a smaller cultivar, reaching about 18 inches in height. Similarly Brush Fire produces bright red flowers above fleshy foliage.

The cultivar Thai Giants is bred for its large flowers and thorns. However, while the display is large, flowering is not as profuse as on smaller varieties.

Also known as Giant Crown of Thorns, California Hybrids are known for their thick stems and large flowers. A popular choice if you are growing outside, Saturnus and Rosalie are both reliable cultivars.

The different cultivars produce flowers and foliage in a range of shapes and sizes. However, they all have one thing in common, the distinctive thorny stems, which gave the plant its name. 

Planting and Repotting

Depending on how quickly it grows, you will need to repot Euphorbia milii once every two or three years.

You may also need to repot your crown of thorns soon after purchase. Shop purchased plants are often allowed to sit in containers that are too small, slowly becoming pot bound and draining the soil of its nutrients.

Repot in a pot that is either the same size or slightly larger. It should have drainage holes in the bottom. Clay pots are more porous than plastic pots. While these are better for drainage, crown of thorns can happily grow in a plastic pot.

Clay or terracotta pots are more porous than plastic alternatives. This helps the soil to drain excess water quickly, preventing plants from sitting in waterlogged conditions.

Adding a layer of pebbles to the bottom of the pot further helps to improve drainage. Fill the pot with well draining or sandy soil. A mixture that is two parts succulent or cactus soil and one part sand or perlite is ideal. This combination provides a well draining soil mix that helps to prevent problems such as root rot.

Make a hole in the center of the pot. It should be large enough to comfortably hold the plant. Carefully remove your crown of thorns from the container. Squeezing the sides of the pot loosens the soil aiding removal. If the plant is particularly difficult to remove you may need to cut it out of the pot.

Position the plant in the hole. It should sit at about the same level as in the previous container.

When you are happy with the position, firm down the soil. Water well with a watering can. A good watering helps the plant establish itself. Allow the water to drain away and place in a favorable position.

Finding the Ideal Position

Finding the ideal position for crown of thorns helps it to thrive. It also makes care a lot easier. A sunny windowsill is ideal. Euphorbia milii should receive 3 to 4 hours of direct sunlight every day.

During the winter months, Euphorbia milii is best placed on a south or west facing windowsill. If you struggle to find a position with enough natural light, try placing your houseplants under a grow lamp.

Avoid placing in a draughty or cool room. The room temperature should average between 65 and 75 ℉. A pleasingly robust houseplant, it can tolerate summer highs of up to 90 ℉ and winter chills as low as 50 ℉.

Originating in the desert, unlike other houseplants, the crown of thorns does not require high humidity levels.

In ideal conditions Euphorbia milii can reach 2 ft in height.

Placing Euphorbia milii in a bright, warm position helps to encourage healthy growth and flowering. 

Planting Crown of Thorns Outside

In the warmest USDA zones crown of thorns can be grown as a perennial. A full sun loving specimen, Euphorbia milii thrives in warm positions. It also tolerates salt spray well. In ideal conditions, crown of thorns can grow into a 3 to 6 ft tall shrub.

Before planting dig over the soil and work in sand or compost. This helps to further improve drainage.

Dig a hole large enough to hold the plant in its current container. Remove the plant from the pot and position in the hole. It should sit at the same level in the ground as in the pot. When you are happy with its position, backfill the hole and water well. Keep the soil evenly moist until the plant has established itself in its new home.

How to Care for Crown of Thorns

As long as your crown of thorns is in a favorable position care is pleasingly straightforward.

When to Water

Knowing how often to water houseplants can be difficult

Water when the plant is actively growing, this is usually from mid to late spring until early fall.

Wait until the top inch of soil feels dry before watering. A soil moisture gauge, such as the Atree Soil Moisture Meter, provides a more accurate way of measuring how wet your soil is. If you are growing in a container water until water begins to drain from the drainage holes in the bottom of the pot.

When Euphorbia milii is not actively growing, reduce the amount of water you give. Just don’t let the soil dry out completely. This can cause root damage.

Applying a layer of mulch helps the soil to retain moisture. This reduces how frequently you need to water. As organic mulches break down they also restore nutrients to the soil, further benefiting your plants. 

How to Fertilize

Only fertilize the plants when they are growing and producing foliage. General purpose fertilizer contains a good mix of nutrients and is ideal for crown of thorns. However there are a number of natural alternatives for houseplants.

Like liquid fertilizers, a water soluble fertilizer, diluted to half its strength, is easily incorporated into your watering routine.. Fertilize every other time you water.

You can also apply plant food that is rich in phosphorus to promote flowering. Avoid fertilizers high in nitrogen. These promote foliage production at the expense of flowering.

Pruning Euphorbia milii

Always wear gloves when pruning. The milky latex sap of the plant is toxic and can harm your skin.

A pleasingly robust plant, you can prune crown of thorns as harshly as you like and new growth will happily emerge.

When you prune, always try to cut away stems at their point of origin. At every cut branch two or three new branches will emerge. This helps to create a bushier plant.

Use a garden scissors to prune away weak, dead or damaged growth. Prune away any branches that are crossing or rubbing against each other. Tangled branches can rub away the bark, allowing diseases and pests an easy access point. They can also prevent air from circulating freely, this can lead to mildew diseases from striking.

Remember to clean your tools after using them. This prevents you from accidentally spreading pests and diseases throughout your collection.

How to Overwinter

Crown of thorns growing in containers can be taken inside in the fall and returned outside in the spring once the last local frost date has passed.

If you are growing Euphorbia milii in the garden, cease fertilizing and pruning about 3 months before your first predicted frost date. Both feeding and pruning can promote new growth. This is more susceptible to damage by winter frosts.

When frost is forecast, water the soil well and cover with a horticultural blanket such as the Haxnicks Easy Fleece Jacket. Use stakes, such as bamboo canes, to create a frame, preventing the blanket from touching the plant.

Remove the covering the following morning after the frost has thawed.

Repeat this process throughout the winter, every time a frost is forecast.

Common Pests and Diseases

Euphorbia milii is pleasingly pest resistant. Regularly check houseplants for signs of mealybugs or aphids. The latter can be washed away with an application of homemade insecticidal soap.

Regularly check Euphorbia milii for signs of infestation. Spotting problems early allows you to quickly solve them before they can cause too much harm. 

Mildew or fungus can develop if the plant is too humid or the soil too moist. Maintaining a regular temperature and good circulation around your houseplants, as well as placing in a light position, helps to keep them healthy.

Leaf spot is a common bacterial disease. It causes spots to form along the veins of the leaf. Spots forming in other parts of the leaf is a sign of a fungal infection or some other form of damage. Spots caused by leaf spot are usually angular in shape and gray-brown in color, often with a yellow halo type edge.

Over time leaf spots grow, merging into each other and creating a large area of dead tissue. Regularly check the flowers and foliage for signs of disease. Cut away affected foliage.

Destroy affected foliage by burning. Never place diseased foliage or plants on a compost heap. Prompt action helps to prevent disease from spreading throughout the plant and to other plants such as begonias, poinsettias and geraniums.

Correctly spacing and watering the soil not the foliage, or immersing containers in water instead of watering from overhead helps to prevent the disease from spreading.

Remember to clean any tools you use on infected plants.

How to Propagate

If you want to increase the number of houseplants that you have, propagation is easily done. The most reliable method is to take stem cuttings.

Stem cuttings are best taken in spring or early summer, when Euphorbia milii is actively producing new foliage. Use a sharp knife or scissors to make a clean cut. This also helps to prevent unnecessary damage to the plant.

Take healthy stem tip cuttings from healthy stems. Each cutting should be 3 to 4 inches in length. Remember to wear gloves when you take the cuttings. This protects your hands from the harmful latex sap.

To stop the sap oozing out, spray the cut areas of the mother plant with cold water. Dipping the cuttings in cold water stops them from oozing.

Store the cuttings on paper in a cool dry place until they dry out and a callus forms over the cut area. This can take a few days.

Once the callus has formed, pot in a container filled with a well draining potting medium. Care for the cutting as you would a larger plant.

Be careful when handling. Not only are the stems covered in thorns, but when cut they emit a harmful milky latex sap. 

An unusually attractive houseplant, the colorful flowers and spiky thorns of Euphorbia milii provide year round interest. Despite being commonly grown as a houseplant, in the warmest USDA zones crown of thorns also thrives in the garden.

Unusual and resilient, crown of thorns is one of the most pleasing houseplants to grow.