Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.
I grew up in a home filled with exotic houseplants. One of them that I found particularly fascinating was the crown of thorns. I was intrigued by an ugly thorny plant that had brightly colored flowers year round.
What is Crown of Thorns?
Crown of thorns (Euphorbia milii) is a thorny shrub that is related to poinsettias. It is native to Madagascar but was brought to the Middle East where it became associated with the crown of thorns worn by Jesus when he was crucified. Other names for the plant include Christ Thorn and Christ Plant. In Latin America it is known as Corona de Cristo.
The shrub is hardy in zones 9 through 11. It cannot tolerate temperatures below 50⁰F. In colder climates, it is grown as a houseplant. It does well indoors because it likes warmer temperatures and dry air, similar to its native habitat in Madagascar.
Outdoors, the shrubs can grow to 3 – 6 feet tall. Grown as a houseplant indoors, they usually only grow 2 feet tall. The stems appear to be woody, but they are actually a succulent and able to store water, making the plants drought tolerant. The stems are covered with 1 inch thorns. Leaves only grow on new stems, falling off as the stems age. The leaves are fleshy and also store water.
Crown of thorns should be handled very carefully. The stems contain a sap that is toxic, causing irritation if you get it on your skin or in your eyes. If ingested, it causes mouth and throat irritation, stomach pain and vomiting. Always wear gloves and try to only handle them by their roots or young, leafy stems.
Crown of thorns blooms year round. The flowers are small and green, surrounded by two colorful bracts that are often mistaken for flowers. The bracts can be red, white or pink.
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How to Grow Crown of Thorns Outdoors
If you are fortunate enough to live in zones 9 through 11, you can grow this shrub outdoors as a striking specimen plant. Always plant this one well away from sidewalks or pathways to avoid contact with its thorns. Give it plenty of space from other plants too so that you don’t accidentally brush against it while tending to your garden. Those thorns are painful! Full sun and well-drained soil are a must. The more sun the shrubs get, the more flowers they will produce. Because they are drought tolerant, too much water will cause root rot, killing the plants.
Once a year, you will need to prune your plants. Wear heavy leather gloves and long sleeves to protect yourself from the thorns. Prune off any dead or diseased stems, as well as dead leaves. This will encourage new growth.
How to Grow Crown of Thorns Indoors
Crown of thorns grows well indoors as a houseplant. It can stay indoors year-round or spend the summer outdoors. Wait to move it outdoors until night-time temperatures are above 50⁰F. In the fall, move it back indoors before night-time temperatures fall below 50⁰F.
Use a pot that is only 1 inch larger in diameter than the root ball. Any larger, and the soil will retain too much moisture causing the roots to rot. It is best to use a potting mix that is designed for cacti, which is well-draining. Water only when the top inch of the soil dries out. When you water, add a balanced fertilizer for houseplants from the spring through the fall. During the winter, the plants are semi-dormant, needing less water and no fertilizer.
Place your plant in a south or west facing window. The more sunlight it receives, the more flowers it will produce. The most common reason for a crown of thorns grown indoors not blooming well is that it is not receiving enough light.
How to Grow Crown of Thorns From Cuttings
The crown of thorns plants available commercially now are all hybrids so they cannot be grown from seed. If you want to propagate your plants, you will have to do so using cuttings.
It’s very important that you wear gloves during this process because the sap in the stems is toxic and causes skin and eye irritation. Take your cuttings from new growth during the spring or summer while the plants are actively growing.
Place your cuttings in warm water for a few minutes to allow the sap to safely drain from them. Then allow the cuttings to dry and callous over for a few days. The callous is necessary to prevent disease or insects to enter the cutting before it develops its roots. Once the cut end has developed a callous, gently push it into sandy, well-draining soil and water sparingly. Roots should start to develop in 2 to 3 weeks. You will know that roots are growing because the cuttings will have new leaves. Plants without roots are unable to grow new foliage.
© 2019 Caren White