Light
Give it full sun, or the sunniest window you have. Light is what keeps it blooming, and it flowers best in full sun. Outdoors in a hot summer, a little midday shade helps.
Water
Let the soil dry out, then water thoroughly. Steady, moderate watering in spring and summer gives more flowers and less leaf drop. Water less in winter, because cold, wet soil can kill it.
Soil & pots
A gritty, soil-based mix that drains fast. The RHS suggests three parts loam-based potting mix to one part grit. Use a pot with a drainage hole.
Temperature
Warm rooms suit it. It reacts badly to temperatures below 35°F (2°C), so it only lives outdoors year-round in zones 10 to 12. Elsewhere it can spend summer outside and come in before nights turn cold.
Is crown of thorns safe for pets?
Toxic to cats and dogs. ASPCA doesn't list crown of thorns. NC State Extension flags it as a problem for cats and dogs, and the RHS says it's harmful to pets if eaten. Pet Poison Helpline says eating it causes mild mouth and stomach irritation.
Propagation
Crown of thorns roots easily from tip cuttings, and it's easy to share. Wear gloves for every step, because the cut ends ooze milky sap.
- Wearing gloves, cut a stem tip a few inches long.
- Let the sap dry and the cut end callus for a few days.
- Set it in barely damp gritty mix in bright light.
- Water sparingly until new leaves show it has rooted.
Common problems
FAQ
Is crown of thorns toxic to cats and dogs?
Yes. NC State Extension and the RHS both flag it as harmful to pets. Eating it usually causes mild mouth and stomach irritation, with drooling, vomiting or diarrhea, and the sap can irritate skin and eyes.
Are the colorful parts really flowers?
No. The red, pink, yellow or white "petals" are bracts, a kind of modified leaf. The true flowers are tiny and greenish, tucked in the center.
Does crown of thorns bloom all year?
It can. Indoors it often blooms from late winter well into fall, and in the tropics it flowers all year. The more light it gets, the more it blooms.
Is the sap dangerous to people?
It's an irritant. The milky sap can cause a skin rash and, in the eyes, damage to the cornea. Wear gloves when you prune it or take cuttings, and keep your hands away from your face.
Why is it called crown of thorns?
From a belief held by some that the crown of thorns at the crucifixion was made from its stems. The stems are covered in sharp black thorns, so it earns the name.
Sources
- Euphorbia milii (Plants of the World Online, Kew)
- Euphorbia milii (Missouri Botanical Garden Plant Finder)
- Euphorbia milii (NC State Extension Plant Toolbox)
- Euphorbia milii (RHS)
- Crown of Thorns (UF/IFAS Gardening Solutions)
- Crown-of-Thorns in the shade (UF/IFAS Extension Nassau County)
- Euphorbia milii (UC ANR, Plant Safely in Drought-Tolerant Landscapes)
- Crown of Thorns (Pet Poison Helpline)
- Toxic and Non-Toxic Plant List: Cats (ASPCA)
- Toxic and Non-Toxic Plant List: Dogs (ASPCA)