Crown of thorns

About Crown of thorns

Crown of thorns is a spiny, shrubby succulent from Madagascar, a cousin of poinsettia, with gray zigzag stems armed with sharp spines, bright green leaves on the new growth and small flowers wrapped in long-lasting red, pink, yellow or white bracts (NC State, UF/IFAS). In a sunny window it can bloom nearly all year and stays under 2 ft, and it shrugs off dry winter air (NC State, Wisconsin Horticulture). Its milky latex irritates skin and is dangerous in the eyes, so it is a gloves-on plant.

Hardy outdoors only in zones 9-11, where it is a small shrub (NC State, UF/IFAS); it is grown indoors everywhere and can summer outdoors in sun, eased into the light gradually, once nights stay well above 35 F, coming in well before frost (Wisconsin Horticulture).

When to plant

Timing is counted from your average last spring frost (and first fall frost). Sow & Tell works out the actual dates for your ZIP code.

Usually bought as a nursery plant or started from a tip cutting whose sap has been left to dry before it is stuck (NC State, UF/IFAS).

Growing it indoors

• Light: Give it the sunniest window you have; it tolerates part shade but blooms less, and lamps on at night can cut flowering, since it blooms best with long, dark nights (Wisconsin Horticulture, NC State).
• Water: Water deeply, then let the mix dry before the next drink; many modern hybrids want water a little more often than the species (Wisconsin Horticulture). Cold, soggy mix rots the roots, so use a pot with a drainage hole and water less in winter (NC State).
• Humidity: It tolerates low humidity and does well in heated homes in northern winters (Wisconsin Horticulture).
• Temperature: It survives down to about 35 F (Wisconsin Horticulture; NC State says it won't take less), but keep it in a warm, bright room and bring summer plants in well before frost.
• Feeding: Feed lightly with a balanced fertilizer in spring and summer only; too much gives soft growth and few flowers, and it is sensitive to boron, so skip fertilizers with added micronutrients (Wisconsin Horticulture).
• Repotting: Repot only when it outgrows the pot, in spring or summer, into a fast-draining cactus-type mix; most kinds stay compact and still bloom in a small pot (Wisconsin Horticulture).

Soil

A fast-draining mix such as cactus mix with extra perlite, pumice, sand or gravel and a little compost (Wisconsin Horticulture).

Pot in a fast-draining mix such as cactus mix with extra perlite, pumice, sand or gravel and a little compost (Wisconsin Horticulture).

Feeding

• Spring and summer, during active growth: balanced fertilizer without added micronutrients. Feed lightly; it is sensitive to boron, so avoid fertilizers with micronutrients (Wisconsin Horticulture).
• Fall and winter: none. Stop feeding when growth slows (Wisconsin Horticulture feeds only in spring and summer).

Overfeeding gives soft, lush growth and few flowers, and wet, cold mix rots the roots (Wisconsin Horticulture, NC State).

Varieties (15)

Pests to watch for

Mealybugs, Twospotted spider mite, Scale insects, Thrips

Diseases and problems

Root and crown rot, Stem rot, Leaf spots, Gray mold (Botrytis)

In the kitchen

Safety: The milky latex in every part contains irritating diterpene esters: it can cause mild to severe skin rashes, can damage the cornea or cause temporary blindness if it gets in the eyes, and blisters the lips and mouth if the plant is chewed (NC State rates it low severity; Wisconsin Horticulture). Wear gloves and eye protection, prune with a knife, and mind the sharp spines around children.

Pets: Not listed by the ASPCA (no crown-of-thorns or Euphorbia milii entry on its plant lists); NC State lists it as a problem for dogs, cats and horses because the latex irritates the mouth and stomach, and the spines can injure pets.