The Crown of Thorns (Euphorbia milii) is a popular succulent shrub cherished for its vibrant, long-lasting blooms and thorny stems. It is a favored ornamental plant for both indoor and outdoor cultivation. Originating from tropical Madagascar, it thrives in warm environments, making cold sensitivity a significant consideration for gardeners.
Cold Tolerance Limits
The Crown of Thorns shows stress below 50°F (10°C). While it can withstand brief exposures to temperatures around 40°F (4.4°C), prolonged periods at or below this threshold can lead to significant damage. Freezing temperatures (below 32°F/0°C) are fatal, causing irreversible cellular damage.
For outdoor planting, the Crown of Thorns is best suited for USDA Plant Hardiness Zones 9 through 11, where freezing temperatures are rare. In Zone 9, protection may still be necessary during severe cold snaps. Gardeners in zones colder than 9 treat Crown of Thorns as an annual or bring it indoors before winter.
Recognizing Cold Stress
Early signs of cold stress include wilting or drooping of leaves and stems. As exposure continues, the affected foliage may begin to turn yellow, then brown or black, indicating tissue death.
The texture of cold-damaged stems and leaves can become soft or mushy to the touch. Eventually, these compromised parts may shrivel and fall off the plant. These signs may not manifest immediately, often appearing several hours to a few days after exposure as the plant reacts to damage.
Protecting From Cold
Protecting Crown of Thorns plants from cold damage is important. For outdoor specimens, the most effective protection in colder zones involves bringing the plant indoors before night temperatures consistently fall below 50°F (10°C). If the plant is too large to move, covering it with a frost cloth, burlap, or even a blanket can offer temporary protection during unexpected cold snaps.
For plants remaining outdoors, selecting a planting site that offers some natural shelter, such as near a south-facing wall or under a protective overhang, can help mitigate cold exposure. Applying a thick layer of mulch around the base of the plant can insulate the root system from freezing ground temperatures. Indoor Crown of Thorns plants should be positioned away from drafty windows or doors, and direct contact with cold glass should be avoided to prevent localized chilling.
After Cold Exposure
If a Crown of Thorns shows cold damage, move it to a warmer location with stable temperatures, ideally between 65-75°F (18-24°C), but avoid placing it immediately next to a direct heat source. Resist the urge to prune damaged parts immediately, as the full extent of the damage may not be apparent for several days or weeks.
Allow the plant to recover and new growth to emerge, typically in spring. Once new growth is visible, or if the damaged sections become crispy and clearly dead, prune these back to healthy tissue using clean, sharp shears. Reduce watering temporarily until the plant shows clear signs of active growth, ensuring the soil is allowed to dry out between waterings to prevent root rot.