The Crown of Thorns (Euphorbia milii) is a durable, woody succulent known for its thick, spiny stems and colorful bracts, which are often mistaken for flowers. Because it stores water in its stems, the plant is naturally drought-tolerant and low-maintenance. Although hardy, its health and ability to bloom depend on meeting specific environmental conditions. Problems usually occur when care deviates from its native, arid requirements.
Diagnosis: Water and Soil Management Errors
The most frequent cause of distress is improper water and soil management. Overwatering is a common mistake that leads to root rot, a fungal condition where roots decay from prolonged exposure to moisture. Symptoms of overwatering include mushy, weakened stems and the rapid yellowing and dropping of lower leaves.
To fix overwatering, stop all irrigation immediately and allow the soil to dry out completely. If the soil is waterlogged or the plant shows advanced root rot, repot it into a fresh, fast-draining succulent or cactus mix. The pot must have sufficient drainage holes, as the plant cannot tolerate standing water.
Conversely, underwatering also causes leaf yellowing, but it presents with distinct physical symptoms on the stems. Insufficient water causes shriveling or wrinkled stems, dry or crispy leaf edges, and dried-up flowers. The fix is a deep, thorough watering until water runs out of the drainage holes. Afterward, return to a “drench between droughts” schedule, allowing the soil to dry completely before watering again. Consistent care is important, as sudden stress can cause the plant to shed all its leaves.
Light and Nutritional Deficiencies
The Crown of Thorns requires high light exposure, needing at least four to six hours of bright, direct sunlight daily to thrive and produce colorful bracts. Insufficient light is the primary reason a healthy plant fails to bloom. Low light levels cause etiolation, or “legginess,” characterized by stretched, pale stems and smaller, widely spaced leaves.
Relocating the plant to a south-facing window, or a west-facing window that receives intense afternoon sun, will correct the issue and encourage bud formation within weeks. If natural light is limited, supplementing with a dedicated grow light provides the necessary intensity. The plant’s nutritional needs are modest, but fertilization is necessary during the active growing season (spring and summer) to support continuous flowering.
A lack of fertilizer results in pale, small leaves and poor blooming. Applying a fertilizer with a high nitrogen ratio encourages excessive green leaf growth at the expense of bract production. The solution is feeding the plant with a balanced or bloom-specific liquid fertilizer, diluted to half strength, once a month during the growing season. A formula with lower nitrogen and higher phosphorus and potassium, such as a 5-10-5 ratio, supports prolific flowering.
Addressing Pest Infestations
While resilient, the Crown of Thorns can host common houseplant pests, each requiring a specific treatment protocol. Mealybugs are identified by small, white, cottony masses, typically congregating where leaves meet the stems. These sap-sucking insects drain the plant’s energy, causing distorted growth and a sticky residue called honeydew. For minor infestations, mealybugs can be manually removed using a cotton swab dipped in rubbing alcohol.
Scale insects appear as small, hard, brown or gray bumps adhering to the stems and leaves, often unnoticed until advanced. These pests also feed on the plant’s sap. Severe cases can be treated by carefully scraping the adult scale off with a fingernail or soft brush. Horticultural oil or neem oil can then be applied to smother remaining insects and their young.
Spider mites are minuscule pests confirmed by fine webbing, usually near new growth, and a stippling pattern of tiny yellow dots on the leaves. Mite populations increase in dry, warm conditions, so increasing the air humidity can help deter them. Treatment involves thoroughly rinsing the plant and applying neem oil, ensuring coverage of the entire plant surface, including the undersides of the leaves. New plants should always be isolated for several weeks upon purchase to prevent the introduction of pests to existing collections.