The Crown of Thorns (Euphorbia milii) is a hardy, drought-tolerant succulent. However, the common symptom of leaves turning yellow—a condition known as chlorosis—is a frequent source of frustration for owners. Leaf yellowing is not a disease itself but rather a sign that the plant is under stress from its environment or care routine. Diagnosing the issue requires a careful review of the plant’s conditions, as the underlying causes range from improper watering to nutrient deficiencies.
The Primary Suspect: Overwatering and Root Health
The most common reason for yellowing leaves on a Crown of Thorns is excessive moisture in the soil. As a succulent, it is highly susceptible to root problems when the soil remains saturated for too long. Root rot begins when the roots are deprived of oxygen in waterlogged conditions, creating an environment where harmful fungal pathogens can thrive. The damaged roots can no longer absorb water and nutrients efficiently, causing the leaves to yellow and often drop off entirely.
To check for this issue, insert a finger or moisture meter into the potting mix before watering again. The soil should be completely dry before rehydration, following a “drench between droughts” approach typical for succulents. If root rot is suspected, remove the plant to inspect the roots; healthy roots are firm and pale, while rotten roots are mushy, brown, and sometimes smell foul. Prune away affected roots, allow the remaining healthy root system to air-dry for a few days, and then repot into fresh, dry, well-draining succulent soil.
Environmental Causes: Light, Temperature, and Seasonal Stress
Light exposure is a frequent trigger for chlorosis, as the Crown of Thorns requires substantial bright light. Insufficient light, such as moving the plant to a dark corner, can cause the lower or older leaves to yellow and eventually drop as the plant conserves energy. Conversely, a sudden move from a shaded location to intense, direct afternoon sun can cause sun scorch, where the leaves yellow or develop burn marks from the abrupt shock.
Temperature fluctuations play a significant role in plant stress. The plant thrives in warm conditions, with an ideal temperature range between 65°F and 95°F. Exposure to cold drafts or temperatures below 50°F can shock the plant, leading to yellowed foliage and weakened growth. Additionally, the plant may naturally enter a period of semi-dormancy during the cooler, darker winter months. During this time, it is normal for some leaves to yellow and drop, signaling a natural seasonal response.
Nutritional Needs and Soil Age
The composition and age of the soil directly affect the plant’s ability to absorb necessary nutrients, manifesting as chlorosis. Over time, container soil becomes depleted of macro and micronutrients required for photosynthesis. This depletion is often compounded by soil compaction, which reduces the air space needed for root respiration and nutrient uptake. Compacted soil also limits the roots’ exploration area, restricting access to water and nutrients.
Nitrogen deficiency, needed for chlorophyll production, typically causes uniform yellowing across the entire leaf, starting with the older, lower foliage. Other micronutrient deficiencies, such as iron or magnesium, can cause interveinal chlorosis, where the tissue between the veins turns yellow while the veins remain green. To correct this, fertilize sparingly during the active growing season (spring and summer) using a balanced or cactus-specific feed diluted to half or quarter strength. Repotting every few years into fresh, well-draining cactus mix helps replenish nutrients and break up compacted soil.