Greetings from the plant kingdom! You may know me as a Christmas Cactus (*Schlumbergera*), and you're likely concerned about my changing color. While my leaves are typically a lush, deep green, seeing them turn red or purple can be alarming. But from my perspective, this is often a clear and honest signal. It's a story written in pigments. You see, I contain more than just chlorophyll, the green pigment essential for photosynthesis. I also produce pigments called anthocyanins. These flavonoids are responsible for red, purple, and blue hues in many plants. Under normal, happy conditions, the green chlorophyll masks these anthocyanins. However, when I experience certain types of stress, my physiology changes, and these vibrant pigments become visible. It's my way of communicating that my environment is out of balance.
The most common reason for my red or purple hue is too much light. In our natural habitat, we Christmas cacti are epiphytes, growing in the dappled shade of tree canopies in the Brazilian rainforest. We are adapted to bright, but indirect, light. When you place me in a location with intense, direct sunlight—especially the harsh afternoon sun—it triggers a defensive response. The excessive light energy can actually damage my photosynthetic machinery, a condition known as photo-inhibition. To protect myself, I produce more anthocyanins. These pigments act as a natural sunscreen, absorbing the excess light energy and dissipating it as heat to shield the sensitive chloroplasts within my cells. So, the reddening you see is essentially me putting on sunglasses. It's a sign that I am getting sunburned, and while it's a protective measure, prolonged exposure can lead to scorching, where my stems turn pale and crispy.
While light is the main factor, other stresses can also encourage anthocyanin production, often working in combination with light stress.
Nutrient Deficiency: A lack of essential nutrients, particularly phosphorus (P), can manifest as a reddish-purple coloration. Phosphorus is vital for energy transfer within my cells. When it's scarce, my growth can be stunted, and the sugar buildup in my tissues can stimulate anthocyanin production. If I haven't been fertilized with a balanced, diluted fertilizer during my growing season (spring and summer), I may not have the resources I need to stay green.
Temperature Extremes: I prefer comfortable room temperatures, much like you. Exposure to cold drafts or temperatures consistently below 50°F (10°C) can be a significant stressor. The cold can slow down my metabolic processes, including the ability to uptake phosphorus, leading to a purple tint. Similarly, extreme heat can compound the stress from high light levels.
Moisture Stress: Both drought and root rot from overwatering can cause problems. Under-watering forces me into survival mode, where reddening can occur. More seriously, overwatering damages my roots, preventing them from absorbing water and nutrients effectively. This creates a physiological drought and nutrient deficiency *inside* me, even if the soil is wet, which can also lead to a purple or red discoloration, often accompanied by wilting.
Seeing my red or purple color is your cue to play plant detective. First, assess my light situation. Please move me away from any south or west-facing windows that get intense direct sun. An east-facing window or a spot a few feet back from a south window is ideal. Next, consider my feeding schedule. If it's been over a year since I was repotted or fertilized, a mild, balanced fertilizer may help, but only during active growth periods. Check for drafts near windows or doors and ensure my pot has good drainage to avoid soggy soil. The good news is that if the stress is recent and not severe, I can often return to a healthy green once the conditions are corrected. However, stems that have already turned completely red or purple may not revert; the change will be visible in my new growth.
