The Thanksgiving Cactus (Schlumbergera truncata) is a popular houseplant known for its vibrant holiday blooms. Discovering its normally green segments, called cladodes, turning shades of red or purple can be alarming, but this color change is a common, non-fatal sign of stress. Unlike a desert cactus, this plant is an epiphyte from the tropical rainforests of Brazil. Its reddening serves as a clear visual cue that environmental conditions need adjustment.
The Biological Reason for Red Pigmentation
The physical mechanism behind the red coloration is the plant’s production of protective compounds known as anthocyanins. These are water-soluble pigments that range in color from red and pink to purple and blue. The Thanksgiving Cactus generates these pigments as a natural defense system against various forms of stress.
Anthocyanins accumulate in the plant’s cell vacuoles, acting as an internal sunscreen for photosynthetic tissues. By absorbing excess light energy, they shield the chlorophyll from damage, preventing photo-oxidative stress. This reaction is similar to how the leaves of deciduous trees change color in autumn.
Environmental Causes: Light and Temperature Stress
One of the most frequent causes of red cladodes is exposure to excessive light, specifically direct, intense sunlight. In their native habitat, Schlumbergera truncata grow in the filtered light of the forest canopy. Placing the plant in a south or west-facing window where it receives harsh, direct afternoon sun can quickly trigger the production of protective anthocyanins.
The reddening is a direct response to the intensity of the light, which overpowers the plant’s ability to use the light energy for photosynthesis. While these plants need bright, indirect light to thrive and bloom, too much direct exposure causes the segments to blush red or purple.
Temperature extremes can also induce this stress response, particularly exposure to cold. Although the Thanksgiving Cactus requires cooler temperatures, typically between 55°F and 65°F (13°C to 18°C), to set flower buds, temperatures that dip much below 50°F (10°C) can cause the segments to turn red or purple. This chilling response indicates that its cellular processes are being negatively affected by the low temperature.
Cultural Causes: Water and Nutrient Imbalances
Improper watering is a significant factor that causes the cladodes to turn red, as both too little and too much water create stress. Under-watering leads to dehydration stress, forcing the plant to conserve moisture, which often results in the cladodes shriveling slightly and turning red or purplish. Since this plant is a tropical epiphyte, it prefers consistent moisture.
Conversely, over-watering can be damaging because it leads to root rot, a fungal disease that destroys the roots. When roots are compromised, they cannot effectively absorb water and nutrients, which creates a form of drought stress for the rest of the plant. This lack of nutrient uptake can also result in the segments turning red or purple.
A specific nutrient deficiency commonly associated with red or purplish coloration is a lack of phosphorus (P). Phosphorus is a macronutrient that plays a role in energy transfer, photosynthesis, and overall growth. When the soil is depleted or the pH level is too high, the plant struggles to perform these functions, and the resulting stress is visually expressed as red or purple pigment accumulation.
Restoring Health and Reversing Reddening
The most effective action for reversing environmental stress is to adjust the plant’s placement. If the reddening is due to excessive light, move the cactus to a location that receives bright, indirect light, such as a few feet back from a sunny window or in a north or east-facing window. If cold is the cause, ensure the plant is away from drafty windows and cold air vents, maintaining a consistent temperature above 55°F (13°C).
To correct water-related issues, implement a proper watering schedule. Allow the top one to two inches of the potting mix to dry out completely between waterings. If you suspect root rot from over-watering, unpot the plant, trim away any mushy or dark roots, and repot it in fresh, well-draining soil.
Addressing a phosphorus deficiency requires applying a suitable fertilizer. During the active growing season, use a balanced, water-soluble houseplant fertilizer, or consider a “bloom booster” formula which is higher in phosphorus (the middle number in the N-P-K ratio). Consistent feeding will help replenish necessary nutrients and allow the plant to resume normal, green growth. The red color reversal is a gradual process; stressed cladodes may remain discolored for many weeks or months, but new growth should emerge green if the underlying problem has been corrected.