Cactus and succulents turn red in reaction to changes in the growing conditions – extreme changes in terms of climate, watering, nutrition, type of media, pests & infections – inducing stress. Changes in the color of leaves, spines and plant body of xerophytes happens in response to this stress.

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We all know that it is Chlorophyll that gives the plants their green color because it does not absorb the green wavelengths of sunlight. It absorbs most energy from wavelengths of violet-blue and orange-red light, and it is a poor absorber of green and near-green portions of the spectrum. That particular light wavelength is thus reflected from the plant, so it appears green.

Apart from Chlorophyll and many other compounds are also present in the plants that help them with the core functions within the plants like making food from sunlight, reproduction, dealing with stress, etc. Among these compounds are also anthocyanins and carotenoids.

In plants, anthocyanins are critical for their role involving reproduction, attracting pollinators & seed dispersers. Carotenoids are essential compounds that are involved in photosynthesis and photo-protection in plants. Apart from these, another very critical role of anthocyanins and carotenoids is in protection against various abiotic and biotic stresses.

Abiotic stress is stress induced through external non-living factors like changes in temperature, water availability, nutrition & salts, soil or media. Biotic stress is stress induced by living organisms like fungi, bacteria, viruses, nematodes, parasites and insects.

Anthocyanins primarily help plants in dealing with abiotic stresses besides their role in reproduction as they attract pollinators and seed dispersers by imparting bright colors to the plants. Anthocyanins also often accumulate in young vegetative tissues and sun-exposed side of plants fruits to protect them from photo-inhibition and photo-bleaching under light stress without significantly compromising photo-synthesis. To some level, the presence of anthocyanins also reduces the infestation of insects and pathogens.

Carotenoids accumulate in plants under different biotic (bacterial and fungal) and abiotic stresses such as extreme changes in pH levels, temperature variations, water and scarcity of nutrients. Carotenoids have protective roles which help the plants to cope-up against these biotic and abiotic stresses. Carotenoids help plants to absorb light at wavelengths from the blue and green part of the spectrum and reflect light in the yellow, orange and red parts of the spectrum making plants appear yellow, orange and/or red and brown.

So, essentially as collectors of expensive cactus and succulents, we need to recognise signs of stress and take precautions, if required. Abiotic stress mostly will not mortally harm the plant as long as the plant does not look distorted, disfigured, pulpy or wilted. In fact, most cactus and succulents face abiotic stress in their native habitat and the changes in their colors are mostly in response to these abiotic stresses. Biotic stress in case of cactus and succulents is mostly fatal and needs instant intervention.

A keen sense of observation of your cactus and succulents over a period of time, will let you know the kind of stress your plant is facing and you can then take corrective actions. Do remember to learn more about the plant to ascertain if they go through any kind of abiotic stress in their natural habitat which is essential to their survival. One classic example of this is the South American cacti like Echinopsis which need the winter exposure of upto 5 degrees Celsius to flower in subsequent spring season and prolong their genetic line. Another typical example is Haworthia that tend to go dormant in extreme hot summer months and limit their photo-synthetic activities to near zero to conserve water and also in some cases shed all of their fibrous roots again to conserve water.

If you enjoy various hues on your cactus and succulents try putting them through some abiotic stress like not watering them for long durations, reducing or not fertilising them or exposing them to direct sun during periods of extreme low watering, especially if your winter temperature reaches near zero degrees Celsius.

In the above picture, you can see two plants of same species Haworthia koelmanorium - one on the left kept inside a greenhouse and one kept outside where night temperature at present goes as low as 5 degrees Celsius. The one on the left is deep watered once in 10 days whereas the one on the right is only misted every 3-4 days and has not been deep watered for over a month now.

Growing in cocopeat, these cactus have not been watered for a month

The sun exposed part of the cactus in the above picture shows clearly how carotenoids have come to the rescue of the plant in dealing with the abiotic stress.

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Why do cactus and succulents turn red?

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