¿Por qué algunas plantas se vuelven granates en invierno? Es una de las preguntas que más nos hacen en HOSTA Botanical Service, y la respuesta es fascinante.
Watching a plant you know well turn deep red, purple, or maroon can raise reasonable doubts: Is it sick? Is it lacking something?
In most cases, the answer is reassuring. This color is not a distress signal, but rather a demonstration of plant intelligence.
Desde HOSTA Botanical Service queremos explicarte la ciencia detrás de este fascinante mecanismo, porque entender a las plantas es el primer paso para cuidarlas bien.
This chromatic shift has a technical name: phenotypic response to abiotic stress. In simpler terms: the plant perceives adverse external conditions—cold, excess light, nutrient scarcity—and modifies its appearance to protect itself.
The protagonist of this process is a pigment called anthocyanin, a molecule from the flavonoid group that the plant actively synthesizes when temperatures drop.
To understand why the plant produces it, one must grasp the paradox of the botanical winter. When the cold arrives, the plant's metabolism slows down considerably; its internal reactions become sluggish or stop almost entirely. However, the sun does not stop. If the plant continued to receive intense solar radiation without being able to process it, its tissues would suffer irreversible oxidative damage—something akin to a burn at a cellular level.
Anthocyanins act as a high-efficiency sunscreen: they absorb excess light before it reaches the photosynthetic structures, protect the remaining chlorophyll, and neutralize the free radicals generated by photo-oxidative stress. The maroon or red color you see on the leaf is, literally, the plant's shield visible to the naked eye.
The process has a second, equally interesting layer. Before entering dormancy—or to survive it in the case of perennials—the plant actively breaks down its chlorophyll to recover the nitrogen and other nutrients it contains. It is a high-precision internal recycling operation.
When chlorophyll disappears, the dominant green stops masking other pigments that were always present but invisible:
Carotenoids reveal the orange and yellow tones we see in many trees during autumn. Anthocyanins, on the other hand, were not there before—the plant manufactures them from scratch in response to cold and light.
There is a third factor that amplifies this production: sugar trapped in the leaves. When night temperatures drop, the leaf's vascular vessels narrow, making it difficult to transport sugars to the rest of the plant. These sugars remain retained in the leaf and, combined with daytime radiation, further stimulate anthocyanin synthesis.
This is why the most intense colors appear in autumns and winters with sunny days and cold nights: it is the perfect combination for the plant to produce its best pigments.
One of the most striking examples of this phenomenon—and especially relevant for those of us living on the Mediterranean coast—is that of the Casuarina glauca, observed in one of our projects.
Two specimens of the same species, with the same genetic material, can look radically different depending on their location. A specimen in Moncada (Valencia) maintains a soft green color throughout the winter. The same tree in El Campello (Alicante) acquires an intense and striking maroon hue.
The difference is not in the genes. It lies in what botany calls physiological plasticity: the ability of a plant to adapt its phenotype—its physical appearance and metabolic processes—to the specific conditions of its environment, without changing its genetic information.
In coastal areas like El Campello, solar radiation is particularly intense. The atmospheric clarity of the Mediterranean and the albedo—the percentage of radiation reflected by the sea toward nearby vegetation—create high-insolation conditions that trigger anthocyanin synthesis more intensely. The plant, in response, invests more energy in building its pigment shield.
The result is a maroon tree that is neither sick nor pathologically stressed—it is simply better adapted to its environment than its inland twin. It is nature being efficient.
The most common case of plants turning maroon in winter in our homes is the Geranium (Pelargonium zonale). It is common for its lower leaves to turn reddish or purple in late autumn, and the cause is not excess light but the difficulty in absorbing phosphorus when the substrate is cold. Phosphorus is essential for energy transfer within the plant.
When the substrate temperature drops below a certain threshold, the roots lose efficiency in absorption, and the plant manifests this deficiency through color changes in the older leaves.
The good news is that this blush is usually reversible: with the rise in temperatures in spring, and if the substrate has adequate nutritional reserves, the plant recovers its usual green color without additional intervention.
Not all winter reddening is a response to light protection. In some species, the color change has a nutritional origin that does require attention.
A similar case occurs with bergenias and some succulents, which develop deep purple tones in winter not due to a lack of nutrients, but as a heat-retention mechanism—a kind of chemical thermal blanket that allows them to survive frosts that other plants would not tolerate.
Understanding the mechanism behind the winter blush changes how you should act. Here are our recommendations:
🌿 Do not prune reddish leaves prematurely. Those leaves are still active, producing protective pigments and recycling nutrients. Removing them before they complete their cycle deprives the plant of resources it needs. Wait until they fall naturally or until the color fades on its own as temperatures rise.
🌿 Protect the roots with mulching. Covering the base of the plant with an organic layer—pine bark, straw, or dry leaves—keeps the substrate temperature above the critical threshold and facilitates the absorption of phosphorus and other nutrients. This is especially useful if you notice reddening in geraniums or potted plants.
🌿 Water carefully and consciously. In winter, evaporation is minimal and the plant’s water needs drop drastically. Water only when the substrate is dry to the touch several centimeters deep. Excess water combined with cold is the most common cause of root rot at this time of year.
🌿 Fertilize before winter, not during. A base fertilization in late autumn—with a balanced fertilizer or a biostimulant rich in boron and molybdenum—provides reserves that the plant will use during the colder months without relying on active absorption, which is limited by low temperatures.
🌿 Observe the pattern of reddening. Generalized reddening in young leaves combined with other symptoms may indicate deficiencies or watering issues. Localized reddening in older leaves at the base, without other symptoms, is usually normal. Learn to tell the difference.
Plants do not speak, but they communicate constantly. The winter maroon is one of their most elaborate messages: a high-precision chemical response to adverse conditions, encoded over millions of years of evolution.
En HOSTA Botanical Service llevamos ese conocimiento botánico a cada proyecto de paisajismo de interiores que diseñamos. Porque elegir bien las plantas para un espacio — y saber cómo van a comportarse a lo largo del año — es la diferencia entre un jardín de interior que siempre luce bien y uno que da problemas en cuanto cambia el clima.
Do you have a plant that has turned maroon this winter and you're not sure if it's normal? Write to us or visit ourcontact page. We would be delighted to help you.
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