Bracts that light up winter
When December arrives, many homes brighten with an intense red that marks the beginning of Advent. It is the colour of the poinsettia (Euphorbia pulcherrima), a plant we often call the “Christmas flower”, although what captivates us are not the petals, but the bracts: modified leaves that the plant uses to protect and highlight its true flowers, very small and discreet. These coloured structures, common in several botanical families, are especially emblematic in the genus Euphorbia, where the cyathia (inflorescences that mimic a single flower) hide a delicate evolutionary ingenuity.
The poinsettia is native to the tropical forests of Mexico and Guatemala, where it can grow as a tall shrub or even a small tree, up to four metres high, and turn red in sync with winter celebrations. This natural link with the calendar is what has made it a universal symbol of Christmas. The plant, however, is much more sensitive than it seems: it suffers from excess watering, from the dry air of heating systems, and from cold drafts. For this reason, it often does not survive much beyond the festive season.
Its characteristic red depends on a key phenomenon: the photoperiod, that is, the amount of light and darkness it receives each day. The poinsettia is a short-day plant, meaning it only develops coloured bracts when it experiences long, uninterrupted nights. In greenhouses, growers make use of this sensitivity and, in autumn, cover the plants for 12–14 consecutive hours to accelerate and intensify the colour change.
Thus, behind this plant so common in homes, lies a series of highly refined biological mechanisms that explain why it changes colour and why it is so sensitive to household conditions. Understanding them helps us appreciate the poinsettia more and enjoy it more wisely during the holidays.
