Poinsettia

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Poinsettia
Poinsettia varieties on sale in England

Poinsettia (Euphorbia pulcherrima) is a red and green plant popular at Christmas time. It grows in Mexico and Central America and is an important export. It belongs to the order Malpighiales.

The poinsettia is a short-day plant. It needs two months of long nights before blooming

Many flowering plants use the pigment phytochrome to sense seasonal changes in day length, which they take as signals to flower. This sensitivity to day length is termed photoperiodism. Short day plants flower when the length of daylight falls below a certain critical level.

It is not actually the period of light exposure that limits flowering. Rather, a short day plant requires a minimal length of uninterrupted darkness in each 24-hour period (a short daylength) before floral development can begin.

Plants make use of the phytochrome system to sense day length or photoperiod. This fact is used by florists and greenhouse gardeners to control and induce flowering out of season, such as with Poinsettia.