Poinsettia Euphorbia Pulcherrima
The botanical name of the poinsettia is pulcherrima and it is a deciduous shrub from Mexico, cultivated for the large scarlet or pink bracts which surround the insignificant flowers in winter. It is forced in large numbers for the Christmas trade and this is how most gardeners acquire it. It is not difficult to retain plants for several years but it is almost impossible to keep them as short and compact as they were when bought as they were sprayed in the nursery with growth-retarding chemicals which are not readily available for home use.
If taller plants are acceptable, there remains the problem of making certain that they produce flowers and bracts, which are only initiated when the night length is sixteen hours or more. Difficulty arises mainly when plants are grown in rooms, for even an electric light, switched on in the evening, can inhibit the formation of the flower buds and their accompanying bracts. The way to prevent this is to cover plants from mid-September onwards with a black out each evening sufficiently early to ensure at least sixteen hours of complete darkness.
After flowering, poinsettias are best kept in a greenhouse with a temperature range of 15 to 20°C (59-68°F) with plenty of moisture in the air and light shading in summer. For two or three weeks in April theis kept almost dry so that leaves fall. The stems are then shortened by two-thirds, watering is resumed and the temperature raised a little to re-start the plants into growth. If desired the prunings can be prepared as and rooted in a warm propagator. Old plants and rooted cuttings can be potted in a fairly rich, soil-based and, after about a month, should be given supplementary liquid feeding every seven to ten days throughout the summer.