Euphorbia pulcherrima Willd. ex Klotzsch (Euphorbiaceae) Poinsettia, Christmas Star, Christmas Flower, Painted Leaf, Lobster Plant, Mexican Flame Leaf. A large shrub with red leaf-like bracts, native of Mexico in the Sierra Madre Occidentale, growing on the margins of forest, flowering in October-January. Shrub to 3m in frost-free climates. Leaves long-stalked to 15cm long, with few shallow lobes and teeth. Bracts 1 0-15cm long, lanceolate in the Euphorbia pulcherrima wild, but larger and broader in modern cultivars. In frost-free climates the shrubs are easily grown in sun or partial shade, given well-drained, slightly acid soil and ample water in summer. They should be pruned after flowering and usually produce numerous small heads. Larger heads are encouraged by thinning the branches. They do well outdoors in the warm parts of California, surviving - 5°C overnight if protected by buildings or a wall and kept dry at the root.
Paler-flowered varieties are said to hold their bracts into spring. Pot cultivation aims to produce a dwarf plant with large heads, flowering in mid-winter. Commercially the plants are treated with a dwarfing hormone. To keep a pot plant going a second year, prune, water very sparingly and keep cool (10°-15°C) after flowering, when the plants may drop their leaves. In early summer, the plants should be repotted in good rich compost, kept as warm and humid as possible, and in good light until autumn. Flower buds are not produced until the long nights of the autumn equinox, at temperatures below 18°C and should then develop through the winter. For instance, at 40-45°N, the latitude of New York, Northern California or Madrid, flower buds are initiated on September 21-25th. In this period, when me buds are being formed, the plants need a minimum of 15°C. If the plants are grown where they get artificial light at night, it is necessary to put them in a dark cupboard or closet for 14 hours each night.