This section is from the book "Commercial Gardening Vol2", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
For many years this brilliant-looking Mexican Spurgewort has been a favourite in the market during the winter months, not because of the beauty of its flowers, which are small and yellow, but because of the whorls of scarlet leaves or bracts at the top of the stems. There is a variety having white instead of scarlet bracts, but it is not likely to be a market plant. There is also a so-called double variety (plenis-siraa), in which clusters of smaller brightly coloured bracts are borne within the larger ones, and, as they colour in succession, the decorative season is somewhat prolonged (fig. 296).
Plants for market are grown as short as possible, usually in 5-in. or 6-in. pots. They are raised each year from cuttings taken between May and June to the beginning of August from stock plants which are cut down earlier in the year (March), and are started into growth in a fairly warm greenhouse. The cuttings are from 4 to 5 in. in length, and some growers only cut them halfway through at first, and then leave them for a clay or two before severing completely from the parent plant. In this way the poisonous milky juice is not wasted so much, and the cuttings root more readily afterwards. The cuttings are inserted in sandy soil and plunged in a hotbed as near the glass as possible, the temperature being from 75° to 80° F. They are shaded for a few days until rooted, but afterwards are given plenty of light to prevent them from becoming leggy - a drawback to be guarded against at all costs.
Fig. 296. - Poinsettia pulcherrima.
When the cuttings are rooted and slightly hardened off, they are placed singly in 5-in. or 6-in. pots in a compost of 2 parts turfy loam, 1 part leaf mould or old cow manure, and 1 part silver sand. The temperature is kept up to about 70° F. at night. The plants are shaded from strong sunshine at first until again established, but otherwise the heads of the plants are kept as close to the glass as possible without actually touching it.
During the summer months they may be placed in a frame, but the lights are always kept on except when watering, as the sunlight through the glass helps the plants considerably to remain dwarf and to develop more highly coloured bracts. As they increase in height it may be necessary to raise the frame by putting bricks under it at each corner. In this way ventilation will be secured from the bottom. Of course, low span-roofed houses, well ventilated and exposed to the sun, will be more useful for growing Poinsettias than frames, and no fire heat will be needed during the summer months. About the end of September, however, the temperature must be kept up to about 50° F. during the night, with a rise of 5 degrees or a little more during the day. If the temperature is much higher than this the bracts will not retain their brilliant colouring for such a long period.
In the event of plants becoming too tall during the summer months - and Poinsettias are naturally inclined to lankiness - the stems may be "ringed" within 6 or 7 in. from the top about the end of August or early in September. If cut about halfway through at this point, and left until a callus has been formed by the coagulated sap, at the end of ten days or a fortnight, the shoots may then be severed completely and placed singly in 3-in. pots in a mixture half loam and sand. They are kept in a close frame for about three weeks, when they will be well rooted, after which they are gradually exposed to more air. The plants are then potted again into 5-in. or 6-in. pots, according to vigour, and are placed close to the glass.
Of course during the whole period of growth watering is attended to carefully, and fully established and maturing plants may be treated to occasional doses of weak liquid manure. The average price is from 9s. to 18s. per dozen.