Growing Sweet Peas under glass has become quite a common practice in recent years. In America, the winter flowering section is largely grown, but great success has not been obtained with them in Britain. The winter-flowering race is different in habit from the ordinary sweet pea. They run up to a height of nearly two feet before they break out freely into side growths, and the flowers are smaller than the summer-blooming sorts. For those who wish to try these winter bloomers, I cannot do better than quote the instructions given in a Bulletin on Winter Flowering Sweet Peas, issued by the Department of Horticulture, Cornell University, U.S.A.
"Winter flowering Sweet Peas grow six to ten or more feet high, and if they are to attain their full development, a house with this amount of headroom is required. They need all the light they can get, as the lighter the house, the more blooms there will be. Sweet Peas may be grown on benches, but solid beds are better since the plants require a deep, moist, cool soil. The roots should be given an opportunity to go down, by properly preparing the soil two or three feet in depth. The seeds may be sown in the beds or benches where they are to stand, or they may be sown in pots. The former method gives the best results [From my experience in England, I should question this. - W.C.].
The Sweet Pea must be kept growing steadily from the day the seed is sown. When water is needed, do not apply near the plants, but between the rows, where it should be given liberally. This treatment is especially important with the early plantings. One reason why water should not be applied too near the plants is that they are especially subject to damping off. This trouble becomes more prevalent in October and November, when there are more cloudy days, cool nights and like conditions. Because of this no soil should be heaped around the stems.
When a good crop is desired for Christmas, the seed should be sown the 20th of August. When sown September 1st, the plants will flower in January. When sown September 15th, the main crop will be in Feb ruary, and when sown in October, the crop will be ready in March. November sowings flower in the latter part of March; December sowings in April; January sowings in April and May; February sowings from May 1st on, and a March sowing in May or June. This gives the time when a reasonable crop can be expected, although flowers will be cut, especially with certain varieties, in a shorter interval than that given.
As soon as the peas are up, a support must be furnished. This may consist of string, wire and string, or wire netting.
Keep the temperature in the early stage as low as possible, giving full ventilation, day and night, as late as possible without freezing. The cooler the plants can be kept while growing the stronger and healthier they will be. In this way the natural outdoor spring conditions are approximated. When the flower buds can be felt in the tips of the growing stems, the temperature should be raised one degree at night, until fifty degrees is attained, which is the proper temperature during December, January and February. On bright days, a rise of ten degrees or even fifteen degrees may be given. On cloudy days fifty-five degrees is high enough, for higher temperatures on such days often promote soft, succulent growth. Plenty of ventilation should be given at every opportunity, as this, with careful regulation of temperature, causes a firm growth.