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.
The florist's China Aster has been evolved during the past eighty years by selection and cultivation from a beautiful Margueritelike composite - Callistephus hortensis - referred to at p. 14. There are now a very large number of varieties, amongst the most popular being those called "Chrysanthemum-flowered", "Comet" in dwarf (fig. 187) and tall forms, "Ostrich Plume", "Pseony-flowered", "Victoria", and several others. The flowers in some are as regular as in a decorative Dahlia, while in others, notably the Ostrich Plume and Chrysanthemum-flowerecl sections, the flower heads resemble those of Japanese Chrysanthemums.
As to colour, there is a great range of variation from the purest of whites to the deepest of blues and purples passing through rose, crimson, mauve, pink, salmon pink, lilac, violet, with intermediate shades. Yellow is a rare colour, and is only just beginning to appear, but is a long way from being perfect.
China Asters are treated as half-hardy annuals. The seeds are sown thinly from February to April in shallow boxes in ordinary good rich gritty soil. They germinate in a few days in a temperature of 60° to'65° F. When large enough to handle easily, the seedlings are pricked out 2-3 in. apart in similar boxes and soil, and are hardened off to be ready for the great sales starting in April and finishing in June, for bedding-out purposes. Apart from this trade in the young plants, a good sale is also done with the plants in flower, and also in the cut blooms later on. The plants are placed in rows 6-9 in. apart in the open borders, or on ground that has already been cleared of Violas, Pansies, Daisies, etc, something like 80,000 plants going to an acre. The soil should be rich and deeply dug or trenched to ensure sweetness; otherwise the Aster disease may attack the plants and cripple them. Where cold frames are available after Violas, Pansies, or other dwarf crops, China Asters may be also planted in them, and will come into flower earlier with the protection of the lights at night. The hoe should be kept going between the plants regularly to keep down the weeds and insect pests, and also to liberate food and conserve the soil moisture - a most important consideration in hot rainless summers.