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.
This section of cultivators is an ever-increasing one, and likely to be so while the interest in the Dahlia continues to grow. Few people realize the tens of thousands of young plants that are raised and sold annually in the spring months. To be successful, the grower must have plenty of land on which to grow his stock, also a large amount of glass, both houses and frames. He must also make large purchases of new varieties, and above all keep his stock true to name, for any grower sending out wrongly-named stock would inevitably fail to hold his customers. As in so many things, to be suecessful in this branch of the business it must be carried out on a large scale, so that young plants can be turned out in their thousands during April, May, and June. To secure this end a start must be made in December, so that the stock house can be filled ready for the first week in the year. Benches on either side of an ordinary greenhouse are best for placing the stools, for they are then near the light, and it enables the cultivator to take the cuttings expeditiously without much stooping. Almost any soil will do for covering the bare benches on which the old tubers are placed in proper order and in divisions, each variety kept distinctly clear of its neighbour and with label securely attached. In covering the tubers with soil each crown or base of the old stem should be well out of it. If a temperature of 55° F. is maintained it will be quite sufficient during the early part of the year, but should be increased as the daylight grows.
The Dahlia will root well provided it gets plenty of bottom heat and not too much top heat. Where plenty of pipes, say three rows of 4 in., are placed under beds 4 ft. wide, this will give all the heat required. As it is essential to keep the top cool, only sufficient heat is wanted to keep out the frost and damp. Some growers plunge their pots in ashes or fibre, but as this adds to the labour bill, and is not necessary, it is more generally dispensed with, and the pots are simply stood shoulder to shoulder on the beds.