This section is from the book "Commercial Gardening Vol4", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
The variety most in favour for frame work in French gardens is the "Marteau" or "Half-long White Forcing", but others may be grown if considered sufficiently remunerative. The seeds are sown early in January on a hotbed with a temperature of 70° to 80° F., the surface being covered with fine spongy mould passed through a sieve. The seeds are not sown broadcast or in drills. Small holes about 1 in. deep and 6 in. apart are made with the index finger, and two or three seeds are dropped into them. The surface is levelled over with a piece of batten and nicely watered, and the lights are covered with mats for four or five days, until the seed leaves appear. After a few days the seedlings are thinned out, so as to leave only one plant to each little hole. About the middle of February this first crop of Turnips will be advanced enough and sufficiently hardy to allow the lights to be taken away to place over a second sowing in other frames. In the event of frost, mats must be placed over the plants at night. By making a sowing of "Marteau" Turnips in January, February, March, and April, four crops can be taken off the beds by the middle of May or thereabouts, using only two different sets of beds and one set of lights. Crop No. 3 will be sown on the same bed as Crop No. 1, and Crop No. 4 will occupy the beds from which Crop No. 2 has been cleared.
Besides the plants mentioned above, others are sometimes grown as catch crops in French gardens. For instance, Strawberries may be brought into earlier maturity by being covered with cloches, and Tomatoes and Marrows are protected early in the year. Egg plants (Aubergines) are also grown, and Seakale, Chicory, and Asparagus are forced in the hotbeds when necessary, while Dwarf Beans were at one time an important crop. Leeks, Onions, Sorrel, and Cabbages are also grown when considered remunerative, and even Dandelions are not despised. The cloches and frames, however, are chiefly used for the most remunerative crops, like Radishes, Carrots, Cauliflowers, Lettuces, Endives, and Melons. There is no reason, however, why early Potatoes, Mint, Violets, Christmas Roses, Tulips, and many other dwarf early-flowering plants might not be grown under the system under carefully-thought-out conditions. The reader will find much more detailed information in French Market Gardening (London, John Murray, 1909). [J. W].