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.
Vast quantities of these are grown, the smaller - rooted varieties, like "Paris Forcing" (Carotte rouge a forcer), "Early Forcing Horn" (Carotte tres-courte a chassis, or G. grelot, C. Toupie), "Scarlet Dutch Horn " (G. rouge court hative, or G. Bellot) being preferred for early framework, while the "Half-long Nantes Scarlet" (C. rouge demi-longue nantaise or G. sans cozur) and the "Half-long Scarlet Carentan" are used for open-air work.
The first crops are sown in the frames in October on finely prepared mould, only a gentle heat from the manure being necessary at this time. On top of the Carrots, Radishes (forcing scarlet white-tipped variety) are also sown. The Radishes, however, grow more quickly than the Carrots, and are picked and marketed before these are any appreciable size. But while both the Radishes and Carrots are germinating, the upper surface of the soil is planted with some "Crepe" or "Petite noire" Cabbage Lettuces which have been raised in advance from seed sown in September. Thus each frame is actually carrying three crops at one and the same time, viz. Carrots, Radishes, and Cabbage Lettuces. Being of a distinct nature, and differing in rapidity of growth, one crop is taken off before it is likely to interfere with the others. Thus, although the Lettuces on the surface are more advanced than the Carrots or Radishes, the latter grow very quickly on top of the slower-growing Carrots, and up between the Lettuces. The Radishes, therefore, are the first crop to mature and to be cleared out of the frame; then come the Lettuces, and when they are mature the Carrots have then reached a stage when they will require all the space to themselves. In later crops, however, young Cauliflowers are planted on the north and south edges of the bed for first crops.
This process of ringing the changes on the same patch of soil with three crops at one time is carried on throughout the year, not only under lights and cloches, but also in the open air during the summer months, and explains (what at first seems inexplicable) how the French maraicher obtains such an enormous quantity of produce from a small area.
With these early crops of Radishes, Lettuces, and Carrots it must be borne in mind that only as many frames and bell glasses are utilized as may be necessary according to the season and the state of the markets. As one batch of frames and cloches are maturing their crops, beds are being made up for succession crops, so that from one year's end to another there is never a break in the supply of produce.