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.
This is still an important crop to the French maraicher, but is not grown so much in frames now as formerly. The seeds of a variety like "Monstrous Viroflay" is sown on prepared beds about August to produce leaves in September; but a crop of Cos Lettuce, or Endive will have been planted over the surface immediately after sowing; or the Spinach may be sown between a crop already standing, but only half-grown. In October such varieties as "Flanders" or "Prickly Longstanding" may be sown for winter work, and some of these plants may be taken up in February or early March, have the leaves cut off, and then be planted in frames between a crop of Gotte Lettuces after a crop of Radishes have been cleared, and while the Carrots beneath are only just showing through the soil. Spinach grown in frames in this way produces beautiful leaves fit for picking about three weeks after planting. The entire plant, however, is pulled, so as to leave room for the Carrots. From the middle of February until the end of July or middle of August a sowing of Spinach may be made on the beds, between or under other crops, at intervals of two or three weeks, so that a supply is kept up all the year round. With plenty of water there is little danger of the plants running to seed even in the hottest summer.