This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
A heavy and well-hearted Lettuce is a good thing, but it is not absolutely necessary that it should be big and heavy to be acceptable as a salad. Besides, Lettuces when forced are rather stubbornly inclined to be open-hearted - a fact which market-gardeners know; and so they tie them up the day before sending them to the market, in order to suit their customers, or rather to deceive them. Still, a Lettuce grown fast, and in a rich soil, makes just as good and tender a salad as any when pulled young, and every leaf is clean and usable. To plant Lettuces 8 or 9 inches apart in frames for a winter and early spring supply, takes up a great deal of space for all the return they give - a fact which every one knows who has to supply a quantity during the winter months. We found this out long ago, for our Lettuce-frame was never at liberty till October; and though planted full at ordinary distances apart, it would not carry on the supply long enough; so we had recourse to thick-planting - that is, we planted autumn-sown plants 4 inches apart between the rows, and the same distance between the plants. The lights are then put on, the plants are well centred and heated to a genial temperature, watering when needful, but otherwise keeping the frame dry.
They soon begin to grow quickly till they meet each other, and with the crush the leaves are thrown up, till each plant is like a cup and begins to heart. Long before they get this length, however, we begin to pull - first taking every alternate row, and afterwards every alternate plant in the rows left. The last plants, of course, get big and hearted before they are needed, but I cannot say they are appreciated more, if as much, as the first tender little plants, which are sent in in quantity. In this way, we have always an abundant supply of first-rate Lettuce all the winter and spring months from a frame of moderate dimensions; for in spite of all that has been said about the French cloche for Lettuce-growing, I prefer a good light frame, which is infinitely easier to manage, more convenient, will accommodate more plants, space for space, and is upon the whole cheaper. Some may think that plants grown in this crowded fashion in the winter months would be apt to damp off, but they are not. As I said before, the frame is kept dry.
When the plants are watered, a fine day is chosen for the job, and the lights are left off till they are dry again; but they cover the ground so thickly that very little watering is required after the end of October. Our spring frame has been crowded since March with fine succulent plants, though continually thinned out; and the supply will last us till the first outdoor batch is well in. I therefore advise to plant thick for such crops if you want quick returns and a plentiful supply. J. Simpson.