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.
There is perhaps no other fruit-bearing plant that submits with greater freedom and success than the Fig to early forcing, and it certainly yields under favourable treatment a very good return in the shape of two crops of fruit annually. In some cases it has been made to produce a third crop by commencing to force early, and prolonging the process late in the season; but although this is possible, it is by no means desirable, for, besides the debilitating influence on the plants, the third crop is never fine in quality.
Where a regular succession of ripe Figs is required from April to November, I recommend that there be a set of plants in pots, and another planted out, as has been treated of. Those in pots should be started about the new year, to ripen their first crop in April and May, and their second in July and August. Those planted out in borders, if started the end of February or beginning of March, ripen their first crop in the end of May and June, and their second will be all gathered before the middle of October, thus keeping up the supply of ripe Figs for at least six months of the year.
In beginning to force those in pots at, say, the beginning of January, it is very desirable that they be supplied with a gentle bottom-heat. Although this is not absolutely necessary, yet they start more freely into growth, the young fruit is less likely to drop off, and it swells better with than without bottom-heat. A house or pit in which Figs can be thus early forced, may be, and generally is, used for other purposes besides. In some cases early Strawberries are forced along with them on shelves on the back wall near the glass; a pot-Vine is fruited on each rafter; and in others all these three fruits are forced in the same house. But there is no doubt that where circumstances admit of their having compartments to themselves, they can be forced with less trouble and more success.
As in early forcing of every description, a lean-to light house, with a good command of both top and bottom heat, is just for Figs. If leaves can easily be got, it does not matter much whether the bottom-heat is wholly derived from a bed of them of considerable depth, say 3 1/2 to 4 feet, or from a lesser quantity in conjunction with hot water circulating below them. So long as a bottom-heat of about 75° can be maintained, it does not matter much which system is pursued.
Supposing that a set of pot-plants are at command in a well-ripened and fruitful state, and that ripe Figs are required by the end of April, by the 1st of January they should be plunged to the rim in the leaves. If there has been any red-spider on them the previous year, let the shoots be well washed with a soft brush and water, and then painted with a little sulphur, soot, and clay, well mixed together in water. Remove any loose soil that may be on the surface of the balls, and replace it with loam and horse-droppings in equal proportions. In plunging them, give them sufficient room to allow the leaves and young growths to expand without crowding. Give a good watering of water at 80°. See that the bottom-heat ranges about 75°, and that the night temperature is kept steadily at 50°, with an increase of 8° or 10° by day, till they show signs of growth, and the young fruit have begun to swell. Then raise the temperature to 60° at night, with a corresponding increase by day. Give air freely on all favourable opportunities, and syringe the trees morning and evening with water a few degrees warmer than the atmosphere of the house.
After the young fruit get to the size of nuts, over-syringing must be avoided, especially in dull weather, as an excess of water at the root, in conjunction with a too free use of the syringe, has a tendency to cause the fruit, especially in dull weather, to become yellow, and drop off before the setting process is past. At the same time, avoid an arid atmosphere, or a check from want of water at the root. Either extreme must be avoided until it be seen that the fruit are out of danger. But with well-ripened wood and bottom-heat, the fruit are rarely lost. As soon as the young growths have made four or five joints, pinch out the terminal bud, and increase the temperature to 65° in mild weather, When the second crop has fairly shown itself, feed the plants liberally with manure-water, as there is then a great demand on the energies of the plant. Manure-water made from sheep's dung and soot, should be given in a weak clear state every alternate watering; or guano, at the rate of a handful to a large garden watering-pot of water, answers well.