This section is from the book "Commercial Gardening Vol3", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
When it is desired to start Vines early, the change must be gradual; it will not do to make a violent alteration in the time of starting. If a house is started a month earlier each year it will be quite change enough. It is a question whether it pays to force Vines very early at the present time. Prices are very low and firing is dear; added to this, no more than half a crop can be expected, and the strain on the Vines soon tells. Some growers force their Vines for a few years and then gradually start them later to give them a rest, taking other houses for the earliest work. Where early work is done the rods must be tied right down low to induce them to start evenly, and in some places they are bent to one side till the end can be tied to the bottom wire about half the length of the rod along; in this way the rod forms a bow, with the middle of it the highest point. As soon as the buds have broken well the rods are tied up in the ordinary position. The easiest variety to force is Black Hambro, and it is the quickest to come to maturity, five months being sufficient under skilful growing and favourable conditions. Muscats can also be forced, but require a month longer to mature. The amber colour in these grapes is all-important, and that is what is so hard to get; but unless it is got the grower will be sadly disappointed with his returns. Air and light and sufficient time to finish must be given.
For forcing grapes a supply of warm water for watering is essential. This can be provided by running one of the hot-water pipes through the tanks. A layer of hot water will be formed in this way, and by stirring up, the cold edge can be taken off the whole. A more up-to-date method is to have the water supply so arranged that the water can be passed through the boiler before passing into the house. By keeping up a little extra fire a good supply of warm water can be got without lowering the temperature of the house by cooling down the hot-water apparatus. Where the pulsometer pump is used, the chill is taken off the water without any further trouble.
Gros Colmar is grown in very large quantities for late work, and Alicante, from the toughness of its skin, is very useful for the purpose. When very late work is attempted the Vines are allowed to start quite naturally; and as long as the colour is obtained before the leaves turn, they are not hurried in any way. Some very poorly coloured Colmars are seen on the market in some seasons, owing to the withholding of a little fire heat to help out the colouring at the end of the growing season.
When the natural covering of leaves falls, if grapes are to be kept for any length of time the houses must be covered with canvas to exclude the light and prevent shrivelling. Under the most careful conditions, one very large grower gives it as his opinion that grapes kept till March will lose weight to the extent of 25 per cent.