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.
As soon as it is possible to see the character of the bunches they must be thinned out, leaving only one bunch to a lateral, and it is best to remove straggly shoulders from the black varieties; the Muscats generally have them left on. The thinning out of the Muscat bunches is generally left a little longer, so that the set can be judged better. As soon as the bunches have berries as large as BB shot, the thinning should be taken in hand. For this purpose there is nothing much better for holding the bunches still while the berries are being thinned than a piece of a bamboo cane about 6 or 9 in. long, cut with a neat little fork at one end and the rest of it shaved down to a fine handle. A good thinner will finish a bunch (fig. 384) at one thinning, and this should be the aim of everyone who undertakes the job. The expense of the operation is considerable, and it is a pity to have to repeat it. No market house should want more than just looking over for a few thick bunches after thinning. To get a bunch just right the thinner must carry in his head an idea of the size the finished berry should be, and enough berries must be left so that when the bunch is cut and laid on a plate it will retain its shape. This is a most important point, for a loose bunch will never travel well when sent to market. Alicantes are a great trouble to thin, as they set so thick that, unless the thinning is carried through quickly, the bunches get so tight that it is almost impossible to thin them. Muscats will bear being left longer than other sorts, as the bunches are of a more straggly character than those of the black varieties.
a, Erect stamens, free setting. b, Deflexed stamens, shy setting.
MUSCAT OF ALEXANDRIA, A WHITE LATE MARKET GRAPE.
ALICANTE, A BLACK LATE MARKET GRAPE.
BLACK HAMBRO'.. A BLACK SWEETWATER EARLY MARKET GRAPE.
BUNCHES OF GRAPES CUT FOR MARKET.
Soon after the thinning is done the berries finish their first swelling and begin to stone. During this process it is useless to try to drive the Vines in any way, and it is usual to reduce the temperature a little. Immediately it is seen that the stoning is finished, and the berries are beginning to swell again, opportunity should be taken at the next watering to give a little stimulant of some kind, and this should be repeated at intervals of a fortnight until the grapes are colouring. This practice is carried out on some of the Worthing nurseries, and the soil there can apparently take water at all times without getting too wet. On a good many soils frequent watering cannot be done, and the only guide to the cultivator in the matter of watering Vines is only to do it when required, and this can only be decided by intelligent observation of the borders. Vines can take a great quantity of water at times, and must never be allowed to suffer for want of it; but too much water - entailing as it does unhealthy border conditions - is worse, if anything. As soon as the berries begin to show colour more air must be given, and a chink should be left on all night until, as the colour gets to be pronounced, free ventilation can be left on night and day except in bad weather. Damping down must also be reduced, and left off entirely in all but early vineries, where considerable fire heat still has to be kept up. In mid-season vineries the fire may be left off altogether unless the berries are seen to be suffering from rot, as they will in damp weather. With late grapes the pipes must be kept just warm; but for-keeping grapes very late, after the leaves have fallen, great care must be exercised with the firing, and no more than enough to keep the air dry must be allowed, or the grapes will shrivel and lose weight to a great extent. A temperature of 45° will be sufficient in such cases.
Fig. 384. - Young Bunches of Grapes 1, Before, and 2, after thinning.