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.
It may appear out of place to call attention to a process which has been in practical operation ever since Grape cultivation was attempted. Grape thinning, though not requiring great muscular exertions, is work that requires much mental calculation, as bearing a direct influence on the ultimate perfection of produce. Large vigorous buuches may be produced, the finest flavour, colour, etc, gained, that nature and art combined are capable of accomplishing; yet where is the symmetrical beauty so desirable, and absolutely necessary, to approach an ideal in Grape-growing when all the rules of right thinning are flagrantly violated? What is more unsightly than the result thereof on our exhibition tables: those lank and disordered masses of Grapes, which, if thinned with a judicious application of forethought, would have formed praiseworthy, handsome, proportionate bunches? The bare shanks and loose form of such are rendered still more obviously defective when associated and contrasted with one of those rigid examples whose crowded berries are so offensively clustered together as to give the impression that the principal aim is an individual test of skins.
The most essential requirement, and a sure guide against such extremes, is a thorough knowledge of the one size that different varieties of Grapes attain. Varieties with short foot-stalks, of which the Black Alicante is the type, require most thinning, while such as Hamburgs, and those with longer foot-stalks, are not so liable to become compressed, though they may appear a little crowded at first. As swelling proceeds they are pressed upwards, and one berry makes room for another. When set, and before ever thinning is attempted, each bunch should be firmly syringed with clean tepid water, allowing them to become quite dry again before thinning is begun. This syringing removes all decayed blooms, unfertilised berries, and other adhesive matter, more perfectly and quicker than the scissors or any other appliance, and not only accelerates and renders more easy the thinning of all varieties, but Muscats and other shy-setting kinds can often be sufficiently cleaned in this manner, so that no more thinning is required.
Where a large houseful has to be thinned with little means of accomplishing it quickly, thinning should start immediately they are set, selecting the thickest set bunches, and relieving them of all that is likely soon to obstruct the swelling of the permanent berries. Gone over in this way as a preliminary thinning, the second and final thinning should be very carefully performed, taking out the smallest berries, and leaving all as equal in size as possible. No bunch should ever be allowed to get so crowded before thinning that the scissors cannot be conveniently inserted without being in danger of piercing the berries: those so punctured sometimes do not show their blemishes until swelling is well advanced, when they have to be cut out with the hazard of spoiling the symmetry of the entire bunch. With the exception of Lady Downes, no allowance should ever be made for thinning out a few berries if too thick just before they are fully swelled. Large blanks are thus formed which are rarely nicely filled up again. Lady Downes should be left rather thick until after the stoning, that a reserve may replace those which generally decay through scalding at this critical period. Rust on Grapes is said to be caused by contact with the hands, hair, etc, while thinning.
This, I think, is not correct. Such friction is, however, not conducive to their appearance. Grapes brushed even while thus small in size bear traces of tarnished bloom at last. A good way of steadying the bunch or supporting the shoulder while thinning, is to use a small forked piece of wood in the form of the letter Y, which can be easily slipped in between the berries without touching them with the hand, or in any way tampering with their bloom. To prevent the berries from falling to the ground and mixing with the soil when thinning, a circular wire 18 inches in diameter is covered with a piece of canvas in a scoop form tapering to a point, four cords are attached to the wire at equal distances, knotted together at whatever length is desired, and united to a hook to suspend it from the trellis, so as to come underneath the bunch and receive all the thinnings. J. M.