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.
While the subject of the rods is under discussion it will be as well to glance at the question of the wiring. There are two ways of doing this: one is to have the wires running horizontally and the other vertically. Each system is largely used, but the vertical way is perhaps to be preferred. The advantages are as follows: the rods are easier to train straight up the roof, as they lie along the wire all the way and can be tied at any point. With a gross-growing variety, like "Alicante", the wire to which the rod is to be tied may be set 6 in. lower than those for the laterals. This helps to prevent the laterals being broken out when tying down is in progress. Lastly, the operation of glass washing is greatly facilitated. The wires for Vines should be set at 18 in. from the glass. In the horizontal method the wires are strained between two lengths of angle iron properly fixed and stayed at each end of the house, and supported at intervals of about 15 ft. up the house by lengths of scroll iron 1 in. by 1/8 in., the length of the rafters, and slung from them by short pieces of the same material. The long pieces are bored at intervals of 9 in. to take the wires. In the vertical method screw eyes are put along the lower edge of the ridge, one for each rod and one for every space between. From these are hung wire hooks long enough to support the wires 18 in. from the glass. A piece of 3/4-in. gaspipe is bolted to the wall at short intervals, with eye bolts passing through the wall and screwed up from the other side. A strong wire is run right along the house under each purlin, generally passing through holes drilled through the purlin standards; these are to support the vertical wires when loaded with the crop. The wires are now fixed to the gaspipe on one side of the house, taken right over the hooks in the roof and strained tight to the gaspipe on the other side, passing over the horizontal supporting wires on the way. Whatever number of rods is decided upon, the first thing to do is to cut back the young canes level with the bottom wire in the horizontal wiring and to where the canes meet the wires at 1 ft. from the wall in the vertical wiring, leaving them just long enough to be tied to the wire. Whatever crop is grown in the houses while the young canes are covering the roof, the house must be treated as much as possible as a vinery, and on no account must red spider be allowed to obtain a footing. Tomatoes are usually grown under the Vines for three years, in spite of the fact that the atmosphere suitable for the one is most unsuitable for the other. However, the Vines must receive the first care and the Tomatoes take second place. Roses do well under young Vines, but pot plants are not the best thing to have, as the frequent watering required is apt to produce sour borders.
The young canes should be allowed to start into growth naturally the following spring. When the buds on the young canes show signs of swelling, a light syringing should be given twice a day to help them to break; but as soon as the leaves begin to open out, the syringing should be stopped and the canes be damped over with a sprayer instead. All the buds should be allowed to break, and when three leaves are formed the growths to form the rods can be chosen from near the top of the canes and allowed to run on while all the rest are stopped at the three leaves. Some growers rub out all buds except those they want for rods at once, but if three leaves are left to each shoot the roots are encouraged at once and the vigour of the cane is not sapped in any way by the extra growth. The selected buds are allowed to grow on up the roof, all side shoots that form being stopped at once. When the young rods reach halfway up the rafter they should have the tips pinched out and then be allowed to run on again. As the season advances they may be allowed to make a few side shoots near the top. When dressing the crop that is growing underneath, the Vines should get their share of the manure. As soon as the under crop will allow, at the end of the season, full air should be given except in stormy weather, to ripen off the canes. When the leaves are dead, and the sap has stopped flowing, cut back the young rods, leaving 3, 4, or 5 ft. of rod according to strength and ripeness. The wood left should be as thick as the forefinger and thoroughly well ripened. The following year this piece may be lightly cropped, and the young canes run up as before and pruned when dormant, according to strength as in the first year, the laterals which will have been formed on the previous year's growth being cut back to one good prominent bud, or two to be on the safe side. When the buds break the following spring the stronger can be selected and the other rubbed out.