Grape vines such as Concord, Fredonia and Himrod, should be pruned every year, preferably in late winter or early spring before the buds start to swell. Later pruning increases the danger of damaging tender buds, and bleeding at pruning wounds is usually greater (it actually causes little or no harm to the vine), but it is better to prune late than not to prune at all. Don't prune the vine while it is frozen.
Left unpruned, the vine may bear more fruit than it can take care of, the grapes are not as large, sweet and tasty, they don't ripen as early as they should, there is poor fruit formation, and the vigor of the vine is impaired.
Pruning a young vine isn't much of a problem. If it is an old vine rambling all over the place, pruning it properly is about as difficult as trying to explain comprehensively how to do it.
If the fruit doesn't mean much to you and if birds get most of it anyway (you can protect it by putting a plastic bag over each bunch as it matures), don't bother to prune until you start getting complaints from the birds about poor quality.
To prune properly, it is necessary to understand the structure of the vine. There is the trunk coming out of the ground. Attached to the trunk are arms, or branches more than 2 years old. Canes grow from the arms. They are last year's growth, are just 1 year old and will be arms next year. The arms have gray shaggy bark, the canes are brown and have smooth bark.
On the canes are fat growth buds which will develop into shoots which bear leaves, flowers and fruit.
The amount of pruning depends largely on the vigor of the vine. In general, a healthy, vigorous vine should have about 40 to 50 buds after pruning (4 or 5 canes, each with 10 buds which will become shoots).
The canes remaining after pruning should be 4 to 6 feet long and about the thickness of a lead pencil. One cane per arm is best, but two or three can be satisfactory.
In addition to the fruiting canes, leave about 6 canes for renewal. These should be as close to the trunk as possible. Shorten the canes so that only 1 or 2 buds remain. These buds will provide next year's canes which in turn will provide the leaves, flowers and fruit.
An old vine can be rejuvenated so that it should bear good fruit again is about two years.
The first step is to remove all dead and extremely weak wood. Then shorten the trunk so as to leave 8 to 10 strong canes. Shorten one-half of the remaining canes, leaving 8 to 10 buds on each, and cut the other half back to 1 to 2 buds each.
This heavy pruning stimulates the development of strong new shoots nearer the main trunk and at the same time provides some fruit the same year. One year later, prune to thin out the canes leaving only the ones for fruiting and renewal.
Fertilizing the vine encourages growth of new shoots and increases quality of the grapes. A good general rule is to give the vine two to four pounds of 5-10-5 fertilizer each year, making the application in late winter or early spring.