In the Washington area, for good quality grapes it is necessary to prune the vince every year.
The average vine bears more fruit than it can ripen properly and one of the main purposes iof pruning is to keep it fom doing so.
If there are too many grapes, they are not as large and sweet and tasty, they don't ripen as early as they should, there is poor fruit formation, and the vigor of the plant is impaired.
Pruning can be done in late February and during March. Later pruning increases the danger of knocking off tender buds as they begin to swell and grow. It is always better to prune, even if late, then to let a grape vine go unpruned.
Professional growers prune any time during the dormant period except when the vine is frozen. The wood then is very brittle and breaks easily. In very cold climates it is considered best to wait until the worst of winiter is over before the pruning.
If pruning is delayed until the the sap begins to flow in early spring, heavy bleeding may result. Bleeding actually causes little or no harm to the vine. In a short time the wounds callus over and the bleeding stops. Life or productivity of the vine will not be reduced.
Pruning a relatively young vine isn't much a problem. If it is an old vine rambling all over almost everywhere, you have a mess. Pruning it properly, is about a s difficult as trying to explain comprehensively how to do it.
If the fruit doesn't mean much to you and if birds het most of it anyway, don't bother to prune, at least until you start getting complaints from the birds about poor quality.
To prune properly, it helps to consider the structure and growth habits of the vine. There is the main stem or trunk coming out of the ground. Arms grow from the trunk. Canes grow from the arms. The canes are last year's growth, they are just 1 year old, next year they will be arms. The arms have gray shaggy bark. The canes are brown and have smooth bark.
Along the canes are fat growth buds. There may be an occasional growth bud on an arm or on the trunk, but most are on the canes.
When the weather is right in the spring, these fat growth buds open and shoots grow from them. Each of these shoots usually bears two or four blossom clusters in addition to leaves. The blossoms open and bunches of fruit develop.
The amount of pruning given a grape vine depends largely on its vigor. In general, a healthy, vigorous vine should have 40 to 50 buds after pruning. Thus, a vine could have 4 to 5 canes left, each with 10 buds.
This could mean removal of half or more of last season's cane growth. A weak vine would have fewer buds remaining, possibly 30 to 40.
Before pruning, the fruiting canes should be 4 to 6 feet long and about the thickness of a lead pencil or slightly larger. One cane per arm is the best rule.
In addition to the fruiting canes, leave about 6 canes for renewal. These should be as close to the trunk as possible. Shorten these renewal canes so that only 1 or 2 buds remain on each.
A strong shoot will grow from each of these buds and these shoots will be next year's canes.
An old vine can be rejuvenated so that it should bear good fruit in about two years. The first step is to remove all dead and extremely weak wood. Then shorten the long trunk and arms so as to leave 8 to 10 strong canes on the vine.
Shorten one-half of the remaining canes so as to leave 6 to 8 buds on each, and cut the other back to 1 or 2 buds each.
This heavy pruning stimulates the development of strong new shoots mearer the main trunk and at the same time provides some fruit the following summer. One year later, prune to thin out the canes for fruiting and renewal.
Fertilizing the vine encourages growth of new shoots and increases quality of the yield.A good general rule is to give the vine about two pounds of 5-10-5 fertilizer each year, making the appliation in late winiter or early spring.