So I wrote in a previous blog that I leave extra buds when we do the first pruning just in case the winter was harsh and killed off many of the dormant buds or in case we were to get a late spring frost. Leaving the extra buds provides some degree of insurance against these possibilities as there are more buds that could possibly survive and produce some grapes or even secondary buds producing a smaller crop. However, if all the buds including the extra ones that I leave produce a good cane with proper flower clusters I would need prune off many of the canes to bring the extra canes back to a "normal" cane count in order to produce quality fruit that will ripen on time. (If I were to leave all the extra canes the vine would be over-cropped and be slow to ripen and may actually not winter well as the vine does not get enough time to shut down into dormancy before the cold winter hits).
So what is normal bud/cane count? I use quotations around "normal" because I'm not so sure there is a normal bud/cane count. You'll often read documents that provide recommended bud counts to leave for various grape varieites. For instance a document may say that for St. Croix to leave 30 buds for the first pount of prunings then and extra 10 buds for each pound of prunings after that to a maximum of 50 buds etc etc. Often these measures dont tell you all you need to know to make a proper determination like what if your vine has 8-10 feet of trellis length. While 10 feet is unusual, at 10 feet of trellis length the bud count I mentioned above would probably leave 5 buds per foot when the vine could probably grow best at about 6-7 buds per row foot. Variables that affect growth include soil type, the fertilizer, length of season, vine type, irrigation, training method etc., as all of these will impact what the optimum balance is between bud/cane count and quality fruit production. So while the bud retention counts per pound of pruning is a good method its really just an approximation - a place to start - then you'll have to tinker with it to determine what is optimal.
So getting back to green pruning - when I did the first dormant pruning in March I may have left 10-12 buds per trellis foot on some vines but in the end I only want to retain 6 buds per trellis foot. So now it is late May and risk of frost is gone, bud break has occured, and the canes are all about in the 3-5 leaf stage. I can see what canes are healthy and what ones are not. Some canes may be stunted or have poor flowers or no flowers at all and others may be fine. In doing the green pruning I want to keep the good healthy canes and prune away the canes that are stunted or with poor or no flowers. I'll prune away until I achieve the desired 6 canes per trellis foot. See the pictues below;
Good cane with 3 nice flowers
Poor (or No) flowers on a cane
These canes shown here are going to be thinned out a bit even though they are all excellent with great flowers. So while all these canes are great its not a choice of which ones are better than the others - in this case I'll prune ones away so that there is greater space between the remaining canes that will allow better sunlight exposure and air flow.