HORTICULTURE
Crop Adjustment in Michigan Juice Grapes
Mark Longstroth District Extension Educator
Juice grape growers in Southwest Michigan have suffered two light crop years in a row with the vines often carrying less than half the crop that they are capable of carrying. As this year's shoots grew and the flower clusters emerged it was clear that we had the potential to set a very heavy crop of grapes in Concord. Instead of 2 clusters per shoot we had 3 or 4 flower clusters on each shoot. After bloom, the average berry count was 43. This is higher than the long-term average of 36 berries per cluster. It is obvious that we are carrying a very large crop. Estimates of the final crop vary from site to site. Estimated yields ranging from 7 to high as 20 tons/acre have been reported. The Michigan average juice grape yield in recent years has climbed to 6 ton per acre. We are running about one to two weeks behind normal and Concord harvest is predicted to begin in early October and run for over a month. Since we usually have killing frosts in mid October this means that most growers will be hard pressed to deliver all their grapes.
Growers should assess the crop that they are carrying and determine if they need to remove part of their crop. Determine the weight of the fruit in the vineyard at this time and then estimate what the final fruit weight will be. At this time the berries weigh about 70% of the final berry weight. Growers can divide the weight of their fruit samples by 0.7 to determine final fruit weight. Some growers are weighing the fruit from single vines and then multiplying by the number of vines per acre to estimate crop. Using this method you need to do more than one sample in your vineyard. Individual vines are too variable for one vine to by a good representative of the vineyard. Several samples are needed to get a good average. Five would be minimum number of samples that I would use for a good average. Another method if you have a harvester is to sample a length of trellis. For nine-foot trellis spacing 43 feet of trellis is equal to a hundredth of an acre. For ten-foot trellis spacing 48 feet is equal to a hundredth of an acre. You can lay out a length of rope and harvest that distance. Weigh the fruit and multiply by a hundred, this will give you the weight of fruit now in the vineyard. If you then divide by the correction factor of 0.7 that will give you an estimate of your final harvest weight. The correction factor continues to change as we approach harvest. Next week the correction factor will be closer to 0.8.
Thinning should be done before veraison, when the grapes change color, soften and start to ripen. If you can have your crop adjusted by veraison then the grapes will ripen normally. If you try to thin after veraison you will need to remove even more grapes. Veraison should be in about 3 weeks. Growers who are thinning are trying to reduce their crop into the 7 to 9 ton/acre range. In some cases this means that a lot of grapes need to be removed. If you need to remove more than 3 or 4 tons, then only try to take part of the fruit in the first pass and come back later and remove the rest. Trying to take off half the crop at one time will damage the vine, reducing its ability to ripen the crop this year and also its ability to harden off for this winter resulting in increased winter injury.
There is still time to estimate what your crop will be and determine how much to remove to allow a timely harvest this fall. In some cases only a slight adjustment may be necessary. Most growers should be adjusting some of their vineyards for no other reason than to provide blending stock to assure that they can blend during harvest. I feel that this grape crop will be hard to manage during early harvest with most sites having low sugar. Anything a grower can do to increase his ability to harvest his grapes early will make it more likely that he will get all his grapes harvested this year.
For more information on juice grape thinning check Terry Bates Home Page off the Lake Erie Regional Grape HomePage