This winter was significantly colder than normal in the northern half of Indiana. Many areas north of “I-70” experienced temperatures between -15 and -20˚F in early January. That is cold enough to cause considerable cold injury to grapes, peaches, and blackberries. See the map below for minimum winter temperatures.
We have evaluated crops at the Meigs Horticulture Research Farm in Lafayette and found considerable injury. Peaches are 100% dead. Most blackberry cultivars appear to be severely injured. Injury in grapes depends largely on genotype (cultivar). Cold hardiness varies greatly among commonly grown cultivars. Cold tender cultivars such as Chambourcin and Vidal have significant damage. In our trials, several cultivars have more than 50% primary bud injury. These include Chambourcin (45% live), Noiret (32% live), Cayuga white (23% alive), Traminette (21% live), and Vidal (18% live). Super cold hardy cultivars like Marquette and LaCrescent are 85-95% live.
Growers need to know how much injury has occurred before they prune so that they can adjust pruning severity. It is important to check buds from your plantings. Our results may differ considerably from yours. Many factors affect relative cold hardiness including vine vigor, crop load last season, late season leaf disease incidence, etc. as well as the temperature that occurred at your location. I recommend growers do a thorough evaluation of their most tender cultivars first, then moderately hardy and finally hardy cultivars. Our general recommendation is to collect a sample of at least 100 buds. I generally take ten 10-node cuttings of each cultivar. Choose canes that are representative of those you will save for fruiting wood this year. Avoid shaded or excessively vigorous canes. Since the cold damage occurred in early January there has been plenty of time for the damage to show up. If samples are taken immediately after a cold event, it is recommended that they be brought indoors and allowed to warm up for 24-48 hours to allow browning of damaged tissues. Check bud survival by slicing through the buds with a sharp razor blade. Start about 1/3 of the way from the tip and slice progressively deeper until you can see the main, primary growing point. With some practice it is easy to also check the secondary growing point as well. The video clip mentioned below shows how to properly dissect buds.
Once you’ve determined if you have cold injury, you can decide if you need to adjust your pruning severity. By leaving extra buds to account for those damaged, you can produce a full crop and maintain vine balance. For instance if 40% of the primary buds are dead, then only 60% of the buds are alive. To assure that we have the desired 50 shoots per vine (on an 8 foot vine spacing), we will need to leave extra buds. To determine how many to leave, divide 100 by the percent live buds. For example, 100/.60 = 167%. So we need to leave 50 x 167% = 84 buds to end up with the appropriate number of live primary shoots and fruit clusters. If more than 50% damage occurred, we recommend pruning lightly and adjusting shoot counts after bud break.
If you have questions about winter injury and pruning adjustment, don’t hesitate to contact me at bordelon@purdue.edu.
The attached video clip (about 3 min) shows how to dissect grape buds to determine injury.
https://mediaspace.itap.purdue.edu/media/Winter+injury+in+grapes/0_vus5dd3n
2018 Minimum winter temperatures