By now, most of us have begun or are nearing completion of winter pruning. Many Midwest vineyards this year have been pruned in response to cold injury, but even though the pruning may be non-standard valuable pruning weight data can still be collected.
What is pruning weight data and why is it important? As a winegrower, our most fundamental task is to balance the vegetative and reproductive (grapes) growth of our vines in order to achieve the highest quality fruit. Quality in this case meaning the optimum sugars and compounds for winemaking. This study showed that there is an optimum ratio of leaf area to crop size to achieve these quality goals, but it is difficult for us to measure leaf area directly. That is where measuring the weight of one-year-old wood from a vine during pruning can give us a close approximation in the field. These “pruning weights” will give us a proxy for the size of the canopy that we can then use with yield data to determine vine balance using a ratio named “Ravaz Index”. There is a good explanation of vine balance on extension.org and more info on taking pruning weights in the Midwest Grape Production Guide starting on page 44. A few things to note about taking pruning weights:
- Make sure to sample enough vines for it to be representative! The more vines you sample, the better picture you will have of your vineyard block.
- By sampling three vines right next to each other from each sample location, you can try to limit the subjectivity of the vines that are chosen.
When collecting pruning weights, it is a good idea to get an approximate count of the number of canes per vine while you are cutting them off. This extra step allows you to read a few more things about the vine that may not be as easy to see with just the pruning weights. For example, if the average number of canes per vine is much greater than the number of buds left last year at pruning, then perhaps there were too many non-count shoots crowding the canopy and a consideration would be to improve shoot thinning in the following season. Additionally, by dividing pruning weight per vine by number of canes per vine you can see your average cane weight. Optimum would be around 0.7-1.4 oz; if this is low then the vines may be over-stressed, perhaps because of over-cropping or other factors, and if it is high then perhaps the vine is out of balance and didn’t have enough crop left on it. This should be considered in addition to the Ravaz Index in order to get the best idea of where the vines stand. A terrific resource for determining what your numbers should ideally be is Richard Smart’s classic book “Sunlight Into Wine” listed in the resource section of this website.
Armed with your pruning weight data, you can make data driven decisions in the year ahead. When it is time to decide how much to fertilize, you may look at your pruning weight data and decide you want to put down a bit more nitrogen in a block that ended last year weak and over-cropped. Pruning weight data reduces the subjectivity and guess work when making crop reduction decisions; using your data you can determine how big of a crop you should leave to balance the vines. By observing trends over time, you can tell if a block is in decline and take steps to identify and remedy problems.
Pruning weights are a great way to track the most foundational principle of our vineyard, vine balance. Collecting good data helps us know where a vineyard stands and how to position it to produce the best fruit in the coming season.