How to Indentify Injury to Grape Buds
When temperatures go down below freezing, grape buds can tolerate temperatures down to about 28 F. If temperatures drop below 28 F, new buds that have emerged are subject to injury. However, the duration of cold temperatures can be hard to gauge without a min-max thermometer that has a paper graph read out of the temperatures during the night. A regular min-max thermometer only provides the minimum temperature during the night or during the period starting when the thermometer was last set. As a result, there is no way of knowing if the extreme cold existed briefly or for several hours.
Please see related story: Viticulture Advice For Warmest Spring Ever From MSU and UW Experts
Primary buds that have emerged and are showing a leaf or two are usually alright unless they are dropping and wilted. According to a chart created by Dr. Tom Zabadal, Professor of Horticulture at Michigan State University, buds that are barely broken or slightly swollen can take a lot of cold before injury occurs. Dr. Zabadal’s chart also shows that buds that have burst or are showing leaves will show some injury at 28F. Checking for bud injury requires cutting open several swollen buds to determine if the center of the bud is brown. If the center is not brown, chances are that these buds (not the ones you sliced open :-)) are fine and will continue to grow normally. Overall, we will all need to watch and see.
I have sometimes wondered if cold that is just on the margin of injury does not damage the tiny cluster that’s compressed within the bud. Sometimes we see clusters that are small or misshapen and it’s reasonable to think that individual compressed clusters may have experienced partial damage despite the fact that the bud itself was OK and continued to grow.
It will continue to be an interesting spring. We are all learning a lot this year.
John Marshall is the owner of Great River Vineyard in Lake City, Minnesota