I'm a chemist and have worked for decades in cancer research. I grow grapes, make wine, and pay attention to how the weather affects those processes.
By: DrZZ , 12:34 PM GMT on January 11, 2014
The big freeze has come and gone. I won't know for sure what the damage is to the vineyard until spring, but if the info I have holds up, the damage should be minimal. It has been very interesting to add the details of my own weather station to thinking about how weather affects the vineyard.
The first really interesting thing was to see how variable the temperature was across pretty small distances. Early morning January 4th my station recorded a low of 6.3F. Just over a mile away (but ~175 ft downhill) the low was 3.7F and a mile farther it was 0.3. The official low for Frederick that day (the airport I presume) was 3. On Jan 7 the stations were more in line with 0.0, 0.2, 0.0 and 1 for the listed lows. It will be interesting to see if I can come up with regularities that will help me know how to adjust the Frederick predictions for my vineyard.
The other part of the winter management decisions is to know what temperatures have what effect on the vines. Again there will be a strong component that is specific to my vineyard, but there are not only generalities, but relatively local data. The sensitivity of vines to damage varies thoughout the winter. If you get a spell of relatively warm weather the dormant buds are more susceptible to colder temps. Various centers (I think Virgina Tech does this, but I couldn't find it on the web just now) will cut samples vines during the winter, subject the buds to contrlled temperatures for lengths of time and then determine the damage. These kinds of measurements yield values around -2F to -4F for when you start seeing bud kill to -8F to -12F for when you start seeing 50% of buds killed. So the zone figures given by the supplier of my vines are probably pretty reasonable estimates for seeing no damage at all.
Another thing to consider when worrying about damage is that most of the buds on the vines now will be pruned anyway. Most of my vines have ~125 buds. When pruned there will be no more than 14-15 buds. So if you are going to prune ~90% of the buds anyway, 50% kill doesn't sound disasterous.
So with all that what did I finally do? As I detailed in the last post, I piled mulch up on the mourvedre. That made sense because not only are the mourvedre the most sensitive to cold, but they are the newest and probably only have 10-30 buds per vine. I did go out with a thermometer Fri Jan 3 and after a few hours of good sunshine (that's recorded too!) the temp in the middle of the pile was 36F while the air temp was 19F. I went out the next morning when the air temp was 9F and the middle of the pile temps were still around 30-32F, so I'm pretty confident that the mourvedre will come through. I decided not to do anythng for the rest of the vines. As the temp never got below zero, in principle, I should have plenty of surviving buds. We will see in the spring. And has calpoppy pointed out in a comment to the previous post, at least I will have some hard data on my own vineyard to guide future decisions.
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