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A mile’s worth!
The Turtle Creek laboratory
In 2004 we began an ambitious effort to study and implement various means of protecting Vitis Vinifera vines in a New England winter.
It is important to understand that this effort stemmed not only from conventional considerations of extreme cold temperatures but from earlier, mostly anecdotal evidence that, in this climate, significantly greater fruit production could be achieved with the fruiting cane placed on the high wire of a vertical shoot positioned (VSP) trellis. We found that high–wire fruiting and renewal zones produced four times the fruit as a conventional VSP trellis and at the same Brix levels. Higher potential yield are always something which can be traded for improvements in quality. While Gladstone and others have dismissed the independent effect of light intensity on ripening (concluding that it is completely correlated with temperature), our experience has been different. A high–wire VSP trellis maximizes light exposure for the fruit and renewal zone, something which is a deficit for the East (we compensate for this with a surfeit of fungal pressure).
There are issues a high–wire trellis presents, however. a Vitis Vinifera vine has an upright growing habit, meaning the shoots want to grow upward and, left alone, will provide excessive canopy and defeat the purpose. Second, it takes more effort to initially get the vines trained to the top wire, especially with low–vigor rootstocks. Finally, there is the issue of trunk renewal. Having the scion responsible for a trunk that comes from a graft union a few inches above the soil to a point about six feet high exposes more trunk length to the ravages of winter. There is already speculation that the need for trunk renewal in the East is a factor in lessening wine quality.
The answer to the first two issues is simply more work and discipline and, frankly, goes hand in hand with a premium vineyard. It should not be ignored that such a high–wire trellis would possess some incremental advantages; netting could be reduced in width and in its application, and annual maintenance might be reduced. Inocula falling from canopy above the fruit above will be reduced and everyone’s backs will suffer less. In addition, there is increasing evidence that the more perennial wood, the higher the wine quality. The third issue of trunk renewal is where our research focused.
If we could move the graft up to the top wire we would not have to worry about Crown Gall and winter freezing / thawing. If we could protect the graft union at that height, it was not hard to imagine extending the protection to the renewal zone where the following spring’s buds would emerge. A solution that protected the vine from winter death might also protect the buds responsible for next year’s crop.
This is where an old friend appeared in a moment of need: Bob Hauslein was responsible for all the heat transfer calculations and models as well as lab testing; this work would not have been possible without his experience and generous contributions. He calculated the heat losses with various materials, the effects of leakage, and the losses from night time radiation as well as selected the salts to evaluate and imagined different closure systems.
Our objective was to protect a vine cane from a minus 20°F event which lasted 10 hours, a seventy year event in this climate.
We looked at heating cables and various forms of insulation but eventually focused on a combination of phase change material and insulation. Phase change materials give up their latent heat of fusion in the process of freezing and will not permit a lower temperature until that energy reservoir is exhausted. Water is a simple example; indeed, water is used in exactly this manner for frost control. As long as the water flowing on the vine in freezing circumstances is not completely frozen, it does not matter how thick the ice is, it will not be below 32°F.
After researching a number of salts, we selected ammonium chloride for several reasons: first and foremost, it has a freezing point of 3.6°F at its eutectic mixture in solution (approx. 18%); it is also used as a fertilizer; and, it has no security issues. It may well turn out that other phase change materials will recommend themselves as we learn more.
For the first test in the winter of 2004–2005, we constructed, by hand, over 4000 feet of sleeves, made in ten foot and three foot sections. This consisted of 6 mil polyethylene, 8 inches wide filled by hand with the ammonium chloride solution at 0.75 pound per foot. This sleeve was sealed at one end and, after getting the air bubbles out on carefully leveled tables, was sealed repeatedly at eight inch intervals to compartmentalize the liquid and distribute it evenly along the length of the sleeve.
We had a cost target for this system per vine that would not exceed the installed cost of a vine and this caused us to discard a number of insulating materials.
To make a long story short, we settled upon a foil–laminated double layer of 3/16 inch bubble wrap. Because the foil is so thin (0.5 mil) and we were not sure our closure system would prove completely effective, we elected to add an additional layer of 1.5 mil aluminum foil around the compartmented sleeves to minimize any temperature gradients; this may not have been necessary.
The solution–filled polyethylene tube was laid on the eight inch foil which, in turn, was placed on the sixteen inch wide foil laminated bubble wrap, taped down repeatedly and cut to length.
We considered many different closure mechanisms: wooden clips, staples, tie wraps, even stitching. What won out in this initial test was a simple garment tagging gun with short nylon tags. In this system, you pierce the foil below the sleeve holding the liquid, through both sides and the preformed “T” of the tag sticks out one side and you retract the gun, leaving the other end to hold the two sides of the sleeve compressed together by the tag. These guns are finicky and it usually took a few punctures in the hand to initiate a new installer but everyone did it and the sleeves were installed without too much hassle.
We really will not know the results until bud break. It is unknown whether there may be bud abrasion; we do not know the effects of three months of no light and no moisture on these plants. We do know that the sleeves stayed in place and seemed little worn when they were taken down. There have been some anomalies in the performance of the phase change material; eg, while in the lab, we saw no evidence of super–cooling, in the field it appears that there is some super–cooling. Curiously, it looks like the solution needs to be “inoculated” through one cycle and, afterwards, the material functions more as expected.
We have the measurements of probes inside the sleeves, next to the vine, and have reproduced the graphs of ambient vs protected temperatures here. In January, we had three good spikes to minus 10°F or lower—you can see what we think the protected vine saw…
More information as we get it.