From Tony Wolf:
Question from the field: We saw unusually warm temperatures in January and the early part of February. Forsythia and daffodils are blooming in my back yard. I’m wondering if grapevines have lost cold hardiness and, with cold temperatures forecast for this coming Sunday (12 February), should I be concerned about the potential for cold injury?
Answer: Our research technician, Kay Miller, has been routinely monitoring bud cold hardiness this winter with some of our on-going research projects. The results are reassuring and the forecast temperatures should not pose a significant problem, particularly if they stay in the +10F zone. The following temperatures are the predicted temperatures (at a particular test date) that would be expected to kill about 50% of primary buds for the stated variety. These temperatures are determined from freezing tests that are conducted in the lab using buds collected from several locations. The Viognier and Merlot buds were collected from a Charlottesville area vineyard on 5 February and tested on 7 February. All others are from Winchester, northern VA, or western Maryland. Here is a recent sampling of some of those data:
| Variety | Test date | MLTE (degrees F) |
| Viognier | 7 February | – 7.1 |
| Merlot | 7 February | + 0.5 |
| Cab Sauvignon | 27 January | – 8.0 |
| Barbera | 27 January | – 9.2 |
| Petit Verdot | 17 January | – 5.8 |
| Albarino | 17 January | – 9.6 |
Again, the temperatures shown for each variety are an approximation (from controlled freezing tests) of the field temperature that would be required to kill about 50% of the variety’s buds (e.g., -7.1 F for Viognier). Thus, as of mid- to late-January, we saw the maintenance of good levels of bud cold hardiness (well below zero Fahrenheit). The limited testing that has been done in early February would suggest that Viognier, at least, is still quite hardy. We haven’t done a lot of Merlot cold hardiness testing, but we know that it’s not a very cold hardy variety – a principal reason why we did not grow much Merlot in the eighties and nineties. Warmer winters (among other reasons) have led to an increase in acreage. I’m not surprised to see the inferior cold hardiness when compared with Viognier collected from the same vineyard. So, I’m not too concerned about 10F temperatures (or warmer). And the colder weather should help maintain the bud dormancy; no one really wants to see an early bud break.
We’ll talk about hardiness and assessing potential bud and cane cold injury at the pruning demonstrations, and it’s also discussed in the Wine Grape Production Guide for Eastern North America. Viognier growers should, as always, cut (section) a sample of buds to determine the level of bud necrosis.
Tony Wolf
Viticulture Extension Specialist
E-mail: vitis@vt.edu
Web: http://www.arec.vaes.vt.edu/alson-h-smith/grapes/viticulture/index.html