Many sources of variability affect bud freezing results on a given day - or in a given year, so the best way to look at these numbers is 'as a group' to indicate relative cold-hardiness. An LT50 of - 15 degrees F does NOT mean the buds are 'hardy down to -15'. Because the breeding plots are not managed like commercial vineyards, the vines' performance may differ from a commercial situation.
Here are a few of the factors that can affect results:
- Results reflect weather conditions in Geneva.
Vines acclimate to cold differently in different climate regimes. For the cold-hardy varieties, for example, one might expect that vines subjected to colder conditions would have lower LT50s.
- Less than optimal disease control.
Some of the varieties are grown in a 'no-spray' vineyard, which can reduce bud hardiness, relative to a 'clean' vineyard.
- Crop load was not precisely adjusted.
Vines may have been undercropped or overcropped, relative to what one might do in a commercial vineyard.
- Some varieties were not harvested.
The fruit remained on some vines until after leaf fall. In a short, cool year like 2009, leaving fruit on the vine could significantly reduce vine hardiness.
- Short-term weather variations affect results.
Warm temperatures a few days before collection can result in higher LTEs
- Midwinter collections only.
No effort was made to measure rates of acclimation and deacclimation during the fall and late winter periods. These can vary among different varieties.
Bud hardiness in your vineyard will probably differ from the midwinter LT50s shown here. The best use of this information is to compare relative hardiness of different selections. Absolute numbers may change, but relative 'ranking' of varieties in the same location is generally consistent over several years.
Further information about the varieties, selections, and their parentage can be found at the Cornell-Geneva Grapevine Breeding and Genetics Program.