On Sept 15th, 2010 at about 4 am the temperature in our backyard was -6c and that ended the season for growing grapes here - think this was near the all time low for this date. Even covering the vines doesn't help when it gets that cold and the berries were frozen solid in the morning.
The best of the St. Croix were showing sugar in the neigborhood of 12-14 Brix, see picture below,
But most of the St. Croix and Edelweise grapes still had a long way too go and were averaging around 10-11 brix, which given the cool weather of late August till now, is not bad. I think about 10 more days of warmer weather would have brought the overall average to the 17-18 brix range. I picked the St. Croix to see what weight came off and we had about 12 lbs over 24 feet of row. Not bad for 3rd year plants.