Oh, it is a very varied designation. MU obviously uses 32 and I think our very own NWS might but officially the USDA says the growing season is not 100% directly related to the air temp vs. ground temp. Since people do not routinely have ground measurement capability, the first hard frost is what they use and they estimate it to 28. I certainly would not trust my plants to always survive if it got to 29 but I would still water them assuming they made it. But when someone gets to 27 I think that ends the discussion!