Pretty interesting data...you can see most Bermudagrass varieties will not survive temperatures below about 22 degrees (F). The most popular bermudagrass varieties used today (on greens) are Champion, MiniVerde and Tifeagle. Of those three, Tifeagle is the most cold tolerant according to this graph. Most superintendents I know managing bermudagrass greens will cover them when temperatures are predicted to get to 25 degrees (F) or colder. Some supers even spread a layer of pine straw over the turf before putting the cover on to provide an additional layer of insulation. This is never an issue with bentgrass greens (like HCC) because it is a cool season species that is much more cold tolerant. It can withstand temperatures as low as -20*F or more. However, with cool season turf, ice damage can be an issue. Winter kill occurs frequently on Poa annua, when ice cover doesn't allow for gaseous exchange between the grass and atmosphere. It can also occur on bentgrass, although this is fairly uncommon because bentgrass can tolerate more days under ice compared to Poa annua. In northern climates, superintendents will attempt to remove the ice cover if it will last 20-30 days or more. This can be done by physically removing it (using shovels) or using black colored sand or fertilizer that will absorb heat and melt the ice.