If you have some milk jugs, soda bottles, anything that can hold water-- put them under the covering too. Preferable all the way around the planting, but any will be better than none. As the water slowly freezes, it gives off heat... often just enough to save a crop. That's the principle behind the Wall O Water.
Another idea that a master gardener friend told me: if you have any of those mini Christmas lights- stick them under the cover-- they give off a little heat. Not enough to burn plants, but maybe enough to save them. She actually used them in her greenhouse, at the top of the dome. It would provide just enough heat to melt some of the snow sticking to the greenhouse and allow the snow to slide off.
