I beg to differ regarding the use of impermeable plastic for covering plants in the winter in Massachusetts. It *does* work. Most of the big commercial nurseries around here that grow potted plants cover their outdoor stock with Microfoam. This is a closed cell polypropylene foam that is around 1/8" thick. Typically it is put over pots or beds after a hard freeze, and removed in March. An additional covering of white plastic film may be used. Note that everything is white to reflect sun, and minimize freeze-thaw cycles. I know home gardeners who have successfully used it, too. I agree that it would be foolish to use plastic in warmer weather, but that's not what we're talking about. Reflectix might work so long as it does not become brittle in cold weather. Microfoam might be more practical and cheaper, however, if you can use a 270 foot long roll. And styrofoam would also work if used properly, as Adam observes. Most nurseries stopped using hay a long, long time ago. --Roy NW of Boston