about a month ago, I planted two fairy maiden grass plants in large (approx 20 gal) cedar planters. They have thrived since then. I am concerned about what to do with them when winter approaches. I have received varying opinions about this. I live in Iowa and the temperature in the winter can reach -20 or lower. If I try to overwinter them outside, will the roots freeze , causing the plant to die? I have other grass species planted around my yard and they seem to tolerate the winters alright. Is there some way that I could mulch in or around these planters to protect the plants? I have a garage I could place them in, but, it is unheated and is probably only 10 - 20 degress warmer than the outside temperature in the dead of winter. Any opinions on this matter would be greatly appreciated.

Also, I have seen several towns that plant trees in large concrete containers along the streets in the business district. How do the roots of the trees keep from freezing in this situation?