It is possible to use the styrofoam covers, but you need to make sure the plants are completely dormant before placing them on the plants.
Mulch the base of the plants with leaves or hardwood mulch.
Also make sure the covers are vented to avoid heating up on sunny days.
It's the root system you are needing to protect in winter.
Snow cover is really the best insulation.
Here is a link with more information.