Hi EllenR,
I overwinter all my hardy potted herbs outside. I'm zone 6 in Pennsylvania. I put rebar hoops with WHITE plastic over the hoops. I DOUBLE the plastic for thickness. My overwintering beds are 5ft wide by 50ft long. You need to use white because you only want to block out certain light. You don't want it dark because even though the plants are dormant they still need a certain amount of light and you don't want to use clear plastic because it can get very hot under clear plastic. The plants should remain cold. Many plants resent having anything laying flat on top of them this is why I use the hoops to raise the plastic off them. Your plants need to sit flat on the ground without solid plastic or a solid surface on the bottom because the plants need to absorb moisture. Even though the plant is dormant and not using water it must be keep slightly moist. I use landscape fabric on the bottom which has tiny holes throughout. If this isn't possible you'll have to water in the winter if the soil drys out. Tender plants like rosemary do not overwinter outside for me in pots. I move them in my greenhouse and keep the temperature slightly above freezing. Please note: Terracotta pots will freeze and crack in the winter. I remove all coverings by the middle of March and fertilize. At that time you can check to see if you need to root prune or move to larger pots.

Thanks!
What are rebar hoops? Is that something I can ask for at the garden center or home center and they will know what I want?
I'm imagining it is something curved to hold up the plastic.

ellenr

Rebar can be found in any home supply store. I believe it's used to reinforce concrete. It comes in long rods and is bendable. I use 10ft rods. I find that it's easier to hold the rods if you hammer 1" steel pipe in the ground every 4ft and put your rebar rod in the pipe and bend it over to another set of pipe. If you only have a few plants you can use translucent storage containers that you can find in Kmart or Walmart. Just turn them upside down and use them like a little greenhouse.