I think you have to check for the zone hardiness for your perennials. If they are hardy to areas above your zone (say, you're in 5 and the plants are hardy to 3) then you have less to do but if they are not (again, you're in 5 and the plants are hardy to 7 or 8 or whatever) then you will have a lot to do. For the less to do situation, because the plant can stand more cold that you receive in your area, a layer of mulch at the top might be sufficient but you might want to wrap the container with burlap, etc. to be extra careful. For the lot of work portion, the best is if you can bring them indoors, even in a unheated basement, etc. The root ball must not freeze. However, if you have to leave them outside, you might try mulching the top of the container, wrapping the containers with bubble wrap AND burlap and then putting some straws or shredded newspaper on top to further protect the plant. You might also want to move the containers to a wall that gets a lot of sun during the winter so that the wall can keep the containers warm through the winter. Good luck.

I have been successful keeping some plants with styrofoam coolers. Because my two patios are cement I set the plant on the styrofoam lid and set the pots in the styrofoam box. I put l pine needles or leaves on top and around the plant in the styrofoam box.

Don't forget to put drainage holes in the styrofoam so the water will not collect inside. As started above put them close to the house. All plants that are hardy to my zone survive.

Styrofoam coolers:

You would put the lid over the plant with the pot in the box? That is an interesting way of doing it =). I don't have access to pine needles (and my bf is horribly allergic to them to boot), but I can get some leaves from a friend. I also read somewhere about shredded newspaper, of which I can get plenty from my daily paper.

I will check to see what the survival zones are, and adjust winterizing from there =) Thanks for the advice!!

I used styrofoam coolers for my bulbs. If your containers are small enough to fit, they will work really well (don't need to wrap burlap and bubble wrap). If you don't want to put holes in the cooler, try inverting the cooler -- i.e. put the plants on the lid and invert the box over the lid. Don't seal the cooler completely, the plants still need air.

The problem you will have with some plants is drying up and dying if they are taped inside the strofoam box. No rain or moisture and when we get really warm days through the winter they may cook. Bulbs do good dry and cool I do not think plants will survive that treatment. Do you have Bulbs?

How about making a tall little house. Make a tower.

First layer is the lid with drainage holes
Second layer your Styrofoam box, again drainage holes
Third another Styrofoam box with the bottom cut out stacked on top of the plant holding box.

As you go up wrap ductape around the box to hold them together. Now you may want to make it pretty. Go to the Dollar Store and get the plastic ivy and decorate your box. They can be stuck on the box with pins.

This keeps plants from the drying winds and also offer enough protection from the weather and still get moisture and air. I use the pine needles because they allow the soil to breath. If I used newspaper I would not pile it up maybe use it as more insulation around the pots inside of the styrofoam.

Depending on how many plants you have this should be pretty cheap. The styrofoam boxes are cheap or free alot of the fish stores in my area will give them to you.

I should say I use this process for my side patio and back deck plants. The side patio is visible from the front of my house so I used the fake ivy. I even put Christmas lights on it for the holiday to look like a big Present. LOL Trust me it works.

I guess I wasn't clear in my description. The box is inverted the plants are not. If you lift off the box, you can then water the plants that are sitting on the lid. I do this primarily for bulbs in containers. I plant them in the container and if the weather gets below freezing, I put the styrofoam box over it to protect the bulbs. I overwinter my plants in a bathroom with a south facing windows and under grow light.

That house sounds like a good idea! I will have to tally up how many I have and see if that's economical for me to do. I can always put it towards the back of the deck where no one can see, since we live on 3rd floor and it all pretty much has to be out by the edge or hanging to be seen =).

Would some little sticks work to keep the styrofoam "up" a bit so it doesn't seal? I'm a visual person, so I'm trying to visualize all these great suggestions with foam containers, since that sounds like the easiest and cheapest solution to my problem. I do hope to be able to bring some in, but I probably won't have room for everything, alas =(.

Keep the suggestions coming, this is helping a lot!

Exactly what plants do you have (aside from the clematis)? In a bunch of cases, cold doesn't necessarily kill a plant but wet and cold would. What helps is to have the plant in the largest pot that you can haul out there (particularly the clematis) or even sink a smaller potted plant into a larger, soil-filled one. The styrofoam containers are also a good idea (I have an old console humidifier housing that I've started using for that purpose and this thread gave me an idea about lining that with styrofoam in addition to the pine needles that I currently use in it).

As a side note, one of my best insulators has been snow and that may help for you and your plants as well, although I've noticed of late, that many in the MW have had a lack of good snow cover for a bunch of winters and then would have their inground plants get brutalized with extreme cold.

I have seen some suggestions over the years for those in somewhat warmer areas overwintering marginal subtropicals outdoors in coldframe structures where they would rig up a light bulb for overnight to keep the temps up on the colder nights and I have used light bulbs near my clematis when it was a baby and after it had sprouted prematurely in spring and a frost was forecast.

As noted, if you have plants that are rated at about 1 - 2 zones colder than where you are, then you'll have an easier time at it if you can't bring them in.

Here is the list of periannuals I have:

? Moss roses
? New guinea impatient
Dianthus
? Shadow dancer betty
? Dahlberg daisies
Hosta
Clematis
Hardy Lily
Delphinum
Creeping Jenny
? Begonia
? Iresine
? Aspargus fern
Strawberries (hopefully, I bought the wrong kind for containers)
Parsley
Orgeano
Cilatnro/coriander
Lemon Mint
Basil (though I think someone said annual on that one)
Rosemary
Garlic chives
Thyme
Green peppers (if they ever grow...I think they're dead)

Those are just the periannuals =). I spent a lot of money and time on my deck, since I could actually do it this year. From tha tlist, I imagine it's easy to see why I want it all to come back next year.

Thanks for all your help, and I hope this list helps you out!