The danger is, the bulbs must be watered; water must reach the bottom of the bulb--but not stay there--for the bulb to initiate root production. It must have roots before the worst of winter sets in otherwise, the bulb, without roots, is useless...the freeze will kill the bulb. With roots, they are protected against anything winter throws at them. The tulip bulb --let's say put down 6 - 8 inches...would have 6- 8 inches under them. The problem then is to try to keep the water in so little soil from freezing and causing the bulb to freeze. Normally the advice is 'dont' try it'...the bulbs cant survive winter's freeze. But, if you can protect the container from the freeze, and the thaw...then freeze, then thaw...then freeze...you stand a better chance of growing bulbs this way. Put a cardboard layer over the outside surface of the container. If its wood, so much the better because it doesn't attract heat nor cold.....when watered, it can absorb water and freeze when temperatures go down. This then transfer cold to the soil...and to the bulbs therein. So, protect the container from freezing. The cardboard layer can help and when you put an aluminum foil cover over that, the sun will be deflected. Snow must not be allowed to sit on the surface. It can melt, then run into the top surface of the soil, where it then freezes. It therefore, cannot be allowed to be hit by sunshine which will do the thawing but cant protect it against nighttime freezing. If you can, put an A-frame...a lean-to effect over the container to prevent rain/snow/snowmelt from entering the container. It must also not be where it is hit by wind which will dry out the surface much quicker. Such good place to put such container is amongst your foundation plants, next to the building. Mulch the surface to keep the temperature more uniform. In any case, place the container where it will get some protection...as next to the building, out of the sun and away from the wind.

I live in a similar zone to you and have many plants that I overwinter in hanging baskets, outside, still hanging out there all winter. They do require care in checking moisture levels and mulching well over the winter or they won't survive - as tapla and Joe point. The key, as tapla says, is to survive zone 5 or below (two USDA grades below your current one). What you don't mention is how much sun you get which would help direct suggestions, and what size baskets. In addition to the plants already recommended - depending on your light - you might consider strawberries, the dreaded barberry, dwarf chamaecyparis (root pruning at regular intervals recomended), hardy native ferns, kinnikinnick, wintergreen, cymbalaria muralis (trailer, grow from seed - even if it dies back it reseeds sometimes too well), dwarf blueberries, cranberry (trailer!). I've grown on all of these, and kept them happy even, in 16" baskets of the type you describe. However, I do treat them to tapla's 5-1-1 mix, and repot with fresh mix every year. I look forward to hearing what you choose.

I have been told that bulbs will not survive planted in above containers here in MN. I know that Bachmans uses forced bulbs in their spring outdoor containers. I have had success planting tulips, daffodils and frittalaria in an 18"-20" d container that I keep in my unheated garage. Crocus did not survive. Have planted a couple of smaller containers of tulips that I am going to experiment with keeping in the garage this winter. Other years I have planted in those containers and sunk them in the garden beds. With a good amount of mulch those can be dug up early and brought into the garage to flower early.

zooba are you planning on leaving the planters where they are for the winter? Or can you move them? I think either way it can be done, but if you can move them (into an unheated garage or shed, or even against a wall with piles of leaves around them, etc) that affords a bit more protection. I used to have a pot ghetto of up to 240 pots, of ALL sizes. Some were large decorative planters, some were new plants in 2-inch pots that I hadn't gotten around to planting yet. For years I would drag them into the garage and drag them out again and had minimal losses. Despite the fact that the ghetto eventually got smaller, I seemed to get lazier, and the pots didn't all make it in the garage every year. Often just on the driveway without any protection. But for the most part they still did fine. Perhaps a few more losses, but those were the smaller pots and smaller, less-established plants. I used to put tulips in pots because I treat them as annuals, planted in fall. One year there was a pot of them right outside the garage door. In mid-winter, I finally decided to drag it in the garage, and lifted it, and it cracked in half, but remained frozen to the ground. I could see some of the bulbs. Welp, there goes those tulips, I thought. But they were fine and bloomed beautifully in spring. So it can be done. I agree with going down a zone if you are planning on keeping them outside. However I also agree with drainage. I once planted about half a dozen pots specifically for winter interest, out on the patio, all a zone hardier. They all looked great in the winter, and then I lost three of them in spring due to drainage issues. So I would look around for plants you like that are hardy to zone 6, maybe zone 5 if you want to leave them for winter interest, and do make sure they have proper drainage. Good luck! :) Dee