HI, Karen -

You posted this in the photo gallery instead of discussions. I can answering it here, but no one else will see it since the photo area doesn't get used a lot. If you want more answers, repost this in the discussion area.

The short answer is that it depends on the type of tree. The general rule of thumb is to use the largest container you can and to plant things at least 2 zones hardier than your area, so for you that would mean things hardy to zone 4 or colder. The advantage of a large container is that it will go through fewer freeze-thaw cycles which are difficult on the tree's root system. Some folks insulate their containers with foam on the inside before planting or surround the container with bubble wrap or bags of dry leaves on the outside if it's in a spot that you don't use in the winter such as a back garden patio. You can also add a layer of green foam staked into the top of the container and decorated with evergreen branches to insulate the top of the container in winter, sort of like the ones in the link below, but with the foam cut as a donut and in two pieces. Here's a second link with more photos of the same process: http://www.deborahsilver.com/blog/what-a-mess/

Alternatively, some folks use really large perennials such as some of the large ornamental grasses or a large sage such as S. guaranitica 'Black and Blue' instead of trees. If they don't survive the winter they can be replaced less expensively than a tree and allow you to make changes each year.

I hope that this response isn't too late for your containers.

Here is a link that might be useful: Winter containers/Dirt Simple/Deborah Silver