It's generally best to overwinter hardy plants outside, even as cuttings (nurseries do this routinely), although with some protection from the brunt of the weather. One approach is to put the plants in a cold frame, but anything that shields them from winter winds (usually out of the northwest) and keeps the root mass from freezing and thawing too often is helpful. Nurseries tend to use an unheated hoop house (like a greenhouse, but with plastic sheeting over arched supports instead of rigid glass walls) for overwintered hardy crops, but they can also heel-in pots as well. Heeling-in simply buries the pot (as if you were planting the plant, just without taking the pot off first) so it can be lifted back out of the ground in spring once the temperature moderates. This helps to insulate the root ball since it's surrounded by soil, so the temperature changes more slowly. A thick mulch layer (perhaps 3 inches deep) over top of the soil can help to add insulation as well as slow down the evaporation of moisture out of the potting mix, since drying out too much in winter can be just as deadly to a hardy plant as too much cold exposure.

Do not water sparingly in winter, as drought stress can reduce winter hardiness and predispose plants to other damage later. Instead, water the same as you would in summer -- whenever it's needed based on checking the soil moisture in the pot, and enough water applied to thoroughly rehydrate the entire container so the excess water drains out -- but the weather coupled with dormancy tends to greatly reduce the frequency of watering. For example, nurseries overwintering hardy plants in an unheated greenhouse might need to water about once a week in winter, versus daily or more in summer. Granted, you'd have to wait for a thaw (if the pots do freeze, which by itself isn't necessarily a problem) in order to water, which is one reason why it's good to check on watering needs prior to a forecast cold snap.

If your greenhouse warms-up too much in winter, try shading it more or venting the extra warmth, as it would be ideal to keep hardy plants dormant in the ~40-degree ballpark. (Above freezing, but otherwise fairly chilly.) You can't do much about midwinter warm spells of course, but the more you can reduce temperature swings for the plants (including the roots), the better. Larger volumes warm up and cool down more slowly than small volumes, so clustering the pots together, or putting loose or bagged mulch around them to add to their mass, can help to insulate them from drastic or rapid temperature fluctuations.

If you have sites available for the plants where you eventually want to grow them, you could even plant them in their permanent locations this autumn (September, ideally, so they have ample time to start establishing more roots). Protection from deer browsing would be important, then, if they visit the area to eat.

Miri