Which perennials can be saved as roots over the winter?
I am thinking like dahlias.
I have heard that Salvia patens roots can be saved, and meconopsis rudis.
Which ones have you come across?

Hi Caroline! You can overwinter pretty much any dormant perennial in a container. If it's a hardy perennial, the root can freeze solid when dormant with no ill effects (although continual freezing and thawing should be avoided). If it's not frost-hardy, then you'd need a storage area that doesn't freeze, but is cool and dry so that the root doesn't start into growth. I've successfully overwintered in my minimally-heated greenhouse various containerized perennials that I didn't have time to plant in the garden before freeze-up, and also some potted sub-shrubs not hardy in my zone, such as lavender, culinary sage, rosemary, and fuchsia. The hardy plants were on the greenhouse floor and froze solid, while the others were raised up on shelving and they got a bit of warmth from the heater (but no more than a couple of degrees above freezing).

I overwinter hardy perennials that are in containers by placing them against a wall and then stacking a bale of straw against them. I often use gallon containers or quart pots, and if they're herbaceous and not shrubby, you can stack the quarts once the foliage is gone. I will frequently grow them on in containers if they need some size to survive in a bed, or if I want to see the real habit of the plant or color of bloom before I plunk it in a bed. Or, if it's a plant that needs "crowd control" like mint, bamboo, gooseneck loosestrife, etc.

If there is space between the pots, I fill with shredded dry leaves. Wet leaves will yield slimy black goop, dry leaves will be almost composted by spring. The straw will be ready for mulching the vegetable garden.

Thank you for good ideas on this.

For storing in the basement, (assuming very cool but not freezing, dry) bulb plants are usually up for it:
Cannas
Colocasia esculenta (elephant ear)
Gladiolus
Amaryllis