How do I get potted perennials through winter without losing them?

Potted perennials are far more vulnerable than the same plants in the ground, and the reason is not the air temperature but the root zone. Soil in the ground buffers cold; a pot is exposed on all sides and its root ball can drop several degrees below ambient. As a rule of thumb, a plant in a container survives about one hardiness zone less well than the same plant planted out.

Three things kill them. Repeated freeze-thaw cycles, which shear roots as ice forms and melts. Waterlogging, because a frozen or saturated pot cannot drain and roots suffocate — more potted perennials die wet than die cold. And desiccation, when frozen soil means the plant cannot take up water while wind continues to pull it out of evergreen foliage.

The fixes follow from that. Move pots together against a sheltered wall, ideally on the north or east side, where they stay uniformly cold rather than thawing every sunny afternoon. Raise them onto pot feet or bricks so meltwater drains freely. Group them and wrap the outside of the pots — not the plants — with hessian, bubble wrap or straw; you are insulating the root ball, not the top growth. For anything marginal, sink the pot into a spare bed or a heap of leaf mould for the winter.

Water sparingly but do not let pots dry completely. Evergreens in particular still transpire on mild days and a bone-dry pot in February is a common cause of spring losses.

Cut back herbaceous top growth after it has died down, but leave hollow stems and seed heads on anything that provides insect shelter until early spring — it costs nothing and matters.