You picked up a grape vine, maybe on an end-of-season sale, and winter arrived before you got it in the ground. It is still sitting in its nursery pot. Can it survive the cold outside? The good news: grapes are tougher than most people think. The catch: what saves them in the ground is exactly what a pot takes away.
It is the roots, not the canes
Grape canes are remarkably cold-hardy. The hardy varieties grown across Canada, like Valiant, Beta, and Frontenac, ride out prairie winters with the top growth fully exposed. Roots are a different story. Grape roots are only hardy to roughly -6 to -10C, and in the ground they never see anything close to that, because a blanket of soil and snow keeps them well insulated.
A plant in a pot loses that insulation. The roots sit at air temperature, so a potted vine left above ground can freeze its roots solid while the canes look perfectly healthy. Come spring the vine leafs out weakly or not at all. Protecting the root ball is the entire job.
Option 1: Heel it in (best in cold zones)
If your soil is not frozen yet, dig a hole in a garden bed and set the whole pot in, right up to the rim. Backfill around it and, once the vine is dormant, mound leaves or straw over the top. The soil around the pot insulates the roots the same way it would for a planted vine. If you already know where the vine will live, plant it properly now instead and mulch it well. Either way, the roots end up surrounded by earth, which is what they need.
Option 2: Cold storage
No diggable ground? Move the pot into an unheated but protected space: an attached garage, a cold cellar, or a shed that stays cold without plunging to outdoor extremes. Aim for somewhere near freezing, around 0C. Wait until the vine has dropped its leaves and gone dormant before moving it in. Check it about once a month and add a little water if the soil is completely dry, so the roots do not dehydrate over the winter. Do not bring it into a heated room. Warmth breaks dormancy, and a vine that wakes up in January has nowhere to go.
In mild regions, it is easier
On the south coast of BC and other Zone 8 areas, winters rarely threaten grape roots the way the Prairies or the interior do. There, sinking the pot into a bed and mulching over it is usually all it takes, and full cold storage is overkill. Your local frost dates are a good gauge of how hard your winter really bites.
Watch for mice
Straw and leaf mulch make cozy winter housing for mice, and they will gnaw bark at the base of the vine. Wrap the lower stem with hardware cloth or a spiral guard, or set traps near your stored plant.
Come spring
Once the hard freezes are behind you and the vine is breaking dormancy, move it back out or plant it in its final spot. A vine whose roots came through winter intact will push strong new growth and be years ahead of a replacement. The same logic applies to other tender potted plants and to vegetables you leave in the ground over winter: insulate, keep them dormant, and let the cold do the rest.
The whole strategy comes down to one idea: give the roots the insulation the pot took away, and keep the plant cold enough to stay asleep until spring.