Overwintering Potted Plants: What Stays Out and What Comes In

The same hydrangea that sails through winter in a garden bed can die in a pot three feet away. That is the whole puzzle of overwintering potted plants, and the answer has almost nothing to do with the plant. It is the container. A pot leaves roots exposed to air temperature on every side, with no surrounding soil to buffer them.

Once you understand that, the decisions get simple. Here is the rule that tells you what can stay outside, what needs a garage, what has to come in, and the one winter mistake that kills more potted plants than cold ever does.

Why a plant in a pot is less hardy than the same plant in the ground

In the ground, roots sit inside a huge mass of soil that holds heat and changes temperature slowly. Deep soil may stay above freezing all winter even when the air is well below it.

A pot has none of that. Three things go wrong.

• The soil volume is tiny. A 14 inch pot holds maybe a cubic foot of mix. It freezes right through to the center, roots and all.
• Cold comes from every direction. Sides, top and often the bottom too if the pot sits on a deck or a patio rather than on soil.
• Freeze and thaw cycles do the real damage. Soil that freezes solid, thaws in afternoon sun, then freezes again heaves roots, tears fine root hairs and splits the pot. Steady cold is far kinder than a swinging thermometer.

So the question is never just how cold your winter gets. It is how cold the root ball gets, and how many times it goes back and forth.

The two zone rule: your shortcut for every pot

Gardeners and extension sources repeat one guideline for containers, and it is the most useful thing on this page. A potted perennial has the best chance of overwintering outdoors if it is rated hardy to one or two USDA zones colder than where you garden.

Put plainly: the pot costs you about two zones of hardiness.

• You garden in zone 6. A plant rated to zone 4 is a good bet in a pot outdoors. A plant rated to zone 5 is borderline and worth protecting.
• A plant rated hardy only to zone 6, your own zone, will probably not survive the winter in a container without shelter.
• You garden in zone 8. Plants rated to zone 6 are safe in pots. Zone 8 plants need help.

Check your zone, then check the tag on the plant, then subtract. That single comparison sorts most of your patio in about ten minutes.

Sort every pot into one of four groups

Group 1: leave it where it is

Hardy two or more zones colder than you, in a large frost proof pot, planted in a spot that does not bake in winter sun. Dwarf conifers, hardy ornamental grasses and tough shrubs in big containers often sit here. Raise the pot on feet so the drainage holes stay clear, water it well before the ground freezes, and walk away.

Group 2: stays outside with protection

Hardy about one zone colder than you, or hardy enough but in a smaller or more fragile pot. These stay out but get moved and wrapped. Details in the protection section below.

Group 3: unheated garage, shed or basement

Hardy plants that go dormant but whose pots will not survive your winter outdoors, plus borderline woody plants like figs and young fruit trees. You want a space that stays cold but not deep freezing, roughly 30 to 40 degrees F. Dark is fine, because a dormant deciduous plant is not using light. Move them in once they have dropped their leaves and gone dormant, not in early fall while they are still growing.

Group 4: comes indoors as a houseplant

Tender tropicals that never go dormant. These come in while nights are still in the 50s, before a cold shock, and live in the brightest window you have. Check for pests before they cross the threshold, because aphids and spider mites explode in dry indoor air.

Timing matters here, and it runs on the same calendar as the rest of the autumn garden. Our guide to what to do before first frost covers what has to happen in which order.

Pot material: what actually survives a freeze

Even when the plant is hardy enough, the container may not be. Water soaks into porous material, freezes, expands and splits it.

• Terracotta and unglazed earthenware. The most likely to crack. Porous by design. Empty and store these unless they are specifically sold as frost proof.
• Glazed ceramic. Often cracks too. The glaze seals the outside, but water still gets in through the unglazed rim and base.
• Plastic and resin. Survive freezing well. Thin plastic can turn brittle and crack if knocked in deep cold, so handle gently.
• Fiberglass and fiber cement. Generally reliable outdoors and the reason they cost more.
• Concrete. Usually fine, though cheap cast concrete can flake at the surface over several winters.
• Metal. The pot survives but conducts cold straight to the roots. Poor insulation.
• Fabric grow bags. They will not crack, but the root ball is almost completely exposed. Only for very hardy plants or move them into shelter.

One easy win regardless of material: get every pot up off the ground on pot feet or a couple of bricks. A drainage hole frozen shut against a patio turns the pot into a bucket, and waterlogged roots in a freeze are a common way to lose a plant you thought was hardy.

Pot size: why about 12 inches is the dividing line

Soil volume buys you time. A large container freezes from the outside in and often keeps a core of unfrozen soil through a cold snap. A small one freezes solid in a night.

As a practical cutoff, pots under about 12 inches across are high risk outdoors in any zone with a real winter, and anything in the 18 to 24 inch range or larger has a genuine advantage. If you have a choice about which pot a borderline plant spends winter in, choose the biggest one.

How to protect pots that stay outside

The goal is not to keep the pot warm. It is to keep the temperature steady and stop the freeze and thaw swing.

• Move pots against the house. A north or east facing wall is better than south. It sounds backwards, but a south wall bakes the pot every sunny afternoon and refreezes it every night, which is the exact cycle you are trying to avoid.
• Group them tightly. Put the toughest plants and biggest pots on the outside of the cluster and the vulnerable ones in the middle. Each pot insulates its neighbors.
• Insulate the sides, not the plant. This is the part people get backwards. Wrapping the branches in burlap does very little. Packing fallen leaves, straw or bagged mulch around and between the pots is what protects the roots.
• Mulch the soil surface with a few inches of bark or leaves. Our notes on mulch depth cover how much is useful and when more becomes a problem.
• Water deeply before the ground freezes. Moist soil holds heat better than dry soil and a dehydrated root ball is far more vulnerable. Soggy is not the goal; evenly moist is.
• Skip the plastic sheeting. It traps moisture, cooks plants on mild days and encourages rot. Burlap, leaves and straw breathe.

The mistake that kills more pots than cold: forgetting to water

Plants moved into an unheated garage or shed are out of the rain. Dormant does not mean dead, and roots still lose moisture all winter. A pot left in a garage from November to March with no water will often come out as a dry stick, and the owner blames the cold.

Check sheltered pots roughly once a month. If the top couple of inches are dry and the soil is not frozen, give them enough water to moisten the root ball and no more. Pots wintering outdoors under snow or rain usually look after themselves, but check them during a long dry spell with no snow cover.

Which tender plants are actually worth saving

Be honest about the effort. Some plants repay it and some are cheaper to replace in spring.

• Worth the trouble. Rosemary, bay, citrus, olive, geraniums, fuchsia, brugmansia, agapanthus, and anything you paid real money for or grew from a cutting that matters to you. Dahlia tubers, canna rhizomes and elephant ear bulbs store easily in a box of dry medium in a cool dark place.
• Usually not worth it. Petunias, impatiens, marigolds, coleus and the rest of the summer annual crowd. They are bred to grow fast for one season.
• The fall mum trap. The potted mums sold as autumn decor are often florist types rather than hardy garden mums, and even hardy ones bought in October rarely have time to root in before winter. Treat them as seasonal decoration and feel no guilt.

A simple autumn order of operations

1. Early fall, before frost. Bring tender tropicals indoors while nights are still in the 50s. Inspect for pests first.
2. Around first frost. Pull annuals and empty their pots. Store terracotta and glazed containers somewhere dry. Finish the rest of the fall garden cleanup.
3. After leaves drop and plants go dormant. Move group 3 pots into the garage or shed. Water them in.
4. Before the ground freezes. Water every pot staying outside, group them against a sheltered wall, mulch and pack leaves around the sides. This is also the window for planting garlic if that is on your list.
5. Monthly through winter. Check sheltered pots for dryness. Water sparingly when the soil is not frozen.
6. Early spring. Move pots back out gradually once hard freezes are done. Do not rush new growth into a late frost.

Frequently asked questions

Do all potted plants need to come inside for winter?

No. Use the two zone rule. If a plant is rated hardy to one or two zones colder than your own, it has a good chance outdoors in a container, especially in a large frost proof pot. Only tender tropicals and plants rated no colder than your own zone really need shelter.

What temperature is too cold for potted plants?

It depends entirely on the plant’s hardiness rating rather than one universal number. For tender tropicals, nights consistently below about 50 degrees F are the signal to bring them in. For dormant hardy plants in a garage, a space that holds roughly 30 to 40 degrees F is close to ideal.

Should you water potted plants in winter?

Yes, sparingly. Pots in a garage or shed get no rain and will dry out and die of drought, which is probably the most common cause of winter losses. Check about once a month and water lightly when the soil is dry and not frozen. Pots outdoors in rain or snow usually need nothing.

Will terracotta pots crack if I leave them outside?

Very likely, unless the pot is specifically sold as frost proof. Terracotta is porous, so water soaks into the clay, freezes and expands. Empty them, clean them and store them somewhere dry over winter, and use plastic, fiberglass or concrete for anything that has to stay out.

Can potted mums survive the winter?

Often not. Many mums sold in autumn are florist varieties rather than hardy garden mums, and even hardy ones bought in October have little time to establish roots before the ground freezes. If you want to try, plant them in the ground rather than leaving them in the nursery pot, water them in, and mulch well. Treat survival as a bonus.

The short version

A pot costs a plant about two hardiness zones. Check the tag against your zone, sort every container into leave it, protect it, shelter it or bring it in, store the terracotta, raise the rest off the ground, and remember that the pots in your garage still need a drink once a month. That is overwintering potted plants in one paragraph.