Overwintering Plants in a Greenhouse: The Complete Guide

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A greenhouse turns winter from a killing season into a holding season. Geraniums you would otherwise compost, dahlias you would otherwise rebuy, potted figs and lemons that cannot survive a hard freeze outdoors: all of them can ride out the cold months under glass or polycarbonate and come back in spring bigger and cheaper than anything at the garden center. But overwintering is not the same as growing. The goal from November to March is survival with minimal stress, and the rules for temperature, water, light, and pests change completely once you stop pushing plants for growth.

This guide covers the whole process for a hobby greenhouse: how cold you can safely run the structure, how to group plants by what they need, how to water a plant that is barely using water, and how to stop the pests and molds that love a closed, humid winter greenhouse. It is written for greenhouse-specific overwintering. If you are leaving perennials in the ground outdoors, that is a different job with different rules.

Why Overwinter in a Greenhouse at All

The economics are simple. A tender perennial that you carry through winter becomes next year's stock plant, a source of cuttings, and a larger, earlier-flowering specimen than a fresh nursery start. A zonal geranium overwintered as a whole plant can give you a dozen cuttings in late winter. A dahlia clump multiplies underground, so the tubers you store in October become more plants in May. A potted fig or citrus that would die in a hard freeze outdoors can live for decades if it gets a protected winter every year.

The greenhouse offers something a garage or basement cannot: light. Many tender plants overwinter far better when they keep some foliage and photosynthesize slowly through the dark months. A basement works for fully dormant material like dahlia tubers or a leafless fig, but evergreen plants such as citrus, rosemary, and geraniums held in the dark tend to drop leaves, stretch, and weaken. The greenhouse gives them daylight and shelter at the same time.

There is also the extension factor. A greenhouse that is only frost-protected in winter still warms up sooner in spring than the open garden, so overwintered plants restart growth weeks earlier than anything planted outside. That head start is the real payoff.

Temperature: The Number That Decides Everything

Before you move a single pot, decide what minimum temperature your greenhouse will hold through the coldest nights. Every other choice flows from that number, and it determines both which plants you can keep and how much heat you will pay for.

Most hobby growers land in one of three bands:

Treat these as the ranges commonly used by growers and extension services, not guarantees for a specific plant. Individual species and cultivars vary, and the right floor also depends on how dry you keep the roots. A dry, dormant plant tolerates colder conditions than the same plant with soggy soil. When a plant's tag or its breeder gives a minimum temperature, trust that over any general chart.

For most people overwintering a mixed collection, frost-free is the sweet spot. It protects the majority of common tender perennials, and each degree you avoid heating saves real money. Heating costs climb steeply as you raise the target. Holding a small greenhouse at a frost-free minimum instead of a cool-house minimum can cut winter heating use dramatically, especially in a single-glazed structure.

Sort Your Plants Into Three Groups

Overwintering fails most often because every plant gets the same treatment. Instead, divide the collection by how each plant wants to spend winter.

Group 1: Dormant and dry

These plants shut down completely. They lose their leaves or die back to storage organs and need little or no light. Dahlia tubers, canna rhizomes, tuberous begonias, gladiolus corms, and deciduous potted figs belong here. The greenhouse is useful for them only if it stays frost-free. Otherwise a cool basement or insulated garage is often better, because a sunny greenhouse can warm up on bright winter days and trigger premature sprouting.

Dahlia tubers are typically lifted after the first frost blackens the foliage, cleaned, dried for a few days, and stored in barely damp peat, wood shavings, or vermiculite somewhere cool and frost-free. Check them monthly. Shriveling means too dry; soft or moldy spots mean too wet. Cut out rot as soon as you see it.

Group 2: Semi-dormant, some light

These plants keep a few leaves and tick over slowly. Zonal and regal geraniums (pelargoniums), fuchsias, salvias, rosemary, lavender in pots, and many tender herbs fit here. They want the brightest spot you can give them, cool temperatures that keep growth slow, and very little water. The aim is to keep them alive without encouraging soft, stretched growth that the low winter light cannot support.

A common approach with geraniums is to cut them back by half to two-thirds in fall, pot them into slightly smaller containers or leave them in their pots, and keep them on the dry side in a bright, cool spot. They will look sparse by February. That is fine. They are not meant to look good in winter; they are meant to be alive and ready to root cuttings in spring.

Group 3: Evergreen and still working

Citrus, bay laurel, olive, and some succulents keep full foliage and continue to photosynthesize, just slowly. They need the most light, the most stable temperatures, and careful watering because their leaves are still transpiring. Citrus in particular tends to react badly to sudden changes. Big temperature swings or letting the root ball dry out completely are common triggers for leaf drop.

Put Group 3 plants in the best-lit position, away from cold glass on frosty nights. Leaves pressed against single-glazed panels can freeze even when the air a foot away is safe.

Getting Plants Ready in Fall

Timing matters more than most people expect. Move plants in before the first frost, not after it. Even a light frost can damage tender tissue enough to open the door to rot later in winter.

Inspect before you move

Every plant that comes into the greenhouse brings its passengers with it. Aphids, whitefly, mealybugs, spider mites, and scale insects that were kept in check outdoors by predators and weather can explode in a warm, sheltered greenhouse. Before moving a plant inside:

• Check leaf undersides, stem joints, and the soil surface with a hand lens.
• Hose foliage off hard to knock down soft-bodied pests, then let it dry.
• Remove dead leaves, spent flowers, and any fallen debris on the soil surface.
• Quarantine anything suspicious on a separate bench for a week or two.

Cut back and tidy

Most tender perennials benefit from a fall trim. Less foliage means less demand on roots, less hiding space for pests, and better airflow around the plant. Cut soft growth back to firm stems. Take cuttings at the same time if you want insurance: rooted cuttings of geraniums, fuchsias, and salvias take up far less space than parent plants and often overwinter more reliably.

Clean the greenhouse first

Fall is the time to wash glazing so winter light gets in, clear out old compost and plant debris, and wipe down benches. Algae and grime on glass or polycarbonate can cost a meaningful share of already-scarce winter light. A clean greenhouse also gives overwintering pests fewer places to hide.

Winter Watering: Less Than You Think

Overwatering kills more overwintered plants than cold does. In winter, low light, cool temperatures, and short days slow a plant's water use to a trickle. Soil that stays wet for weeks at cool temperatures invites root rot, fungus gnats, and crown rot.

• Water in the morning so foliage and the soil surface dry before the cold night.
• Water only when the pot is genuinely dry an inch or two down. For many dormant plants that may be every few weeks, not every few days.
• Lift pots to judge weight. A dry pot is noticeably lighter. After a few weeks you will know by feel.
• Use room-temperature water, not ice-cold water straight from an outdoor tap. Store a few jugs inside the greenhouse so they warm to air temperature.
• Keep water off leaves and crowns of plants prone to rot, such as geraniums and succulents.

Evergreen citrus is the main exception to the dry rule. It still needs steady, moderate moisture. Let the top of the mix dry slightly between waterings, but do not let the whole root ball go bone dry.

Light and Day Length

In much of the northern United States, winter days drop below ten hours for weeks at a time. Growers call this stretch the Persephone period, a term popularized by Eliot Coleman for the time when growth slows to almost nothing regardless of temperature. Overwintering plants live through it rather than grow through it.

You do not need grow lights to overwinter most tender perennials. Good natural light through clean glazing is enough to keep Group 2 and Group 3 plants alive. Where supplemental light earns its place is with evergreen plants that tend to drop leaves in the dark, such as citrus, and in greenhouses shaded by trees or buildings. A modest LED fixture on a timer extending the day for a few hours is usually plenty. You are preventing decline, not driving production.

Arrange plants so the ones that need light most sit at the south-facing side and at bench height, with dormant material under benches or at the darker end. Rotate pots every couple of weeks so plants do not lean toward the glass.

Air, Humidity, and Condensation

A closed winter greenhouse becomes damp fast. Cold glazing condenses moisture, water drips onto foliage, and still air lets botrytis (gray mold) take hold. Botrytis is the signature winter disease of the overwintering greenhouse. It shows as fuzzy gray growth on dead leaves, spent flowers, and damaged stems, and it spreads to healthy tissue from there.

Three habits keep it in check:

1. Move air constantly. A small circulation fan running whenever the heater is on keeps air from stagnating and helps leaves dry.
2. Ventilate on mild days. When the midday temperature allows, crack a vent or door for an hour or two to swap damp air for drier outside air.
3. Remove dead material every week. Pick off yellowing leaves and fallen petals before mold can colonize them.

Bubble-wrap insulation on the inside of glazing reduces heat loss but also increases condensation behind it and cuts light. If you use it, choose horticultural bubble wrap made for greenhouses, leave the south roof as clear as practical, and keep circulation fans running.

Winter Pests and Diseases

Pest activity slows in cool greenhouses but rarely stops. The ones to watch through winter:

Rodents deserve special mention. A warm greenhouse full of stored dahlia tubers, bulbs, and seeds is exactly the shelter mice and voles look for when temperatures drop. Store tubers in closed containers, keep seed packets in sealed tins, and set traps before you see damage rather than after. Our Best Greenhouse Rodent Control guide covers exclusion and trapping in detail.

Gear That Makes Overwintering Easier

You can overwinter with almost nothing beyond a frost-free structure. These tools earn their place when budgets allow:

• A thermostatically controlled heater sized for your structure, ideally with a thermostat you can set to a low frost-free point. Electric fan heaters give even heat without combustion moisture.
• A backup plan for power cuts. Winter storms and heater failures kill more collections than cold snaps alone. See our Greenhouse Winter Power Outage Plan.
• A wireless temperature sensor with alerts, so you know about a failed heater at 2 a.m. instead of at breakfast.
• Frost cloth or horticultural fleece to drape over vulnerable plants on the coldest nights for a few extra degrees of protection.
• Heat mats under a propagation tray for late-winter cuttings.

Waking Plants Up in Spring

As days lengthen past the Persephone period, plants start growing again whether you want them to or not. Guide the transition rather than letting it happen randomly.

• Increase water gradually as new growth appears. Growth first, then water, never the reverse.
• Start feeding lightly once plants are actively growing, with a dilute balanced fertilizer.
• Repot or top-dress plants that have filled their containers.
• Take cuttings from geraniums, fuchsias, and salvias as soon as new shoots are long enough.
• Start dahlia tubers in pots in the greenhouse several weeks before your last frost for earlier flowers.
• Harden off everything before it moves outside. Plants that spent winter under cover are tender to wind and full sun.

Citrus and figs can go back outdoors once nights are reliably above their comfortable minimums. Move them to a sheltered, partly shaded spot for the first week or two before full sun.

If the overwintering bug bites, the natural next step is using the same greenhouse to grow food through the cold months. Our Winter Lettuce & Greens in a Greenhouse guide covers varieties and timing for the Persephone period, and Heated Greenhouse vs Indoor Grow Lights compares the cost of heating a greenhouse against running lights indoors.