Winter transforms a greenhouse into a vital refuge—protecting plants from frost, snow, and freezing temperatures while allowing you to continue growing through the coldest months of the year. Whether your greenhouse is unheated, minimally heated, or fully temperature-controlled, winter gardening requires a different mindset: slower growth, careful moisture management, and strategies for capturing or supplementing heat.
This page will guide you through how to prepare, manage, and use your greenhouse in winter so you can protect tender plants, grow cold-tolerant crops, and maintain a healthy environment all season long.
Understanding Winter Conditions in a Greenhouse
Even an unheated greenhouse is significantly warmer than the outside air during sunny days—but temperatures can drop rapidly at night.
Typical Winter Behaviors
- Warm, bright days: solar heat builds and plants grow slowly
- Cold nights: temperatures may approach outdoor lows
- High humidity: condensation forms on cold surfaces
- Reduced evaporation: plants dry out more slowly
- Slower metabolism: plants require less water and nutrients
Knowing how your greenhouse behaves in winter helps you plan protection and choose the right plants.
Preparing the Greenhouse for Winter
Preparation is key to keeping plants healthy through cold weather.
Check for Drafts & Heat Loss
Seal gaps around:
- Doors
- Vents
- Foundation edges
- Glazing panels
Use weatherstripping or caulking to reduce heat loss.
Clean Thoroughly
Remove:
- Dead leaves
- Old soil
- Debris under benches
- Algae from surfaces
Clean spaces reduce mold, pests, and disease in enclosed winter conditions.
Insulate Where Appropriate
Common insulation methods include:
- Bubble wrap on glazing
- Thermal screens or reflective curtains
- Insulated north walls (to retain heat)
- Mulch on greenhouse beds
Insulation helps stabilize nighttime temperatures.
Temperature Management in Winter
How you manage temperature depends on whether your greenhouse is heated or unheated.
Unheated (Cold) Greenhouses
These rely solely on solar gain and insulation.
Protection Strategies
- Use frost cloth or row covers inside the greenhouse
- Add water barrels to store thermal mass
- Group plants for shared warmth
- Double up on coverings during extreme cold
- Use raised beds or benches (cold air sinks to the floor)
Unheated greenhouses are perfect for overwintering hardy plants and growing cold-tolerant crops.
Plants That Thrive in Unheated Winter Structures
- Spinach
- Mache (corn salad)
- Arugula
- Tatsoi
- Mizuna
- Kale
- Hardy herbs (parsley, chives)
- Hardy annual flowers (poppies, calendula, sweet peas)
Growth will be slow but steady.
Minimally Heated (Cool) Greenhouses
These maintain temperatures just above freezing—often between 35–45°F.
Benefits
- Protects tender perennials
- Keeps citrus or Mediterranean plants alive
- Prevents damage to potted ornamentals
- Increases crop range
Strategies
- Use small electric heaters with thermostats
- Add thermal mass (water barrels, stone paths)
- Close vents earlier in the afternoon
- Use insulation only where needed
A lightly heated greenhouse balances energy use with plant protection.
Fully Heated (Warm) Greenhouses
These maintain stable temperatures for tropical plants or winter vegetable production.
Requirements
- High-quality insulation
- Reliable heating system
- Automatic venting
- Regular temperature monitoring
Plants Suitable for Warm Winter Conditions
- Tender perennials (geraniums, fuchsias)
- Citrus trees
- Tropical ornamentals
- Succulents and cacti (with low humidity)
- Winter tomatoes or peppers
- Year-round herbs
These systems offer the most flexibility but also require the highest energy input.
Humidity Management
High humidity is one of the biggest winter challenges in greenhouses.
Why Humidity Builds Up
- Condensation forms on cold surfaces
- Plants transpire moisture more slowly
- Less ventilation is used in cold weather
How to Reduce Humidity
- Vent on mild, midday winter days
- Space plants for better airflow
- Avoid watering late in the day
- Use fans to keep air circulating
- Remove condensation drips from overhead structures
Keeping humidity in check reduces mold, mildew, and fungal disease.
Watering in Winter
Plants need far less water in winter, but they still need careful monitoring.
Best Practices
- Water in the morning
- Let the soil surface dry slightly between waterings
- Avoid splashing foliage
- Use room-temperature water on tender plants
Overwatering is a common cause of winter plant failure.
Overwintering Plants
Greenhouses are ideal for protecting tender plants through the winter.
Plants Commonly Overwintered
- Geraniums
- Fuchsias
- Dahlias (tubers stored dry)
- Citrus
- Rosemary and other borderline-hardy herbs
- Half-hardy annuals
- Potted perennials
Overwintering Tips
- Reduce watering significantly
- Trim lightly (avoid heavy pruning in winter)
- Maintain airflow
- Protect roots with mulch or foam insulation
Overwintered plants often rebound strongly in spring.
Growing Winter Crops
Even unheated greenhouses can produce cool-season crops.
Best Crops for Winter Growing
- Spinach
- Lettuce (especially winter cultivars)
- Mache
- Mustards
- Kale
- Green onions
- Pea shoots
- Herbs like chives, parsley, and cilantro
Growth slows dramatically during low-light months, but harvests continue.
Common Winter Challenges
All greenhouse gardeners encounter winter issues. The key is preventing them early.
- Overheating in Winter Sun: Surprisingly common—open vents on sunny mornings.
- High Humidity: Use ventilation and fans to keep moisture down.
- Frost Damage on Cold Nights: Use frost cloth or supplemental heat during extreme cold.
- Mold & Mildew on Leaves: Increase airflow and remove affected foliage.
- Pest Activity: Aphids, whiteflies, and fungus gnats may remain active; inspect regularly.
Winter in the Greenhouse in Your Garden
Your greenhouse becomes an essential partner during winter—protecting plants, enabling off-season harvests, and keeping the gardening year alive even in cold weather. With thoughtful preparation, careful temperature and humidity management, and the right crops, your greenhouse can thrive despite short days and frost outside its walls. Over time, you’ll learn how your particular structure behaves in winter and how best to support your plants through the coldest months of the year.