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How to Heat a Greenhouse: The Complete Guide

How to Heat a Greenhouse: The Complete Guide

For gardeners in cold climates, keeping a greenhouse warm throughout the winter months is essential for growing plants throughout the year. However, effectively heating a greenhouse can be a challenge, especially when relying on non-electric heating methods.

This comprehensive guide will provide greenhouse owners with strategies and solutions for maintaining ideal temperatures during cold weather. We'll cover everything from passive solar techniques to old-fashioned compost piles and hotbeds. 

Key Takeaways

  • Prevent heat loss by insulating walls, sealing cracks, and adding polycarbonate panels or double-pane glass to improve glazing insulation.

  • Orient your greenhouse to maximize winter sun exposure for passive solar heating. Keep the interior glazing clear.

  • Hot compost piles inside the greenhouse can raise temperatures by 10-20°F through heat released during decomposition.

  • Affordable electric convection heaters offer spot heating while hydronic systems evenly distribute warmth through hot water pipes.
an image of a seedling tray with a germination mat underneath

    Understanding Heat Loss in a Greenhouse

    Before deciding how to heat your greenhouse, it's important to understand the ways that heat is lost during cold weather. This will help you pinpoint the best methods for retaining warmth.

    How Greenhouses Lose Heat in Winter

    Greenhouses are prone to rapid heat loss in winter through:

    • Conduction: Direct transfer of heat through the greenhouse covering and frames via contact with the cold outside air. Polycarbonate and glass glazing conduct heat faster than materials like plastic sheeting.

    • Infiltration: Outside air penetrating inside through cracks, vents, and openings. Any breezes or drafts will pull warm air out.

    • Convection: Warm interior air rises and escapes out ridge vents or peaks in the greenhouse, drawing cooler air in from below.

    • Radiation: Heat is radiated outward through the greenhouse covering, especially at night when thermal radiation increases due to the temperature difference between interior and exterior environments.

    Ideal Greenhouse Temperature in Winter

    Maintaining an ideal temperature inside your greenhouse during winter is crucial for healthy plant growth. Most plants thrive with daytime temps between 65-75°F (18-24°C) and nighttime temps around 55-65°F (13-18°C).

    However, optimal temperatures depend on the types of plants you are growing. Tender plants like basil and tomato need consistent warmth, while hardier greens and root crops can withstand colder conditions. Getting to know your plants' preferences will help guide your heating efforts.

    an image of black barrels inside a greenhouse

    11 Ways to Heat Your Greenhouse

    Heating a greenhouse without electricity requires clever passive heating techniques along with structural enhancements to retain warmth. Here are 11 effective options for naturally raising temperatures in your greenhouse during winter.

    1. Passive Solar Heating

    One of the simplest ways to harness free heat is through passive solar gain. Orient your greenhouse to maximize winter sun exposure and install solar panels if possible. Keep the area directly inside the glazing clear to allow sunlight to penetrate deeply and be absorbed by the interior.

    2. Hot Compost Piles

    Decomposing organic matter in compost piles produces tremendous heat. Building a hot compost heap inside your greenhouse can raise interior temperatures by 10-20°F. Maintain high temperatures by turning the pile regularly to introduce more oxygen.

    Do not place piles directly touching plastic glazing, which can melt. Hot compost also contributes humidity and CO2 for plant growth.

    3. Thermal Mass and Black Barrels

    Thermal mass refers to materials that absorb and store heat energy. Installing black containers filled with water inside your greenhouse creates thermal mass to soak up heat during the day. These barrels release warmth at night after the greenhouse cools. For maximum effect, paint the barrels black to enhance solar absorption and prevent light reflection. 

    4. Heating With Animals

    Introducing small livestock into your greenhouse, like chickens or rabbits, can provide a notable boost in warmth. Animal bodies give off radiant heat while they breathe out warm carbon dioxide and humidity for your plants.

    Just a few animals can raise interior temperatures by several degrees. Be sure your greenhouse has adequate ventilation and supervision when using this method.

    5. Underground Heating Systems

    Constructing an underground heating system like a walipini to harness geothermal heat is an extremely effective DIY heating technique. Underground greenhouses or heat tunnels built into a slope use the Earth's natural warmth for heat gain. Installing intake pipes that draw hot air from compost piles or solar-heated barrels will further increase warmth.

    6. DIY Hotbeds

    Hotbeds are essentially bottom-heated garden beds, an age-old technique that is still highly effective. First, dig down about 2 feet below your greenhouse floor to create a pit bed. Fill the pit with fresh manure, straw, and other compost materials.

    Cover the bed with soil and allow the compost to heat up to roughly 80°F. Hotbeds release heat from decomposition upwards through the soil for several months, giving plants a warm bottom zone.

    7. Insulating Walls and Covers

    Preventing heat loss is the foundation for keeping a greenhouse warm naturally. Insulate any surfaces that are heat-conductive. Add polycarbonate twin-wall panels, rigid foam boarding, or double-pane glass units to improve glazing insulation.

    Seal all cracks, gaps, and openings with caulk to stop infiltration. Add thermal curtains and shade screens to provide an extra heat-retaining layer. A well-insulated greenhouse can maintain temperatures up to 15°F warmer.

    8. Wood Heat

    Wood stoves and burners have long been used to add supplemental heat to greenhouses. Small wood-burning stoves with exhaust vents provide radiant warmth that can heat a 200 sq ft greenhouse by 50-60°F on a single load of wood.

    Look for thermostatically controlled models to maintain ideal temperatures. Ensure your greenhouse has adequate ventilation for wood heat. Ash residue can also provide beneficial nutrients if mixed into beds.

    9. Candle-Heated Plant Pots

    This unusual off-grid heating tactic involves placing candles or tea lights underneath plant pots to provide gentle bottom warmth. Group pots together on trays filled with an inch of pebbles or sand, with a candle below each pot. The trapped rising heat from the candles' flames transfers through the pebbles into the pots. This kitschy technique can raise soil temperatures by 4-5 degrees.

    10. Horticultural Fleece

    Covering plants with horticultural fleece helps insulate them against cold air. The fabric traps heat given off by plants and soil to create a warmer microclimate. Use hoops or fencing to prevent the weight of snow from compressing the fleece onto plants.

    Adding horticultural fleece can raise temperatures around plants. It also protects against frost and wind damage.

    11. Germination Mats

    Heating mats made for seed starting provide targeted warmth right where plants need it most. Germination mats come in various sizes and can be placed beneath flats or pots. They provide constant mild heat around root zones when electricity is connected. Mats are available in wireless models, making them easy to reposition as needed. 

    an image of an electric heater inside a greenhouse filled with plants

    Choosing the Best Heating System for Your Greenhouse

    Along with the passive solar and non-electric DIY heating methods already covered, some additional heating systems may be necessary to maintain warmer temperatures if your climate gets quite cold. Here's an overview of some supplemental greenhouse heating options.

    Comparing Heating Options: Pros and Cons

    Electric Heaters: Affordable and widely available, but rely on electricity. Fine for small or intermittent heating.

    Wood Stoves: Provides free heat and nutrients but requires dry firewood supply and ventilation.

    Propane & Natural Gas: Efficient and consistent heat, but need fossil fuels and proper exhaust venting.

    Geothermal: Provides stable year-round temps but requires digging underground trenches.

    Hot Water: Hydronic systems evenly heat via hot water pipes but involve a somewhat complex setup.

    Composting: Releases steady gentle warmth completely free but has limited temperature control.

    Heating System Upfront Cost Operating Cost Heat Distribution Maintenance
    Electric heaters Low Moderate Spot heating Low
    Gas heaters Moderate Moderate Whole greenhouse Moderate
    Hydronic High Low Even (whole greenhouse) High
    Wood stove Low Low Zone heating High
    Compost heat Low None Gentle, gradual Low

    When selecting a greenhouse heating system, it is also important to consider the overall design and layout of your greenhouse. Referring to our greenhouse buyer's guide can provide useful tips on choosing the right style and glazing materials for your climate and heating needs.

    an image of a wall mounted electric heater inside a greenhouse

    Electric and Non-Electric Greenhouse Heating Systems

    Common Electric Heaters

    • Wall-Mounted Electric Convection Heaters - Affordable, portable forced-air units offer spot heat.

    • Electric Radiant Strip Heaters - Long heating strips mounted overhead to provide ambient warmth.

    • Electric Baseboard Convection Heaters - Give concentrated floor-level warmth.

    Common Non-Electric Heaters

    • Natural Gas Unit Heaters - Provide whole greenhouse heating via forced air or radiant tubes.

    • Propane Radiant Tube Heaters - Emit directional radiant heat from long overhead tubes.

    • Hydronic Heating Systems - Feature hot water boilers and a network of water pipes warming the soil and air.

    • Wood Stove Heaters - Offers zone heating from single wood-burning stoves.

    When selecting a greenhouse heating system, it is also important to consider the overall design and layout of your greenhouse. Referring to a greenhouse buyer's guide can provide useful tips on choosing the right style and glazing materials for your climate and heating needs.

    Key Considerations When Selecting a Greenhouse Heater

    • Output and heating capacity matched to your greenhouse size

    • Budget and fuel source

    • Desired methods of heat delivery like convection or radiant

    • Automatic thermostats and appropriate safety features

    • Placement and arrangement needed for even whole-house heating

    Ensure any greenhouse heating system you install is properly sized and located to create uniform temperatures throughout your growing space.

    Conclusion

    Heating a greenhouse without electricity may seem daunting, but is absolutely achievable through thoughtful passive design and strategic insulation paired with time-tested solar techniques, thermal mass, and compost heat. Prioritize preventing heat loss above all when planning your greenhouse heating. With this comprehensive set of proven options, you can enjoy growing lush plants year-round even in cold winter climates.

    FAQs About Heating a Greenhouse

    How does bubble wrap help in heating greenhouses?

    Bubble wrapping is a great way to heat a greenhouse in winter as it acts as an insulator. When used within the greenhouse, it keeps the heat inside and prevents the heat from escaping, thereby maintaining a consistent temperature inside the greenhouse on cold winter days.

    What are some tips for heating a greenhouse without electricity during the winter?

    To heat a greenhouse without using electricity during winter, you can make use of passive solar techniques, use items like bubble wrap to insulate and hold the heat inside the greenhouse, and you can also build a compost pile within the greenhouse which breaks down and generates a lot of heat naturally.

    How to use compost to heat a greenhouse?

    Composting materials like grass clippings, leaves, and kitchen waste can help heat a greenhouse. As the compost breaks down, it generates a significant amount of heat. This heat can be used to warm up your greenhouse naturally and create a conducive environment for your plants, especially during the winter days.

    Can you grow vegetables in an unheated greenhouse in winter?

    Yes, many hardy root vegetables and leafy greens can survive with a bit of protection in an unheated or minimally heated greenhouse throughout winter. Insulating the soil, bedding plants together, and using cloches or fleece are key.

    What’s the best way to heat a small greenhouse?

    The best way to heat a small greenhouse would depend on your specific conditions. However, using a combination of passive solar heating, insulation, and generating heat with a compost heap can be quite effective while being cost-efficient. These methods help provide a steady source of heat for your small greenhouse.

    Previous article When Can I Put Plants in an Unheated Greenhouse?
    Next article How to Winterize a Greenhouse: The Complete Step-by-Step Guide

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    About The Author

    Andy Wu - Resident Expert

    Andy Wu - Resident Expert

    Andy Wu is the resident backyard products expert and hails from Atlanta, Georgia. His passion for crafting outdoor retreats began in 2003.

    As a fellow homeowner, he founded Backyard Oasis to provide top-quality furnishings and equipment, collaborating with leading manufacturers.

    His main focus is on sheds and generators!

    In his spare time he like to hike the tallest mountains in the world and travel with his family.

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