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What is a Cold Frame Greenhouse? Everything You Need to Know

What is a Cold Frame Greenhouse? Everything You Need to Know

A cold-frame greenhouse is an invaluable tool for any gardener looking to extend their growing season. This mini-greenhouse without a bottom allows you to grow plants in a protected environment, sheltered from external factors.

In this comprehensive guide, we'll cover everything you need to know about cold-frame greenhouses, from what they are to how to build and use them effectively. You'll learn how cold frames can help you grow earlier in spring, later into fall, and even throughout the winter in some cases.

Key Takeaways

  • Cold frames are mini greenhouse structures that help extend the growing season. They shelter plants from cold weather and frost without needing heating or electricity.

  • With a cold frame, you can start planting earlier in spring, grow later into fall, and protect delicate plants year-round.

  • Sunny location, proper ventilation, and temperature monitoring are crucial to prevent overheating inside the cold frame.

  • Many types of vegetables, herbs, flowers, and seedlings like lettuce, spinach, and cabbage thrive in cold frames in cool weather.

  • Cold frames allow gardeners to customize microclimates and grow plants not normally suited for their climate zone or location.

an image of a cold frame greenhouse with plants inside

    What Exactly is a Cold Frame Greenhouse?

    A cold frame is a small, enclosed structure that acts like a mini greenhouse. It typically sits directly on the ground or on raised garden beds.

    The main components are:

    • Transparent roof - Usually made of glass, plastic sheeting, or polycarbonate panels to let in sunlight.

    • Short side walls - Typically 1 to 2 feet tall, made of wood, brick, bales of straw, or other materials.

    • Lid or roof vent - Allows heat and humidity to escape. Can be manually propped open or automated.

    Unlike a real greenhouse, cold frames lack electricity, running water, and heating systems. Their simplicity makes them an affordable option for home gardeners looking to extend the growing season.

    "Cold frame" refers to their ability to protect plants from cold temperatures. The transparent roof acts as a barrier between plants and the environment. During the day, the enclosed space warms up like a greenhouse from absorbed sunlight. At night, the lid can close to retain heat inside.

    Ventilation prevents overheating on sunny days. Most cold frame designs incorporate manual, self-regulating vents that prop open once temperatures climb too high.

    Overall, cold frames create a favorable microclimate that's 5-10°F warmer than the ambient outdoor temperature. This sheltered environment allows you to plant earlier in spring and extend your growing season into fall.

    The Many Benefits of Using a Cold Frame Greenhouse

    There are several key benefits that make cold-frame greenhouses a useful addition to any garden:

    Extending the Growing Season

    One of the primary benefits is the ability to extend your frost-free growing season. The enclosed space protects plants from cold snaps and light frosts.

    Depending on your climate zone, cold frames can add 4-8 weeks on either end of the normal gardening season. In warmer zones like 9-10, they may allow gardening year-round.

    With a cold frame, you can safely:

    • Plant seedlings or seeds weeks earlier in spring before your last expected frost date. The warmer interior lets seeds germinate and get a head start on growth.

    • Grow tender crops like lettuce, kale, carrots, and radishes further into fall and early winter. Protect them from freezing nights and harsh weather.

    • Start harvesting crops 1-2 months sooner than normal in an unprotected garden.

    Protecting Plants from Cold Weather

    In addition to frost protection, cold frames shield plants from other cold-weather conditions like snow, wind, and heavy rain.

    The enclosure keeps precipitation out and prevents delicate plants from becoming damaged, broken, or uprooted. Snow accumulation and icy winds are also blocked.

    For example, cold frames can extend the strawberry season by protecting blossoms from late spring frosts. Lettuce, spinach, and other greens will last longer into winter versus being exposed.

    Hardening Seedlings and Delicate Plants

    Before transplanting seedlings outdoors in spring, it's important to gradually acclimate or "harden" them to outside conditions. This hardening-off process toughens up the plants.

    A cold frame is ideal for:

    • Hardening off seedlings started indoors or in a greenhouse. Slowly introduce to sun, wind, and cool nights over 1-2 weeks.

    • Protecting newly transplanted seedlings as they establish roots and adjust to the garden.

    • Sheltering delicate, cold-sensitive plants that struggle with direct exposure.

    Overwintering Plants

    In cold climates, gardeners often overwinter plants by moving them indoors under grow lights or in a greenhouse. But some plants can be overwintered right in a cold frame.

    • Tender perennials like fuchsias and geraniums can survive in a cold frame with extra insulation like straw around the sides.

    • Many hardy greens like kale, chard, and some Asian greens will tolerate cold frames throughout winter in zones 5-6 or higher.

    • Dormant plants, bulbs, corms, and tubers can be buried in the soil of an unheated cold frame over winter.

    Who Can Benefit from a Cold-Frame Greenhouse?

    Cold frames are useful for several groups:

    Home Gardeners - Extend your growing and harvesting season, protect delicate plants, and experiment with new varieties. Even in milder climates, cold frames offer flexibility.

    Small Farm Growers - Get crops to market earlier and increase yields. Protect specific plants or customize conditions in sections of your fields.

    Plant Nurseries - Harden off seedlings before sale. Keep plants sheltered and avoid losses. Customize microclimates as needed.

    Urban Gardeners - Maximize limited growing space. Try plants not normally suited for your area. Keep plants warm in tiny yards surrounded by cold concrete.

    an image of a wooden cold frame greenhouse with its lid open and plants inside

    How to Use a Cold Frame Greenhouse

    Getting the most out of a cold frame depends on proper setup and maintenance. Follow these tips when you use cold frames in your garden:

    Location and Site Preparation

    Choose a sunny spot in your garden that receives at least 6 hours of direct sun daily, especially in winter. Avoid shade from buildings, trees, and shrubs.

    For freestanding cold frames, level the ground if needed so the structure sits securely. Position the enclosure with the long side facing south to capture more light.

    For in-ground frames, dig trenches or beds for the walls to nestle into. Amend the soil with compost for drainage and nutrition.

    Building a DIY Cold Frame

    Many options are available for purchase, but you can also build a custom cold frame from simple materials:

    • Use cinder blocks, straw bales, bricks, or boards for short side walls, about 12-24 inches high.

    • For the top, look for old windows at salvage shops. Plexiglass, rigid plastic sheets, and corrugated polycarbonate panels also work. Attach hinges to prop open.

    • Make sure the lid allows airflow when propped open at least a few inches. Include automatic openers if possible.

    • Face the structure south and angle the top toward the south by about 10-15 degrees for more direct sun exposure.

    • Insulate sides with straw bales, bags of leaves, or other materials if keeping plants through winter.

    Using and Maintaining the Cold Frame

    Once built and in place, use these tips to maintain ideal conditions inside a cold frame:

    • Ventilate properly on sunny days - Open lids, ends, or vents to prevent overheating.

    • Water when needed to keep the soil consistently moist but not saturated.

    • Monitor temperatures daily and add or remove insulation as needed.

    • Adjust manual vents and lids based on weather conditions. Close tightly at night.

    • Remove dead leaves or plants to prevent disease and pests.

    • On warm days in winter, open vents for an hour to prevent dormant plants from prematurely breaking dormancy.

    a cold frame greenhouse with its lid open and lettuces planted inside

      What to Grow in Your Cold-Frame Greenhouse

      One of the most common cold frame gardening questions is what can I grow in it? Let's look at suitable crops for each season:

      Early Spring (Feb. – April)

      • Cold-hardy greens and vegetables - Lettuce, spinach, Swiss chard, kale, carrots, beets, onions, radishes

      • Herbs like parsley, cilantro, dill

      • Early annuals and flower seedlings are to be transplanted later

      Late Spring (April – May)

      • Warm weather crops after your last expected frost - Tomatoes, peppers, cucumbers, melons, squash, eggplant

      • Annual flower and vegetable seedlings to harden off

      • Basil, dill, and cilantro for continual harvest

      Autumn

      • Second crop of lettuce, spinach, kale, Asian greens, carrots, beets, radish

      • Hardy herbs like parsley, sage, thyme, cilantro

      • Cover crops like fava beans, clover, vetch

      Winter

      • With added insulation, try growing microgreens, spinach, lettuce, kale, Swiss chard, Asian greens

      • Parsley, thyme, oregano, dill, chervil

      • Garlic, shallots, and onion sets planted in the fall

      • Certain hardy perennials if properly insulated

      Here is a table summarizing some of the best options for plants in a cold frame by season:

      Early Spring Late Spring Autumn Winter
      Lettuce Tomatoes Lettuce Microgreens
      Spinach Peppers Spinach Spinach
      Swiss Chard Cucumbers Kale Lettuce
      Kale Melons Asian Greens Kale
      Carrots Squash Carrots Swiss Chard
      Beets Eggplant Beets Asian Greens
      Onions Basil Radish Parsley
      Radishes Dill Parsley Thyme
      Parsley Cilantro Sage Oregano
      Cilantro Annual flowers Thyme Dill
      Dill Vegetable seedlings Cilantro Chervil
      Flower seedlings Cover crops Garlic/Onions
      Hardy perennials

      As you can see, cold frames allow you to grow a wide variety of plants. Focus on cold hardy, frost-tolerant vegetables and herbs along with flowers and transplants.

      an image of wooden cold frame greenhouse with its lid closed and plants inside

      Designs and Styles of Cold Frame Greenhouses

      Cold frames come in many different designs, styles, and sizes to suit your needs:

      Freestanding vs In-Ground Styles

      • Freestanding - Structure sits on an existing garden bed or surface. More portable and movable.

      • In-Ground - Side walls sink into a dug-out trench in the ground. More permanent but better insulated.

      Materials

      • Wood - Cedar, redwood, pine, etc. 

      • Plastic - Polyethylene, plexiglass, corrugated panels. 

      • Glass - Old windows, glass panels.

      • Straw Bales or Bricks 

      When selecting materials for your cold frame, consider this comparison of the pros and cons of different options

      Material Pros Cons
      Wood Durable, sturdy High maintenance
      Plastic Cheap, easy to find Less durable
      Glass Maximum light Can break, expensive
      Straw bales Cheap, plentiful Poor insulation, temporary
      Bricks Readily available Heavy, less insulation

      Ventilation

      • Manual - Hinged lid, swinging windows, or propped open end. Requires daily monitoring.

      • Self-Regulating Vents - Uses springs, pistons, or sensors to open vents at set temperature. More convenient.

      • Fans - Electric fan systems for forced air ventilation.

      Size

      • Mini (2-4 square feet) - For starting seeds and small plants. Easy to move.

      • Medium (8-25 square feet) - For hardening off seedlings and growing greens.

      • Large/Walk-in (100+ square feet) - Higher cost but allows growing larger plants.

      an image of a cold frame greenhouse with a woman tending to the plants

        Tips for Getting the Most Out of Your Cold Frame

        Follow these tips throughout the seasons for maximum success:

        • During winter, insulate the cold frame with straw, leaves, or blankets to retain more warmth.

        • On sunny winter days, prop lids open for an hour to provide fresh air and prevent premature sprouting.

        • In spring and fall, ventilate the cold frame when temps exceed 80°F inside to avoid overheating.

        • To conserve moisture, add a layer of compost or mulch on the soil surface inside the frame.

        • Monitor for pests like aphids and diseases like damping off. Remove any infected plants immediately.

        • Prevent weeds from taking over open soil by using landscape fabric or a thick mulch layer.

        • Check soil moisture every few days and water when the top few inches become dry. Prioritize consistency.

        • In summer, shade cold frames with shade cloth if temps will exceed 90°F inside.

        Conclusion

        As you can see, cold-frame greenhouses are useful structures that can extend the growing season by weeks or even months in many climates. With so many designs and customizable options, there are cold frames suitable for most garden settings and budgets. Whether purchased ready-made or built from DIY plans, a cold frame is a relatively simple way to boost your gardening productivity. Follow the above tips and your cold frame will produce healthy plants and bountiful harvests. Just be sure to match plants to your particular climate and conditions.

        FAQs

        How can a cold frame be used to extend the growing season?

        A cold frame can be used to grow cool-season crops like lettuce, spinach, and root vegetables a few weeks earlier in spring and later into fall. The enclosed space protects tender seedlings and plants from cold nights and light frosts, allowing you to plant before your last frost date. Cold frames extend the season by trapping solar energy during the day to create a warmer microclimate inside.

        What is the best way to keep your plants warm in a cold frame during cold nights? 

        To keep your plants warm inside a cold frame during cold nights, make sure to close the lid or vent tightly. You can add an extra layer of insulation like blankets, burlap sacks, or straw around the sides. Avoid old glass windows which don’t insulate as well. Monitoring temperatures and making adjustments helps maintain ideal growing conditions inside.

        How do you build a cold frame to grow plants?

        Cold frames are typically built low to the ground, either sunk into garden beds or built on top of raised beds. Use materials like wood, plastic sheeting, straw bales, or bricks for the sides. Make sure the lid or roof is transparent to let in light (glass or plastic sheeting). Building the back taller than the front helps maximize light exposure and solar heating. Face the structure south or southeast to get the most sun.

        Can a cold frame purchased from a garden store be used all year? 

        A cold frame purchased from a garden store can't typically be used year-round unless you add heating and insulation. Cold frames are commonly used in spring and fall because they lack the warmth and protection to sustain tropical plants through frigid winters. However, they can be used for overwintering dormant plants and hardy greens that tolerate near-freezing temperatures at night.

        What is the best way to grow a garden in a cold frame?

        The best way to grow a garden in a cold frame is to plant cool-season vegetables that can tolerate cooler nighttime temperatures in the 40s or 50s F, such as lettuce, kale, spinach, radishes, carrots, beets, and Brussels sprouts. Make sure to ventilate the cold frame during the day to prevent overheating. Add insulation like straw around the sides to retain heat at night when temperatures dip below freezing.

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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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