Extend Your Growing Season with Cold Frames & Cloches

Gardening doesn't have to stop when the first frost hits, or wait until late spring. You can add weeks, even months, to your growing season using simple, effective tools: cold frames and cloches. These structures create mini-greenhouses, shielding plants from harsh weather and giving them a head start. It's a smart way to get more out of your garden, whether you're starting seeds early or harvesting late into the fall.

Quick answer: Cold frames and cloches extend your growing season by creating a warmer microclimate around plants, protecting them from frost, wind, and pests. Cold frames are larger, stationary boxes perfect for hardening off seedlings or growing cool-season crops. Cloches are smaller, portable covers for individual plants, ideal for quick frost protection. Using them can add 6-8 weeks to your productive gardening time, letting you harvest longer and start earlier.

Understanding Cold Frames

A cold frame is basically, a bottomless box with a transparent lid, typically angled to catch maximum sunlight. It's a centuries-old gardening trick, still effective today for a variety of tasks. You'll find cold frames made from wood, metal, or even recycled materials like old windows. They're usually placed directly on the ground, allowing plants to root into the garden soil while benefiting from the protected environment above.

What Cold Frames Do for Your Garden

Cold frames serve multiple purposes that extend your gardening season significantly. First, they're perfect for hardening off seedlings. This process slowly acclimates young plants (started indoors, perhaps under best grow lights for seed starting) to outdoor conditions, reducing transplant shock. You'll move seedlings into the cold frame for a few hours each day, gradually increasing their exposure over 7-14 days. This makes a big difference in their survival rate once planted in the main garden.

Second, cold frames let you start cool-season crops much earlier in the spring. Think lettuce, spinach, radishes, and peas. You can sow these seeds 4-6 weeks before your last expected frost date, getting a jump on the season. A study by the University of Maine Extension reported that cold frames can raise soil temperatures by 10-15°F (6-8°C), making early planting possible. They also extend the harvest of these crops into late fall, sometimes even through mild winters.

Choosing and Building a Cold Frame

You've got options when it comes to cold frames. You can buy pre-made kits, which typically cost between $80 and $200, or build one yourself. DIY cold frames are often more cost-effective. For a basic wooden cold frame measuring 4 feet by 2 feet, you'll need about $50-$75 worth of materials, mainly lumber and a polycarbonate sheet or old window. The key is to ensure the lid is transparent and allows sunlight through.

Placement matters. Choose a spot that gets full sun, ideally facing south, to maximize heat retention. You'll also want to ensure good drainage. Proper ventilation is also a must; on sunny days, temperatures inside can quickly climb above 90°F (32°C), scorching your plants. Prop open the lid slightly to let excess heat escape, especially if it's over 40°F (4°C) outside.

Utilizing Cloches for Individual Plant Protection

Cloches are smaller, often portable covers designed to protect individual plants or small groupings. The word "cloche" comes from the French word for "bell," reflecting their traditional bell-like shape. They're quicker to deploy and more versatile for targeted protection than cold frames. Many gardeners find them indispensable for protecting tender plants from unexpected frost snaps or early season pests.

Benefits of Using Cloches

Cloches offer immediate, localized protection. If an unseasonable frost is predicted, you can quickly cover your most vulnerable plants, like newly transplanted tomatoes or peppers. They create a warm, humid microclimate that helps young plants establish themselves faster. This can boost early growth by 15-20%, according to some studies, because the soil stays warmer, stimulating root development.

Beyond frost, cloches act as a physical barrier against pests. Slugs, snails, and even some birds can be deterred by a simple cloche, keeping your young seedlings safe until they're strong enough to fend for themselves. They also shield plants from strong winds, which can desiccate tender leaves and stunt growth.

Types of Cloches and How to Use Them

You can find a wide array of cloches, from glass bells to plastic tunnels. Glass cloches are beautiful and effective, but they're heavy and fragile, often costing $20-$40 each. Plastic cloches are lighter, more durable, and generally cheaper, around $5-$15 per unit. You can also make DIY cloches from repurposed plastic bottles, milk jugs, or clear plastic containers. These cost next to nothing and work surprisingly well for short-term protection.

To use a cloche, simply place it over the plant you want to protect, pushing it slightly into the soil to prevent wind from blowing it away. Remember to remove cloches during the day once temperatures rise above 50°F (10°C) to prevent overheating and ensure good air circulation. Just like cold frames, proper ventilation is key to avoiding fungal diseases. You'll also want to monitor the soil moisture, as cloches can reduce evaporation, meaning you might not need to water as frequently.

Integrating Cold Frames and Cloches into Your Garden Plan

Combining cold frames and cloches offers a layered approach to season extension. You'll use cold frames for larger-scale seedling hardening or growing entire rows of cool-season crops. Cloches, on the other hand, become your go-to for targeted protection of individual plants, especially those that are more sensitive or were planted later. Think of the cold frame as your mini-nursery and the cloches as your plant bodyguards.

For instance, you might start broccoli and cabbage seedlings in a cold frame in early spring. Once they're hardened off, you'll transplant them into the garden. If an unexpected late frost is forecast, you can quickly cover the young broccoli plants with individual cloches. This strategy ensures your plants get the best possible start and protection throughout unpredictable weather periods.

Consider where you'll place these structures. Cold frames are typically semi-permanent, so choose a sunny, well-drained spot that's easily accessible. Cloches are more flexible; keep them near your garden beds for quick deployment. This integrated approach can dramatically increase your garden's productivity and resilience. You'll be harvesting fresh greens and root vegetables weeks before your neighbors.

Sources

University of Maine Extension. "Cold Frames for Season Extension." Published 2023.
The National Gardening Association. "Extending the Season with Cloches." Accessed 2025.
Oregon State University Extension Service. "Hardening Off Transplants." Published 2024.

FAQ

What materials are best for building a DIY cold frame?

You'll want durable materials for a DIY cold frame. Treated lumber (like cedar or redwood) for the frame and polycarbonate panels or old windows for the lid work well. These materials resist rot and provide good insulation. You can typically find lumber for $30-$50 and panels for $20-$40 at most hardware stores.

How do cold frames protect plants from frost?

Cold frames protect plants by trapping solar heat during the day and insulating them at night. The enclosed space creates a microclimate that can be 5-10°F (3-6°C) warmer than the outside air. This prevents frost damage, especially when temperatures drop below freezing, helping tender seedlings survive.

Can cloches be used for more than just frost protection?

Yes, cloches offer more than frost protection. They shield young plants from strong winds, heavy rain, and even some pests like slugs and snails. They also help warm the soil, encouraging faster germination and early growth for vegetables like lettuce and spinach.

How often should I ventilate my cold frame?

You should ventilate your cold frame almost daily when outdoor temperatures are above 40°F (4°C). Prop the lid open a few inches to allow excess heat and moisture to escape. On very sunny days, even if it's cool, internal temperatures can quickly rise to over 90°F (32°C), which can cook your plants. Close the lid completely before sunset to trap warmth for the night. This prevents overheating and reduces the risk of fungal diseases.

What's the ideal size for a backyard cold frame?

A common and very practical size for a backyard cold frame is about 4 feet long by 2 to 3 feet wide. This size is large enough to hold multiple trays of seedlings or a small row of cool-season vegetables like spinach or radishes. It's also manageable to build and move if needed, and typically fits within a budget of $75-$150 for materials.