What Is A Cold Frame? (Plus 3 Key Things To Know)


Are you one of those plant owners that gets antsy for the growing season towards the end of every winter? If so, owning a cold frame could indulge your spring fever and let you get started on seeds even earlier. 

A cold frame is a small outdoor structure with a clear lid that protects plants from the elements while allowing sunlight to pass through. They are used to grow seeds, overwinter plants, and extend the growing season for certain crops.

While cold frames shield plants from harsh weather, they can get hot and humid if you don’t take the necessary precautions. This article will talk about the specific uses of cold frames, and how to use them properly. 

What Is A Cold Frame?

Cold frames are small boxes that are built low to the ground, without bottoms, and have transparent roofs. These structures are often built as a small greenhouse extension – usually along the southern wall. They are used to shelter plants in the garden from wind and cold weather. 

cold frame
A cold frame is a small box, built low to the ground with transparent glass or plastic roofs to keep seeds and plants warm.

Cold frames are typically made from wood, but you can use any material that is weather-proof, readily available, and cost-effective for you. Hardwood is ideal since it is more resistant to water damage and generally lasts longer than softwood. 

The top (cover) of the box should be made of sturdy transparent material, like glass or plastic. Old storm windows are commonly used for cold frames if you’d like to repurpose something you already have.  The thicker the material, the more insulation your plants will receive. It’s a good idea to add hinges so that you can easily open the frame to add, remove, or ventilate plants. 

What Is The Purpose Of A Cold Frame?

Cold frames have several different purposes:

To Extend The Growing Season

Cold frames stay warmer than the air surrounding them and provide insulation for plants, so you can grow things in cooler temperatures than the plants can typically tolerate. 

cold frame 2
A cold frame extends the growing season by allowing you to start seeds earlier in the year.

To Harden Off Seedlings And Transplants

When you start seeds inside, you must gradually introduce them to outdoor sun, wind, and temperature fluctuation. Usually, gardeners do this by bringing them outside every day for increasingly longer periods, until they are ready to live outside full-time. This process is known as “hardening off”. 

Alternatively, you can sow your seeds directly into a cold frame and transplant them outside when the seedlings have matured. Or, if you want to wait until the summer is over to transplant them, you can keep the cover open once it’s warm enough. 

cold frame 3
You can also use a cold frame to help you harden off seedlings and transplants to prepare them for life outdoors.

To Provide Shelter For Winter-Hardy Plants 

Cold frames are a great way to provide additional shelter for plants such as vegetables or herbs, even if they can technically survive outside independently. The added protection of the structure gives plant owners peace of mind knowing their plants won’t be blown over or subject to drastic temperature changes. 

What Are The Benefits Of A Cold Frame?

Gardeners who have cold frames on their property see many advantages of them, including: 

Protection From Early/Late Frosts

Any outdoor gardener is familiar with the nerve-wracking, unpredictable weather at the beginning of autumn and towards the end of winter. The Farmer’s Almanac and local weather services can give us their best prediction of the first and last frost dates each year, but Mother Nature doesn’t always play by the rules. 

frost
A cold frame provides protection against late spring frost and early fall frost.

Cold frames protect annuals and tender perennials in early fall from succumbing to an unexpected frost. In late spring, they can protect newly planted annuals or seedlings you accidentally planted too early. 

Gives You More Time & Larger Yields

For gardeners that grow plants for food or profit, cold frames are one of the best ways to make the most of the growing season. Since you don’t have to worry about off-season freezes, you can stretch the growing season to the maximum. 

Protection From Pests

Cold frames are an eco-friendly way to protect plants from a gardener’s worst adversaries. If the cover is closed and adequately sealed, critters can’t munch on what’s inside.

aphids
A cold frame can provide protection against common garden pests (such as aphids) when plants are still young.

Controlled Soil

Using an enclosed structure that you add soil to allows you complete control over the soil, its nutrients, and how much water it receives. This is particularly beneficial for regulating the nutrients your young, sickly, or tender plants receive. 

What Do You Put In A Cold Frame?

Cold frames are multi-purpose structures and can house many different types of plants. Here are some of the most common examples:

Seedlings

A cold frame is an excellent alternative if you don’t have the setup or space to start your seedlings indoors shortly before spring starts. You can start your warm season crops in a cold frame no more than six weeks early using a cold frame, depending on the weather in your area. 

tomato seedling
You can take a tray of seedlings that you started indoors and put it into a cold frame when weather permits.

Once the weather is consistently warm, the danger of frost has passed, and the seedlings are strong enough to withstand wind and rain, you can acclimate them further. Do this by increasing the amount of time you have the lid open each day until the top is open all the time. Finally, transplant the seedlings directly outside before the temperature outside is consistently over 50 degrees Fahrenheit. 

If you want to keep the seedlings in the cold frame for longer, be sure to open the cover when the temperatures rise, or the plants will struggle.  

Cold Season Vegetables 

Vegetables like lettuce, broccoli, and carrots are good candidates for cold frames. For best results, plant your vegetables in late summer or early fall to give them a chance to mature before the weather is consistently cold. 

lettuce seedling
Lettuce likes cool weather and it won’t grow too tall (until it bolts in warm weather), making it a good candidate for cold frames.

Overwintering Plants

Depending on where you live, there are usually plants (edible or ornamental) that are on the cusp of being cold-hardy in your area. You might be able to keep them perenially if you overwinter them in a cold frame. Your local cooperative extension office may be able to advise you on plants to overwinter in the structure. 

Do You Water Plants In A Cold Frame?

You will still need to water your plants if they are in a cold frame, but doing so requires more attention to detail than those that are planted directly in the garden. The amount of water your plants need in a cold frame depends on the following:  

  • Certain plants prefer more water than others. Ferns, for example, need consistently moist soil to thrive.  On the other hand, desert plants like cacti like their soil to dry out before watering. 
  • Cold frames in dry, hot weather dry out faster. When humidity levels are higher, plants don’t require watering as frequently. 
  • Plants that receive a lot of sun during the day are more likely to dry out quicker. Check the soil moisture of cold frames in sunny areas daily.
  • If you’re simply storing plants in a cold frame to keep them alive during the winter, they won’t require as much water. Plants not actively growing during the cold months are dormant, meaning their water and nutrient needs are significantly reduced. 
watering can
You still need to water plants in a cold frame. Although the cold frame may retain moisture, the plants will still use water as they grow.

Do Cold Frames Need Ventilation?

Ventilation is essential for a successful cold frame. Since these structures are often used when the weather fluctuates drastically, you’ll need to decide whether to open or close the frame’s lid depending on the weather. Many seasoned gardeners recommend opening the top when the temperature is 40 degrees Fahrenheit or higher during the day. A removable cover is also useful for hardening off seedlings. 

cold frame 2
Make sure that you have some sort of ventilation for your cold frame to prevent it from overheating.

Failing to provide ventilation could burn your plant when the weather is warm or the sun is intense, and can invite diseases and pests associated with excess humidity. To automate the task of opening and closing the lid, you can purchase a vent opener to raise and lower the cover based on the temperature. 

What Is The Difference Between A Cold Frame & A Greenhouse?

It’s easy to confuse a cold frame with a greenhouse since the structures have similar features and purposes. But there are several key differences:

Size

Cold frames are historically smaller than greenhouse since they are used for different purposes. Greenhouses are designed to hold more plants and are generally big enough to walk inside and fit shelves or racks full of plants. 

cold frame
A cold frame is typically much smaller than a greenhouse.

Temperature

Greenhouses usually have wooden or metal frames only, and the rest is made with glass or plastic. Since more sunlight is allowed in, greenhouses are naturally warmer than cold frames. Since they are larger, there’s also the option to use a heater during colder seasons. 

greenhouse wedding
A greenhouse often has metal as part or all of its frame.

Air Quality

Due to the increased space in a greenhouse, gardeners have the ability to control the climate within the structure. This includes the use of a heater, air purifier, humidifier, or dehumidifier, for example. 

Conversely, cold frames are subject to the environment created by the structure, including moisture-related issues from increased humidity. This is not usually an issue since cold frames are mostly used temporarily in cold weather.

Conclusion

Cold frames are an inexpensive, practical solution to many roadblocks you might run into while gardening. Whether you’re a professional gardener or a casual hobbyist, a cold frame will come in handy at some point. 

You can learn more about garden cloches (which also help to keep small plants warm) here.

You can learn about how to keep a greenhouse warm in winter here.

About the author:
Kathryn is a plant enthusiast and freelance content writer who specializes in home and garden topics. Based in New York, you can get in touch with Kathryn at https://kathrynflegal.journoportfolio.com/.

Kathryn F.

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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