Cold Frame Gardening: How It Works, Pros/Cons & Planting Tips

As a dedicated gardener, do you know the disappointment that comes with the end of the growing season? Are you considering building a cold frame?

Cold frames are easy to use and allow you to continue growing into the cooler seasons.

While they provide gentle protection to the plants during fluctuations in weather, they do not supply any heat on their own. 

What is a cold frame? A cold frame is a structure built to keep the ambient temperature higher for cold-season crops or dormant tropical varieties. Best built with a southern positioning and translucent top, it provides frost protection and uses solar energy to raise the ambient temperature and keep the heat inside.

While it may sound complicated, using a cold frame is simple and comes with many positive attributes!

Read on to understand why having a cold frame is beneficial, all the basics of using a cold frame, and the best planting tips for you to succeed in growing during those bumper seasons!

What Is a Cold Frame Used For?

A cold frame can be used to extend the growing seasons on either side of the spring and summer months by 2 or 3 months.

It captures the sun’s thermal energy, which then warms the air in the box and heats up the contained growing environment. 

How Does a Cold Frame Work?

A cold frame will have a fully transparent or semi-transparent roof made of plastic, plexiglass, or glass that allows sunlight into the growing box.

The sunlight warms the air, and the frame keeps the warm air within the closed box, thus raising the temperature of the growing environment. 

How Effective Are Cold Frames?

Cold frames are very effective at capturing sunlight to heat your crops and soil to a growable temperature.

While it’s great to use for growing in the bumper seasons of the calendar, cold frames have some minor downsides to consider as well. 

Cold Frame Benefits

  • A cold frame doesn’t require much space.
  • It can be utilized to grow in every season. 
  • You can grow crops year-round. 
  • You can store dormant tropical varieties.

Cold Frame Disadvantages

  • May require an inconvenient location in your yard to fully gather sunlight effectively. 
  • Immovable from the ground. 
  • There is no heating element as with a hotbed.

Does a Cold Frame Have a Bottom?

Most cold frames are built without any wood or structural bottom.

However, some people find it beneficial to put one or two layers of landscaping fabric down before your topsoil or potting mix to prevent roots and weeds from growing up into your planter. 

Cold Frame vs. Greenhouse

A cold frame is different than a greenhouse because cold frames are built low to the ground and are not able to be walked into.

A cold frame works in the same way a greenhouse does, keeping the heat and moisture in; it’s just not as large.

What Is a Cold Frame Greenhouse?

A cold-frame greenhouse is similar to a regular cold frame, but all of the sides are made of translucent material.

Most cold frames have wood sides and a transparent top, but a cold-frame greenhouse is made of the same translucent material on the sides and top. 

What Is a Cold Frame for Seeds?

Most seeds require higher heat and moisture to germinate.

A cold frame for seeds is a great way to help germinate your seeds earlier in the season than regular in-ground plantings and for general germination throughout the season. 

How Big Should a Cold Frame Be?

A cold frame can be any size you want; however, you want to make sure you can access the garden to care for it regularly.

It will also require a door (the top), and you do not want that to be too large or cumbersome to open. A good size would be about 3 feet by 2 feet or 3 feet by 6 feet with two doors. 

What Is the Best Position for a Cold Frame?

To ensure that your cold frame maximizes its ability to warm the interior air and soil, place it in a southern-facing location.

Also, be sure to place it away from any structures or trees that will shade the planter.

Does a Cold Frame Need To Be in the Sun?

A cold frame doesn’t have to be in the sun since it will still offer protection to the plants no matter where it is placed.

However, to utilize your cold frame to its greatest ability, I recommend placing it where it can receive full or at least partial direct sunlight throughout the day. 

Why Are Cold Frames Slanted?

Slanting the roof of the cold frame allows it to be positioned in the strong sunlight. Angling the roof toward the sun as it crosses the horizon will allow direct sunlight to penetrate and warm the area. 

How Warm Does a Cold Frame Get?

Since the cold frame relies on sunlight alone, you can expect it to be approximately 5 to 15 degrees warmer than the exterior temperatures and even more in the summer.

Keep the temperature between 50 and 75 degrees Fahrenheit for the best growing conditions. 

Can a Cold Frame Get Too Hot?

Since the sun can heat up your cold frame extremely well, it can also create too hot of an environment. Be sure to vent your cold frame a few inches on warm days, or you’ll end up frying your crops!

Three cold frames made out of brick with the tops removed for ventilation.

Does a Cold Frame Protect From Frost?

A cold frame can protect from frost because it is a mostly closed environment.

Frost will only accumulate on leaves and surfaces that are freezing, and this box will keep the ambient temperature high enough to prohibit frost from affecting your crops.

Can You Use a Cold Frame During Winter?

With a cold frame, some gardeners are fully able to grow in winter with snow on the ground!

If you establish your plants before the chill sets in or you are growing an overwintering variety, you can still grow in your cold frame during the winter months. 

How Cold Is Too Cold for a Cold Frame?

There is no right or wrong answer when it comes to a temperature that is too cold for a cold frame.

If the soil isn’t completely frozen, then you should be able to continue growing. Try cool-season crops like kale, beets, broccoli, cauliflower, and most other greens. 

Does a Cold Frame Need To Be Airtight?

You’ll want ventilation for your cold frame, so it shouldn’t be airtight.

It is good to secure the openings, but if it is airtight, your plants will suffocate and there is a higher probability of fungal diseases and viruses. 

How To Keep a Cold Frame Warm at Night

If the temperatures are dropping more quickly than expected, you may need to add additional heating to your cold box.

The quickest way to heat your cold box is by filling 2-liter bottles or other containers with hot water and placing them in the box.

Alternatively, you can use a thermal light to heat the interior of the frame and then wrap it in burlap, hay, and a tarp to insulate it. 

What To Grow in a Cold Frame?

You will have limited possibilities when growing in your cold frame, but you’ll gain more versatility than growing outside in the ground alone!

Grow carrots, lettuces, spinach, bok choy, kale, arugula, beets, radishes, chard, scallions, and parsley in your cold frame for best results. 

Cold Frame Planting Dates

In the spring, you’ll be able to plant into your cold boxes as soon as the soil is workable. This can be as early as February but may not be until late March or April when the ground becomes soft enough. 

You’ll want to have the majority of your vegetables seeded by August and September to ensure they grow to a large enough size and are established before the cold temperatures arrive. 

How To Make a Cold Frame

Cold frames are simple to build, and everyone in colder climates should have one!

Begin by deciding what materials you would like to use to build your box. There are kits available online, but you can also easily make yours in a weekend using the guide below. 

The frame can be made of wood, plastic, or metal while the top needs to be translucent and can be made of a window, pane of glass, or plastic.

You can also make all of the sides translucent to allow more light to penetrate inside. 

Steps to Build a Cold Frame

  1. Build a frame the size of your translucent top. Most people create a cold frame that is 2 feet by 3 feet or a 3-foot square. 
  2. You’ll want to build a frame with an angle so the top can fully gather sunlight. A 20- to 30-degree angle would be perfect.
  3. Dig the frame into the ground for stability, or build it completely above ground. 
  4. Affix hinges to the lid so the translucent top can functionally open and close. 

Conclusion 

Every gardener becomes disappointed once the fall and winter come, taking away their ability to grow with the ground freezing.

Luckily you can extend your growing season with a cold frame! Try this easy project to help keep your backyard bounty growing while the weather is less than favorable.