How to Make a DIY Cold Frame For Cold-Weather Growing

A project that may be on your to-do list and one that many of you have been asking for is how to build a DIY cold frame for cold-weather growing! Growing more food into winter without a greenhouse has been one of my goals, but winters here can get down to 0°F. 

With some ingenuity and the power of the sun, many crops can still be grown outside well into the colder months. Come along as I explain how I built an easy cold frame DIY with a poly plastic panel lid, from start to finish, over my raised garden bed. 

Winter Gardening

In the winter, I enjoy reading a stack of books to get inspiration for garden planning while sitting in my pajamas, drinking hot tea watching the world get covered in a blanket of snow. But I enjoy it even more if I can still eat fresh vegetables from my garden!

Building a DIY cold frame is just one of the methods for gardening in the winter that I have employed over the years so I can do just that. I also recommend using frost blankets for plants and winter sowing cloches to extend your growing season.

What Is a Cold Frame & How Does It Work

A cold frame is a bottomless box with a transparent, slanted lid that is placed over garden plants to protect them from cold temperatures. Much like a greenhouse, it lets sunlight in, traps heat inside, and protects plants and soil from harsh winter weather.

Cold frames require sunlight to keep warm. The soil and the air inside the cold frame are heated by the sun during the day and release the heat at night.

They can typically extend your growing season by up to 4 weeks later in the fall and up to 4 weeks earlier in the spring. Some people use localized heating from an external source, such as heat tape, to try to extend it even further.

Best Materials To Build a Cold Frame DIY

You may have seen these materials used to make a DIY cold frame:

• Wood

• Treated Lumber

• Cinder Blocks

• Strawbales

• Windows or Polycarbonate Panels

Wood

The most common, and arguably the best, material used to build the frame is wood. Wood is readily available, easy to work with, and looks attractive. Using wood gives you the flexibility to make it exactly the size and shape you want.

You can increase the lifespan of untreated wood by painting it or coating it with boiled Linseed Oil when you build it, and again each year after. Boiled Linseed Oil is all-natural and penetrates into the wood, helping to protect it. (Linseed oil also works great for restoring old gardening tools!)

Treated Lumber

Treated lumber is durable and long-lasting, but more costly. Avoid any treated wood containing creosote or other dangerous chemicals. Pro-Tip: Treated lumber from before the early 2000s was treated with copper arsenate, a dangerous chemical that contains arsenic. Modern-day treated lumber is pressure-treated with safe chemicals that shouldn’t be harmful if they leach into your soil.

Cinder Blocks

Cinder blocks are sturdy and can be long-lasting for someone living in a drier climate. They are porous, which means they will hold onto air and moisture, acting as an insulator. 

In Michigan's climate, the porous structure of cinder blocks holds onto moisture, which freezes and expands during the winter, resulting in broken cinder blocks after just one or two years. That’s why I don’t recommend using them for garden beds or cold frames in colder climates. 

Pro-Tip: Cinder blocks may leach lime into the soil, raising its pH. If you choose to use this material to build a DIY cold frame, I recommend regularly testing the soil pH and adjusting it as needed.

Strawbales

Bales of straw can be arranged to form the four sides of the cold frame; usually, a large window or glass door is placed over the top to serve as the lid. Strawbales are cheaper than other materials but usually last only one or two growing seasons, and mold can become a problem if they stay too wet.

Another disadvantage of most strawbale cold frame designs is their flat tops. Having a slanted top is very important to let in the maximum amount of heat and light! 

You don’t want the top too flat or too steep, or the sun can come in and bounce right back out. If you get the angle right, the sun will come in and bounce around inside the cold frame, warming it up. 

Windows or Polycarbonate Panels

Other materials like repurposed windows or glass doors are also popular choices for making a cold frame DIY, but they are more difficult to construct into an air-tight box and cannot be cut to size.

Polycarbonate panels can be used to construct the entire cold frame or to make the lid on a wooden cold frame.

Best Locations to Put a Cold Frame

The key component is placing your cold frame in a location that gets a lot of sun! Position it so the angled top faces the direction of maximum sun, which is usually south-facing. This should give you the longest sun exposure, but west-facing or east-facing can also work.

Supplies Needed

There are two main parts to a cold frame: the base and the angled lid. In my design, the base is a carbon copy of my raised garden beds so that it fits directly on any of my beds. 

There are so many more ways to make a DIY cold frame than just the one that I am showing you. This is just the easy, yet sturdy design that I prefer and have had success with. 

To build my 4-foot by 10-foot cold frame design, you will need the following lumber: 

• One 10-foot 2x12 Board 

• Two 10-foot 2x8 Boards 

• Three 10-foot 2x4 Boards

• One 12-foot 2x4 Board 

• One 8-foot 2x8 Board 

• One 8-foot 2x12 Board 

• One 8-foot 2x4 Board 

Here's a list of the tools and supplies you will need:

• 6 Mil Poly Film - I used a 10x25-foot plastic drop cloth in the video to make the transparent lid of my cold frame because it was very inexpensive and easy to get from a local hardware store, but you can also get 10x25 greenhouse poly film. Pro-Tip: The transparent top could also be made from a variety of materials, such as polycarbonate paneling, or repurposed glass doors and windows. 

• Scissors - You will need scissors to trim the poly film to size.

• Chalk Line - A chalk line is a quick and easy way to mark boards for cutting.

• Hammer - I used a hammer and nails to construct my cold frame, but you could also use a drill and screws.

• Framing Nails - Some 2 ⅞” 10d construction framing nails worked great for this project.

• Hinges with Screws - A 3-pack of door hinges is needed to install the lid on the cold frame.

• Drill or Screwdriver - You will need a drill with a Philips head bit or a Philips head screwdriver to install the door hinges. 

• Saw - This project can be made with a minimal amount of cuts, but you will still need a skill saw or miter saw to cut the wood.

• Tape Measure - Grab a tape measure for accuracy, and remember, measure twice to cut once!

• Staple Gun - You’ll need a heavy-duty staple gun and staples to attach the poly film to the lid.

How to Build a DIY Cold Frame Step-by-Step

Let’s get to assembling a cold frame! Pro Tip: The dimensions of this build can be easily adjusted to any size that suits your needs.

1. Measure & Cut - To build a 4x10-foot cold frame, you will need to cut the 8-foot-long 2x8 and the 8-foot-long 2x12 each in half with a saw, resulting in four 4-foot-long sections.

2. Measure & Mark Angled Cuts - Take the two 4-foot long pieces of 2x12” board. Measure 4 inches up the side of the first board, secure the chalk line there, then measure from that point to the far top corner, and snap the chalk line. It should leave a perfect straight line for you to cut along. Repeat the process on the second board. 

3. Cut Along the Chalk Line - Make the angled sides by using a saw to cut along the chalk line on each board. 

4. Construct Flat Base - Lay the two 10-foot-long 2x8” boards and the two 4-foot sections of 2x8 on a flat surface, forming a rectangle. Make sure that shorter side boards cover the butt ends of the long boards on all corners.

5. Check Square - This is a good time to verify that your rectangle is square and that each corner is at a 90° angle. A simple way to check is to use your tape measure to measure the length of the interior diagonals from corner to opposite corner. Both interior diagonals should be identical when the rectangle is square.

6. Secure the Corners With Nails - To secure a corner, pound in 3 nails through the side board and into the adjoining end of the long board (as shown in the image). Repeat for all four corners.

7. Construct the Angled Base - Lay the 10-foot-long 2x12” board, the two angle-cut 4-foot sections of 2x12, and the 10-foot-long 2x4 on a flat surface to form a rectangle. Make sure that the side boards cover the butt ends of the long boards on all corners. (Check the square before nailing the same way you did in step 5!)

8. Secure the Corners With Nails - Secure the tall corners with 4 nails each and the short corners with 2 nails each. As you did with the base, nail through the side board and into the butt end of the adjoining long board (as seen in the image).

9. Measure & Cut Stays & Lid - Measure and cut one 8-foot 2x4 into four 16-inch long sections sections. Measure and cut the other 12-foot 2x4 in thirds to create three four-foot-long 2x4s.

10. Measure Location of Stays - Place the angled base on a flat surface with the angled cut side down. Set the flat base directly on top of it. Measure 6 inches in from the corner on all long sides, and mark. This will be the outside edge of the stays.

11. Nail on the Front Stays - On the short side, align the outside edge of a 16-inch 2x4 with your mark and the butt end against the ground. Hammer four nails through the stay into the base to hold it together, two into the angled portion and two into the flat portion. (The ends of your stays should stick up 4 inches over the top.) Repeat for the other front corner.
    

12. Nail on the Longer Stays - On the back side of the cold frame, align the outside edge of a 16-inch 2x4 with your mark. The end of your stay should stick up 4 inches over the top like they did on the front. Hammer four to six nails through the stay into the base to hold it together, two to four into the angled portion and two into the flat portion. Repeat for the other back side corner.

13. Move Cold Frame to Location - Move the cold frame to the location in your garden that you want it. You can push the stays into the ground or attach them to a wooden raised bed if you built it to match your raised bed.

14. Construct the Lid - Lay two 10-foot-long 2x4 boards and the two 4-foot sections of 2x4 on a flat surface to form a rectangle that is the same size as the base. Make sure that shorter side boards cover the butt ends of the long boards on all corners and secure each corner with two nails. 

15. Measure & Cut Lid Brace - Measure the inside width of your lid across the center. Cut the third 4-foot 2x4 to fit snugly inside the lid as a brace. Secure it in place across the center of your lid with nails.

16. Attach the Lid With Hinges - Place the lid on the cold frame. On the tall side of your base, mark where each hinge will go: one in the center, and the other two should be directly above the stay on each end. Place your hinges one at a time with the hinge pin facing out, centered between the lid and the base and in line horizontally. Mark the location for each hole and pre-drill with a small bit (if you have a drill). Use the Philips-head screwdriver to tighten the screws and hold the hinges in place.

17. Staple on the Poly Film - Roll out the poly film and fold it in half once. Lay it over the lid and staple it on with your staple gun, ensuring the plastic is taut before securing it! Trim off any excess. The double layer creates an insulating air gap, and each layer of poly film adds about 3° or 4°F of frost protection. 
    

18. Seal Air Gaps - Wooden raised beds are not air-tight, and air gaps allow heat to escape during cold nights, reducing the efficiency of your cold frame. There are lots of options for sealing air gaps if you choose to, but stay away from fiberglass insulation, as it tends to shed and can get on your food.