kitchen garden how-to
Published October 30, 2023 by Nicole Burke

How to Make a DIY Cold Frame or Mini Greenhouse

Filed Under:
cold season
cold climates
frost
frost protection
cool season
winter garden
how to make your own mini greenhouse

Cold Frames Can Extend Your Growing Season If You're Gardening in a Cold Climate

Cold frames, also called mini greenhouses, are a great way to protect your favorite cool season and cold season plants from snow, sleet, and ice if you live in a colder climate.

These 5-sided structures will sit on top of your raised-bed garden during the colder months of the year. The roof is angled so that snow and rain can slide off instead of collecting on top. If you face the angled roof toward the South, you can also capture heat from the sun during the day so that your plants stay nice and cozy inside.

Using a cold frame allows you to extend your growing season in the fall and winter. That means you can take more harvests from plants that like cool weather during the colder months. Once you reach a period where you have below-freezing temps around the clock, your plants will die back or go dormant for the winter, even inside a mini greenhouse. Once warmer weather arrives, those plants that were protected inside the structure will bounce back much faster than those that were left out in the cold. That means a head start on your spring growing season!

You'll also notice that your soil in your raised beds will be workable and warm much sooner. You can even use your cold frames to harden off cool season seedlings in the spring.

If you'd like to garden later into the fall and earlier in the spring, cold frames are a great investment. You can find done-for-you cold frames online, or you can follow the 10 steps below to build your own cold frame. We'll be following along with Gardenary-certified consultant Kelly Crotty as she constructs a cold frame for the raised bed pictured below.

let's build a cold frame to protect this raised garden bed from frost and snow

DIY Cold Frame Supply List

Materials to Build Your Own Cold Frame

These are the materials to put on your shopping list:

  • 2" x 2" wood pieces
  • clear plastic sheeting
  • a pack of 2-1/2" decking screws for frame
  • a pack of 1-1/4" decking screws for sheeting
  • 2 hinges per frame
  • 2 handels per frame (optional)
materials needed for DIY mini greenhouse

Wood

The 2" x 2" wood pieces will create the frame of your structure. Kelly is using 2" x 2" x 8' pieces of pressure-treated pine to create her cold frame. Do a little research to figure out what's the best wood option for your area. Note that 2" x 2" pieces are actually 1-1/4 inches thick. We recommend checking out wooden balusters, which are made for railings. These 2" x 2" by 36" pieces (again, 2" is really 1-1/4") are great for shorter cuts.

Plastic Sheeting

Kelly is using corrugated plastic sheeting to create the sides of her cold frame. Plastic sheeting can be found in the roofing section of your local hardware store. You can also buy glass or plexiglass and have them cut to your specifications. If you'd like to upcycle old windows, just know that the dimensions of the glass will obviously play a big part in determining the measurements of your cold frame.

You might see some cold frames constructed with wood sheeting around the 4 sides. You can do that if you'd prefer as long as you have a clear material on the roof to let in sunlight.

Hinges and Handles

Hinges and handles allow your roof to be easily opened and closed. Make sure that both the hinges and handles you buy are galvanized so that they won't rust.

materials needed for DIY cold frame

Tools and Supplies to Build DIY Cold Frame

Here's what you'll need:

  • tape measure
  • saw
  • drill
  • impact driver (optional)
  • large clamps
  • utility knife or razor
  • a pencil and permanent marker
  • safety goggles
  • gloves
  • right angle ruler
  • Dremel with plastic cutting wheel
  • wood glue
  • Caps and Cracks insulating spray foam sealant
  • work table


Notes:

We recommend using a miter saw if you have one because you'll be making several angled cuts.

The drill is used to create pre-drilled holes, and the impact driver is useful for driving in the screws.

The clamps are especially useful if you're constructing your frame by yourself and need an extra "hand".

Cutting plastic creates a pretty big mess, so keep that in mind when deciding where you'll be building your cold frame.

It's nice to have a work table so that you can easily adjust the frame as you're working. You'll also be drilling screws on the bottom of the frame; positioning the frame to hang over the side of the table makes drilling underneath easier without flipping over the entire frame.

tools and supplies to build cold frame

Steps to Build Your Own Cold Frame

Step One: Measure and Gather Supplies

If your raised bed has top trim like the Gardenary Signature Raised Bed does, then your cold frame will sit right on top of this trim during the colder months. Otherwise, it'll sit inside the edge of the raised bed, right on the soil. Measure accordingly.

Measure the length and width of the raised bed you'll be placing the cold frame over. If your bed is fairly long (over 8 feet), you might consider building multiple frames to connect together across the bed.

Kelly's bed is 104 inches long and 34 inches wide. Because her bed is over 8 feet long, Kelly decided to build two 52-inch-long cold frames to cover the 104-inch-long space.

Write down your measurements and tape that piece of paper to your workstation near your saw for easy reference. Remember to always measure twice, cut once. We'll be doing some measurements as you go along so that you don't have to do serious math here. Don't hesitate to walk over to your raised bed and re-assess or double check as often as you need while you're working.

Here's Kelly's constructed frame with the terms we'll be using to refer to each piece.

how to build a cold frame

Step Two: Build the Base of the Structure

All right, folks, put your safety goggles and work gloves on. Let's get started!

Begin by cutting the short horizontal base pieces to the width of your raised bed. Use your pencil to mark the wood where you'll be cutting. Cut the two pieces side by side at the same time so that your cuts are exact.

Measure, mark, and cut two pieces to be the long horizontal base pieces.

DIY cold frame how to

Begin assembling the base. Whenever you're attaching a piece, apply wood glue to one end. Pre-drill your holes with the drill and attach wood pieces using the 2-1/2" decking screws and the impact driver.

how to build cold frame

Make sure to double check your measurements to ensure you have the right pieces going on the outside of the base.

Continue around to form the 4-sided foundation of the frame.

how to build cold frame for winter garden

Step Three: Add Angled Vertical Pieces

Adjust your miter saw to a 32° angle to make angled cuts for the four angled vertical pieces.

Measure the two short vertical pieces based on the minimum height of the cold frame. Mark the wood with your pencil.

Cut two short pieces side by side on that 32° angle.

Repeat for long angled vertical pieces, which should be, as their name states, longer than the front pieces. How much longer will depend on how wide your raised bed is.

Pre-drill holes on the bottom of your frame to attach vertical pieces from below. Assemble four vertical pieces using wood glue and 2-inch screws.

Make sure the angled cuts are all facing the same way (that will be the front).

cold frame DIY

Step Four: Finish Top of Frame

Measure the inside of the long horizontal base piece. This is your measurement for the long horizontal top pieces.

Adjust miter saw back to 0° and make cuts for top horizontal pieces. Cut the 2 boards at the same time.

Attach the long horizontal top pieces, making sure to line the pieces up with the bottom of the angled cut, not the top.

how to build cold frame

Measure the inside of the short horizontal base piece. This is your measurement for the short horizontal top pieces.

Make cuts for short horizontal top pieces. Cut the 2 boards at the same time.

Attach the short horizontal top pieces, lining them up with the top of the short angled vertical piece. These pieces will go partway up the long angled vertical pieces.

cold frame directions

Step Five: Add Vertical Supports

Measure the internal distance from the base to the top in the back so you know how long your back vertical support piece should be. Cut one piece to fit.

Find the halfway point to attach this vertical support piece. Mark the center with your pencil. Attach the back vertical support.

step-by-step directions to build cold frame

Measure the interior distance between the bottom and top horizontal pieces in the front. This is how long your front vertical support piece should be. Cut the piece to fit.

Find the halfway point to attach this vertical support piece. Mark the center with your pencil. Attach.

instructions to build cold frame for winter

Measure the internal distance between the top and bottom short horizontal pieces. Cut a piece of wood to fit.

Find the halfway point between the two angled vertical pieces and mark with your pencil. Attach the side vertical support.

Repeat for the other side.

step by steps to build cold frame

Step Six: Build the Roof

It's time to measure for the roof that will be attached to your structure with hinges. Measure from the top of the long angled vertical piece to the short, back to front. Add 3 to 4 inches to this measurement to allow for an overhang in the front. This will allow you to lift the lid up and down with or without a handle.

Then measure the inside width of the short angled vertical pieces in the front.

DIY winter frost protection for plants

Cut two pieces at a time for assembly of the roof. The 36" baluster pieces ended up being the perfect length for the short sides of Kelly's roof.

Move the base out of the way so that you can assemble the roof on your workstation. Assemble the roof the same way you assembled the base.

cold frame for frost protection

Once the outer sides of the roof are assembled, measure the interior distance from front to back for a middle support piece. Find the halfway points on the top and bottom of the roof. Mark those center points with your pencil. Attach the internal support piece.

Your cold frame structure is now complete! Wait to attach the roof for the time being.

DIY cold frame build

Step Seven: Add Plastic Sheeting to the Roof

It's time to add the plastic sheeting, starting with the roof.

Most plastic sheeting is 26 inches wide by 8 feet long. Your sheeting can always be wider than your roof. In fact, having a little overhang will help snow and rain run off.

Kelly only needed to cut her plastic sheeting to be the proper length for her cold frame roof. She cut two equal sections for the roof.

Measure out the length to be cut and mark with your Sharpie. Clamp the sheeting to the work table to prevent movement while you're cutting. Use the Dremel to cut the sheeting along the measured line. Lay out the cut piece on the frame.

Measure and cut the second roof piece. Lay out the second piece on the frame and line it up with the first piece. They should overlap in the center a bit. Play around with the pieces to determine how they line up best.

how to cut plastic sheeting for DIY cold frame

We've found it's best to pre-drill holes in the wood to try and avoid breaking the plastic sheeting. This is especially important if it's cold outside during construction. If the plastic does break, you can patch it with spray foam later.

Pre-drill holes all along the outside of the top frame and along the support piece. Attach the sheeting using 1-1/4 inch screws.

Move the roof off to the side.

diy cold frame

Step Eight: Add Plastic Sheeting to the Main Frame

Now you're going to add the sheeting around the sides of your main frame. Start with the back. Measure sheeting, mark the length needed to cover the back of the frame, cut, and attach.

Measure and cut a piece of sheeting for one of the sides of the frame. You can cut the top of this sheet on an angle to match the slope of the roof, or you can leave the top as is. It's a little more difficult to cut the sheeting on that angle, so Kelly leaves her plastic as is. It really depends on your aesthetic preference. Just make sure your sheeting is as tall as the long angled vertical pieces so that you don't have two gaping triangles on either side of your cold frame letting warm air escape.

Attach.

Flip your frame over and repeat on the other side if you're only building one cold frame. If you'll be piecing together two frames, leave this side blank so that it will fit up against your second frame. (Make sure to mirror this construction when you build your next frame.)

Measure the length of the sheeting needed to cover the front of the frame. This piece you'll need to cut along the top so that it doesn't stick up above the frame. Cut and attach.

DIY mini greenhouse for garden

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Step Nine: Attach Roof with Hinges

Line the roof up on top of the frame.

You'll attach two hinges to the back (the taller side) so that the lid can open and close. Use a Sharpie to mark where you'll drill holes for the hinges. Pre-drill the holes. Attach the hinges using the included screws.

Attach handles in the front if you're using them.

You're so close to the finish line!

how to build cold frame with easy open lid

Step Ten: Fill in Gaps and Edges

Use the foam spray to seal the ridges created by the waves in the plastic sheeting, plus all of the edges and corners. You can also use the spray to repair any breakages in the sheeting caused by drilling. Kelly recommends propping your cold frame up on cinder blocks in your yard so that you don't make a mess on your work table or concrete when the foam oozes out of crevices. This foam doesn't look the best, but it's what insulates all those open spaces. Your goal is for your cold frame to be as airtight as possible.

The foam needs to cure overnight. You can use a razor blade or utility knife the next day to clean up the edges if you'd like.

how to make airtight cold frame

Celebrate!

You did it! You now have a cold frame customized to the exact dimensions of your raised garden bed.

Here are a couple other things to consider.

Since the plants inside your cold frame or mini greenhouse are covered, you will have to supplemental water every now and then. Drip irrigation is a great idea to provide water to your plants until they go dormant (which is also when you'll want to put up your hoses for the winter).

Here you can see the drip irrigation keeping Kelly's celery and brassica plants happy. Note how she nestled two cold frames side by side in the middle of her raised bed.

diy cold frame

Make sure you also have a way to prop your cold frame or little greenhouse open to allow air flow on warmer days (over 45°F). Your cold frame will keep the air around your plants a good 10° warmer than the outside air, so ventilation on warmer days is important to avoid scorching your plants. Keep a couple pieces of wood left over from your cold frame construction to use as props.

Watch your weather carefully and adjust accordingly.

DIY cold frame for gardening in cold climates

Enjoy a Longer Growing Season This Winter!

I hope you found this step-by-step guide to building a customized cold frame useful. Thanks so much to Kelly for providing how-to tips and directions. (You can learn more about Kelly's Cleveland-based garden consulting business here.)

Now your plants can stay snugger than a bug in a rug all winter long.

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How to Make a DIY Cold Frame or Mini Greenhouse