Making a Cold Frame

With Guy Esposito MD

We have all seen greenhouses along the road in farms and nurseries. Although some gardeners build and use a version of a greenhouse, a cold frame will be enough for your home gardening purposes. Everybody in the Northeast who wants to maximize garden growing time should have a cold frame. Using two simple and inexpensive cold frame types, I am able to start my plants a few weeks earlier and keep them growing a few weeks later. This means that I am eating fresh garden lettuce and other vegetables from May to December – nine months a year. You can do this in your garden as well by using one of these inexpensive and easy to make versions of a cold frame.

Like a greenhouse, a cold frame is a transparent or clear roofed enclosure of some sort. Unlike many greenhouses, it is built low to the ground, but they are both designed to protect plants from the colder weather. The clear or transparent top admits sunlight and prevents heat from escaping, especially in the evening. For all intents and purposes, a cold frame functions as a miniature greenhouse, except it is a lot less expensive and time consuming.

I use two types of cold frames, the first being a lightweight version that is reusable. The one I have has lasted me 5 years and is still going strong. The second type of cold frame that I use is a temporary version that you install wherever you need to use it – directly on your garden or if you are protecting your seedling plants in a sunny area of your yard.

Ingredients

The Reusable Cold Frame

The reusable cold frame is a permanent structure using cedar strapping (landscape grade) for the framework and bendable plastic poles for the roof structure, all secured with screws. It is so lightweight that almost anyone can pick up the frame and move it around.

I use mine to protect the trays that we transplanted the seedlings into last week. They are allowed to rest and grow in there while they get ready for transplanting to the garden soil. Since the size of the structure is fairly large, 8 feet by 4 feet, it will hold a large number of trays. They will grow much more quickly in this environment and be fully protected from the cold or bad weather.

To make one of these reusable frames you will need three pieces of the strapping cut into 8-foot lengths, and three pieces cut into 4-foot lengths. You will also need three pieces of bendable plastic piping cut into 6 foot lengths and fifteen 1 and ½ to 2-inch wood screws to fasten it all together. Lay down two of the 8 foot pieces of strapping and connect them to both ends of the three 4-foot lengths. (Place a 4-foot piece on both ends of the 8-foot pieces and one across the middle.) After screwing these pieces together, bend and attach the three pieces of plastic piping at the junction where the pieces of strapping are attached, and screw the piping into the strapping. To finish the frame, lay the other 8-foot piece of strapping across the top of the three pieces of piping (in the center at the top of the curve) and screw the piping into the strapping. The frame will wiggle but be secure.

When you place the frame where you want it, line the entire bottom area (4 feet by 8 feet) with black plastic and anchor it with bricks. This will protect the plants from the coolness and any freezing on the ground.

Lastly, get a sheet of 3-mil clear plastic and lay it all across the frame, securing it at either end and all along the sides of the frame with more bricks. When you want to slide a tray into or out of the cold frame simply remove a brick anywhere along the sides and reach in.

When it is sunny the clear plastic lets in all of the rays and it will get very hot – very quickly. On a sunny 50 degree F., windy day it can get up to 100 degrees F. inside the cold frame. You need to watch this carefully and be prepared to do a couple of things. One is to temporarily open up one end of the plastic roof, folding it back and to the sides. (If you do this be sure to secure the plastic with the bricks.) Another option is to create some vents by making some permanent slits into the top of the plastic. Still another is to cut open some windows on one or both ends.

Instructions

Temporary Cold Frame

The temporary cold frame is a simpler structure, using 8-foot pieces of bendable wire, a clear cover and some bricks. You could use this for the same purpose as the reusable cold frame, but I use it to put directly in the garden as protection for the plants already in the ground.

Instead of using the sheets of clear plastic for a cover, I use an economical, reusable, convenient, and effective product that is called floating row cover, also known as a frost blanket. (Floating row cover is often called by the manufacturer's name: Reemay, Agronet or Argyl.) This lets in a diffuse type of light, with much less heat, but has some of the same protection from colder weather. It also will not get as warm because the material lets some of the heat back out of the cold frame. You will find that it gets the inside 3 or 4 degrees warmer and protects the inside from temperatures up to 3 or 4 degrees colder.

To make a temporary cold frame simply bend the pieces of wire into a hoop shape. (Placing each end into the ground, spacing each end of the wire about 4 feet apart, depending on how high you want the roof.) Place a new wire about every foot until you have reached your required length. You can then cover the top of the frame and secure the material with bricks, just as you would the reusable frame. It will look a little flimsy but it will work fine.

As I mentioned, you will find that both of these structures are very useful not only in the spring to get your garden started earlier but also in the fall to extend your garden’s harvest. If you try one or both of them they will become an indispensable part of your gardening experience. You can also reuse all of the materials for several years if you are careful to put them away for the winter.

As a final note, it is easier to make and install both of these cold frames with the help of another person, but as you can see in the video you can also do it yourself – even on a windy day.

Guy Esposito is an orthopedic surgeon whose other passion is vegetable gardening. From his early medical school days until the present, he has been growing a variety of vegetables in his home garden. He is the head gardener for the PBS series Ciao Italia (www.ciaoitalia.com)  which for the past twenty-one years has featured  in each season several episodes based on his garden. His garden has also been featured in print in Yankee Magazine and Northeast Flavors. He has given public seminars on the health benefits of a lifestyle based on home-grown vegetables. When not attending to his duties as Chief of Surgery of the Wentworth-Douglass Hospital in Dover, New Hampshire, he can always be found in his garden.