Imagine a snowy day in December. You go out to the garden, uncover the cold frame, and pick some nice fresh greens for dinner. This is something I do in my USDA Zone 6 garden.
Cold Frames are a great way to extend your harvest. They’re easy and cheap to build and give a lot back for the money. Cold frames allow you to eat fresh food for a longer period of time.
A cold frame is simply a box or an enclosed space that is designed to protect plants from cold weather and wind. Typically a cold frame has no bottom, solid sides, and a glass or plastic top. There are many ways to make a cold frame so you are sure to find one that grabs you.
This article will cover a number of topics so that you will be ready and able to build and use a cold frame in your garden.
We will cover…
Benefits of Cold Frames?
Selecting A Site
Using The Cold Frame
Heat Loss From Glass
Insulating
Over Heating
Lots of examples
What Are The Benefits of Cold Frames?
Stretch The Season
Cold frames extend or stretch your normal growing season by protecting your plants from cold temperatures and dehydrating winds. A cold frame retains warmth so your plants keep growing.
Photo by Moormand
Cost
Cold frames are the cheapest garden “tool” you can build. I have built many out of scraps and reclaimed materials. Wood leftover from a project and a salvaged window are all you need. Oh, and some nails.
You can spend $30 and make a nice one with an easy lift up top or spend that on straw for a super-insulated plant place.
Selecting A Site
Face South
The top of your cold frame does best if it’s at an angle. In order to gather the most sun, the cold frame should face south. Or at least southeast.
Place your cold frame in the garden. It should be on rich soil that drains well. Situate the cold frame in a spot that will not be in your way such as a back corner. That way when you go about your garden chores or spring tilling it will not be in the way.
Can I Put A Cold Frame on My Patio?
Yes, a cold frame will do well on a patio and receive the benefits of being protected by the house. In addition, the patio boards or stones will make a nice micro-climate.
Make sure your cold frame gets adequate sun and is not shaded by the home. You may prefer to have a bottom if you are going to place it on the patio. Use exterior grade plywood or treated pine boards. Cedar boards would work well also but they do cost more.
You can also buy many styles which would look very nice by the home.
This style is great for the patio.
Using The Cold Frame
Your cold frame works as a solar collector. Even on cloudy days, it will warm up under the glass. You can use your cold frame to extend your harvest, grow salad greens in winter, and start or harden off seedlings in spring.
The soil in your cold frame should be dug deeply and amended. Work in some peat, well-aged compost, and even some seed starter if you are putting the plants in the ground. Your soil should be loose and friable. I also add some sand if I’m growing root crops.
Are you placing your cold frame over an established garden area? You may be making a nice warm spot where fungus, disease, and pests can multiply. To prevent this, allow your cold frame to heat up without venting. The goal is to get a soil temperature of 180 F and keep it there for at least an hour. This will kill most pathogens.
Heat Loss From Glass
The ideal temperature for plants inside the cold frame is 75 F during the day. There are several simple hacks you can do to moderate the temperature in the cold frame.
In late fall and early spring, the nighttime temperatures become quite cold. If you are extending your harvest during fall and winter it is best to plant frost-hardy crops. Lettuce, kale, spinach, onions, radish, and turnips are all good choices.
Keeping heat enclosed in your cold frame at night will help protect your plants. Place weather-stripping where the wood and glass windows meet. You can also cover the frame with some straw or a blanket.
Heat can also escape from the sides especially if they are wood. You can bank soil or straw up against them to block airflow.
Timing can be your friend. If the day is sunny and you are venting the cold frame, close it about 3 pm. This will allow it to heat up before the sun starts to drop. About 4 o’clock place a blanket over the cold frame to trap the heat inside. This is especially useful in spring when you are starting seedlings.
Insulating
Cold Frames for cold climates.
You may live in Zone 4 or 5 and are looking for a cold frame that will protect your plants in a colder climate. If so start by digging down two feet and at least 4x6. Place two inches of small gravel at bottom of the hole. Then use cement blocks next to each other to form a foundation.
Cover the cement blocks with a piece of two-inch Styrofoam insulation. Set your wooden frame on top of the insulation. You will want your cold frame to sit down in the earth at least six inches some stain or paint the boards to protect from moisture.
Over Heating
The temperature under the glass can heat up surprisingly high. On a sunny day where the temperature is 0 degrees Fahrenheit outside the temperature inside the cold frame could reach 100 F. This temperature extreme is not good for your plants.
Having a way to vent your cold frame is important. This may be as simple as lifting the window and putting a block of wood between the window and the frame to allow for ventilation. You can also use a garden stake to prop the window open.
If you are away from home during the day you may want to invest in a thermostatically controlled vent lever.
An automatic vent lever helps a lot if you work away from home
Examples of Cold Frames
Straw Bale
My all-time favorite is the straw bale frame. They are fabulous for fall and winter crops because of straw’s insulating properties. Making one is stunningly easy: just take 4-6 bales of straw and form a square or rectangle.
You can cover the straw bales with old windows, storm doors or drape plastic over it. You’ll need to ventilate this model manually by pushing a window back or removing it for the day.
I place my straw bale cold frames right in the garden in a spot I have cleaned up and tilled. Normally, I would also have let the chickens scavenge the area to remove any nasty pests because I don’t want to create a positive environment for bugs in my cold frame.
Then I am ready to plant fall and winter crops. I’m in zone 6, so I usually do this process in September. I start cold weather seeds for fall in one of the barn sheds so that they do not get too hot in the late summer and are ready for planting in early fall.
When the cold frame is set up and ready to go, I transplant the seedlings into the soil. At this point, I don’t need the lid, except maybe on a cold night. As the weather cools I make it part of my evening chores to go and adjust the lids to protect the plants.
Graphic designed by Chris McLaughlin
Traditional Wood Cold Frame
A cold frame built from wood is easy to put together. I have made several from scrap wood left over after building projects.
Author, Ame Vanorio, is the director of Fox Run Environmental Education Center and has been farming organically for over 25 years.