A cold frame is a bottomless box with a skyward-facing window. Like a miniature greenhouse, a cold frame lengthens the gardening season by protecting plants and seeds from the moderately cold temperatures and drying winds of late fall and early spring. With the addition of a simple heater, a cold frame can be used nearly year-round to grow cool-season flowers and vegetables, and to give summer plants an early start.
The low-cost, easy-to-build cold frame presented here takes one or two weekends to build and uses widely available materials. You can place it on a deck or patio to grow plants in pots, or you can place it over a garden bed.
- 4-x-4-foot sheet of 1/2-inch exterior plywood (or 2 2-x-4-foot panels) plus scraps of 1/4-inch plywood for corner braces
- 18-gauge perforated steel angle — approximately 3 feet
- 2 1x3x8-foot clear pine for the top frame
- Hardware: 25 1/4-inch-diameter by 1-inch-long hex bolts and nuts; 16 washers sized to fit the hex bolts; 4 4-inch steel L-brackets; 45 3/4-inch No. 8 wood screws; 3 3-1/2-inch steel or brass hinges. Expert tip: Buy a few extra bolts and screws.
- 6-mil poly sheeting, clear, to cover the top of the cold frame
- Tools: Long ruler or tape measure; saber saw or circular saw; hacksaw; power drill and 1/4-inch bit; screwdriver (or screwdriver bit for drill); staple gun, or a hammer and 3/4-inch roofing nails
- Optional materials: 1 or 2 4-x-8-foot sheets of 3/4-inch rigid foam board insulation (Foamular or similar). 2 or 3 100-bulb strings of miniature holiday lights. Vinyl tablecloths or blankets.
Raised cold frames are becoming popular because they are easier to access and do not require their user to squat or kneel. Make sure if you are designing a raised cold frame you make your legs strong enough to support your chosen growing medium.
Cold frames are most useful for hardening off seedlings that have been started indoors or in a heated greenhouse. Most plants should not spend their entire lives in a cold frame. As always, first decide what you want to grow, then choose your greenhouse strategy based on your crop’s heat and space requirements.
Insulation is critical to keeping temperature swings from overwhelming your cold frame inhabitants. The easiest way to insulate your cold frame is with pieces of 3/4- or 1-inch-thick rigid foam insulation board. This material can be cut and shaped easily with a utility knife. Expert tip: Make sure you’re using extruded polystyrene insulation, not the crumbly white insulation.Expert Tip
To improve the insulation power of the lid, cut a 2-inch-wide strip of insulation and tape it to the inside of the poly film before attaching it to the lid frame; this will help maintain an air space between the two layers of poly.On cold nights, place some pieces of insulation atop the lid, then cover everything with an old plastic tablecloth. Taken together, these steps will help keep temperatures in the cold frame from dropping too much.You can also moderate temperature swings by adding water-filled milk jugs inside the cold frame. The water warms during the day, then releases that heat slowly at night.If you’ll be using the cold frame on a deck or patio, consider setting it atop a couple of layers of the rigid foam board insulation.
You can heat your cold frame with special warming cables or mats. A simple way to raise the air temperature in a well-insulated cold frame approximately 10 degrees is by plugging in a string of 100 miniature holiday lights (still in their box). You may need to use several sets to heat the cold frame on very cold nights. If you’re using the cold frame on a deck or patio, put scraps of 1/2-inch plywood under the lights to prevent damage to the surface or insulation. If you can keep the temperature at 40 degrees or higher, you can grow many cool-weather plants like lettuce, radishes, and pansies.
Using Your Cold Frame
Resist the urge to fill up your new cold frame right away. Like a new baby, a new cold frame demands a lot of attention at first and can be very demanding. You’ll need to be watchful of the weather, deciding early in the day whether to crack the lid a bit or keep it closed. And you’ll have to be religious about covering the cold frame at night if the temperature is going to drop below freezing.
A great tool for helping you decipher your cold frame’s personality is an electronic remote thermometer. They have a sensor you can put in the frame, and a remote read-out that you keep inside. Most have functions that let you review the high and low temperature over any time period.
Once you get to know your cold frame, you can start using it to grow just about anything you’d grow in a greenhouse — provided it fits under the lid. By far, the best role for your cold frame is to give you an extra-early head start on growing plants for the coming gardening season.