When you garden in a place like Maine, it’s natural to want to squeeze every available day out of the short growing season. Typically, there are 155 days between the last and first frost of the year in the state, but growers can eek out extra growing time with cold frames.
Cold frames are fairly simple to make and can increase the growing season by one to three months. In the spring, gardeners itching to get seeds in the ground can start now in cold frames.
In fact, starting lettuce, other greens, radishes or scallions now could have you harvesting them before the start of the regular planting season even begins, according to information from University of Maine Cooperative Extension. These cold-hard plants will mature faster when planted in a cold frame.
In their most basic form, a cold frame is a bottomless box with the back higher than the front and with a clear top that can be opened to provide ventilation. The idea is to let the sun in to keep the plants inside the box warmer than the outside temperature.
The ventilation is important because on really sunny days the inside can get hot enough to burn your plants if you don’t allow a bit of cooler air to circulate in.
To build your own cold frame, you need to construct a four-sided box that’s open on the top and bottom. You can make the sides using new boards from the hardware store, scrap lumber you have acquired, planks from old pallets, cinder blocks or hay bales.
In one case, an enterprising Maine gardener turned a discarded dog house into a cold frame using recycled nails and old windows.
The materials should be free of insects and should not have been treated with chemicals.
There is no right or wrong size. Build according to how much you want to grow and the materials you have to build. What is important is to make sure the back of the cold frame is higher than the front. Ideally, the front to back slope should be 1-inch per foot back-to-front.
Next you need to place or install the clear lid. For this you can use plexiglass, clear plastic or regular glass.
Plastic materials can be found at garden supply or hardware stores. If you’d prefer glass, scavenging and upcycling old windows, sliding glass porch doors, glass shower doors, storm doors or storm windows is a good option.
Either lay your cover on top or attach it to the back using hinges. For venting purposes, you can use blocks of wood to prop the cover open.
Situate your cold frame in a well-drained area with the front facing south to southeast and add about 6-inches of good planting soil. Finally, attratch or place a thermometer inside so you know when it’s time to open or close the lid. The ideal temperature inside a cold frame is between 65 and 75 degrees Fahrenheit.