Extend the Growing Season With a Cold Frame

Our growing season is short, why not extend it with a cold frame. Cold frames act like mini greenhouses, protecting plants from frost, freezing temperatures and stormy weather. During the day, the sun heats the ground under the cold frame. That heat is trapped inside the cold frame at night, keeping the plants warm.

Cold frame in use

With a cold frame, you can start growing annuals and vegetables several weeks before they can safely be planted outside. In the spring a cold frame works great for starting seeds, hardening off flats of seedlings, or planting seedlings early in the garden. You can also extend the growing season in the fall by planting second crops of cold weather vegetables in late summer, and harvesting them weeks after the other vegetables are gone.

Anyone can build a cold frame with little or no cost; they are easy to build from recycled materials. I made my cold frame out of two old windows I got for free, and scraps of lumber I had in the garage.

A cold frame is simply a box that is higher in the back than the front, with a clear top and open bottom. The top can be made with an old shower door, patio door, windows, clear plastic, or any other durable, transparent material. The base of the cold frame could also be made out of several types of recycled materials such as wood, cinder blocks, brick or straw bales. The size of the box will be determined by the size of the top. The bottom of the cold frame is open and it either sits on top of the ground, or can be buried into the soil a few inches. There are many different design plans available on the internet.

The best location to place a cold frame is facing south or west. Putting the cold frame up against a structure such as a house, garage or fence will offer more protection and keep the cold frame warmer at night. On extremely cold nights, the cold frame can be covered by a blanket to help keep the heat in.

Venting the cold frame

In my experience, the biggest challenge of using a cold frame is venting it. A cold frame must be vented to release excess heat and keep the seeds or plants from overheating. It's amazing how hot it gets in there on a sunny day, even if it's cold outside.

You can vent the cold frame manually by propping open the lid on sunny days, and closing it on cold nights. Otherwise an automatic vent controller could be purchased, which will open and close the lid at specific temperatures. It's a good idea to keep a minimum-maximum thermometer in the cold frame to monitor the temperature fluctuations.

Cold frame with windows removed

Once the weather warms up enough that the plants can survive outside, you can either remove the entire cold frame, or just the lid. I remove the windows from my cold frame and leave the base in the garden year round. The windows are easy to store in the garage until I need them again in the fall.

Cold frames are an easy and inexpensive way to extend your growing season. Every gardener should give cold frame gardening a try.