Sizes of cold frames
This is simple in that it all depends on your needs. If you want to move the cold frame then it should be small and light. If it will be permanently in place then length is less critical. I have seen cold frames that are 10 feet long, but the frame shouldn’t be too wide, say no more than 2 to 3 feet for ease of access and to limit the difficulty in covering and keeping it warm inside. All four sides can be the same height, for instance, if you were using bales of hay, which is great for raising seedlings and hardening off those plants grown inside the house. Cold frames with back walls that are taller than the front seem to be used more to extend the growing season. But these can be dual purpose by also using them to raise and harden off seedlings.
Placement of the cold frame
You want to make sure the cold frame catches the sun as much as possible. Ideally, place cold frames facing south or southwest toward the winter sun. This is especially true of those with a back wall that is taller than front because you want to make sure the sunlight reaches that back wall as well as the soil, thus maximizing the amount of heat absorbed during the day. Otherwise, place the frame anywhere you get sun most of the day. For the most basic of cold frames, the soil is absorbing the heat, while those built with blocks or rock have an increased ability to keep the interior warm on cold nights.
Planting in the cold frame
If you are using the cold frame to harden off seedlings in the spring, just place the flats or containers inside the cold frame. If you have a moveable cold frame you can get an earlier start by planting directly into the soil and placing the cold frame over the planted spot. Then remove the cold frame once the day and night temperatures are warm enough for the plants to go it alone. If using it to extend the season, plant in the soil as you would any garden plot. As with most gardening, when planting in the soil directly you want to work and amend the soil so it has the right balance of organic matter and nutrients.
Issues to consider with
a cold frame
While a cold frame helps you grow plants when it’s colder outside, let’s start with an issue often not initially considered. That is having the interior of the cold frame heat up too much during the day, which can damage the plants you have worked hard to grow. As such, it’s necessary to pay attention to the weather and learn how temperatures go up and down in your cold frame.
An easy way to monitor this is by using an indoor-outdoor thermometer. Place the remote sensor in the cold frame and the LCD display inside the house where you can see the temperatures change. When the inside of the cold frame heats up during a winter day you will need to raise the lid or row covers to let out some of that heat. You don’t want the inside temperatures to rise much over 75 degrees Farenheit, which is easy to do on a warm, say 50 degree, winter day. The thermometer will also let you know how cold it’s getting so you will know when it’s OK to plant for those “let’s get an early start” endeavors, or if you need to add another heating source, like the milk jugs painted black mentioned in the first article (Dec. 29, 2016) for some extra warmth at night.
Whether planting directly into the existing soil or in pots and flats, don’t forget to monitor the soil moisture as it can dry out quickly.
Finally, don’t be surprised if you find yourself asking why you didn’t build a bigger cold frame or more than one, as you see the value in them!
For those wanting a more advanced way of extending the growing season, check out my article on building a low tunnel, a structure in-between a cold frame and a full-fledged green house. It can be found on The Times Independent's website, www.moabtimes.com, from September 2009 — this information never gets old.
Thought for the day: “What good is the warmth of summer, without the cold of winter to give it sweetness.” —John Steinbeck
Previous articles are available at The Times-Independent website, www.moabtimes.com. Have an idea you’d like Mike to consider writing about? Want more information about these topics? Call the Utah State University Extension Grand County office at 435-259-7558 or email Mike Johnson at email@example.com.