It’s completely possible to have fresh salads in the middle of winter! Not only possible, but downright easy.
Folks have been growing in cold frames for centuries, and whether you live in a region where winters are consistently cold and chilly, or a mix of mild temps, building cold frames into your gardens are a wonderful way to extend your season.
A cold frame is simply four walls that are used to trap heat and shelter plants, and a transparent lid that allows light to get in. What kind of walls and what kind of lid you use is up to you, and there are many easy ways to put some together. When you maintain the right temperature, heat, light, moisture and element exposure within the cold frame, your plants will thrive.
Unless you’re a grower that happens to live in a tropical area that isn’t subject to winter’s cold, you really should think about building a cold frame. Here are a few good reasons why:
The more you can grow at home, the less you’ll have to buy at the store! Homegrown greens are far superior in nutrition and taste to their store-bought counterparts. That’s a great reason to grow your own, but let’s do the math a bit. For one $3 packet of lettuce seeds, which has at least a few hundred seeds per packet, can in theory yield you a few hundred heads of lettuce! Especially if you’re buying organic produce, that’s less money than one head of lettuce in the grocery store!
For just pennies, you can head out to your garden and pick your own.
Plus, the cold frame can be used to start cuttings from favorite perennials and ornamental garden plants. I also use mine every season as a safe place to stratify perennials that I want to plant out the following spring.
Extend the growing season
The main reason for using a cold frame is to extend the growing season.
Fresh spinach, lettuce, kale, and the like are possible all winter long, provided the sun’s warmth is properly harnessed. The capacity of the earth to hold heat, combined with the warmth of the sun, keeps the growing environment ideal for cool weather crops.
You can also get a jump on growing for the summer season by starting annual seeds in the cold frame.
It’s the perfect transitional spot for seedlings to get them ready a bit earlier than typically possible. You can harden off seeds from delicate plants by starting them inside before taking them out to the garden. In order to do so, you’ll need to keep track of the weather and plan accordingly.
Before you move young seedlings into the cold frame, be sure that they have multiple sets of leaves that have rooted well.
Before planting them into the frame, begin the venting process starting the with the warmest time of day, keeping the venting period shorter on the first day and then gradually increasing the time.
You’ll know that your plants are acclimating when new growth, dense and dark foliage appear.
Sowing seeds early
Cold frames make an ideal place to sow early spring seeds. You can sow directly into the soil beneath your cold frame, or you can sow into flats that you place into your cold frames. For early spring crops like broccoli and cauliflower, I like to plant into flats, which allow me to start many plants at once and then plant them directly into my garden beds when the plants are about 6 weeks along and the days are getting longer. As with all seed starting, timing is key.
If you want have a cold frame or two just for greens, you can plant directly into the soil under your cold frame because they are so easy to manage spacing. With greens you can sow heavily and eat the thinnings as they grow, leaving space for full sized plants.
Once past the Persephone period, you can start to sow greens seeds anywhere you have already harvested mature greens through the winter, filling holes and making the best use of the space.
Usually you can only sow seeds at a specific time of the year, if you’re sowing them right into the soil, or into flats. But when the seeds are protected by a cold frame, they can be sown a bit earlier.
Seed choice is important here. Selecting crops that thrive in cool weather is key, so experiment with most greens like spinach, lettuce, kale, and collards. Root crops like carrots, beets, turnips, and radishes. Some herbs would be good choices too, such as parsley, dill, and cilantro.
Prep the soil in your cold frames a few weeks before planting. The planting area needs to be evenly moist so you’ll need to begin venting the cold frame in order to remove dampness after seedlings start to germinate. This helps to protect them from extreme weather conditions and temperature fluctuations.
Deciding where to put your cold frame
Before buying or making your cold frame, you’ll need to decide where to put it.
The ideal place is a sunny, south-facing spot that has good drainage and also offers at least some protection from the wind and other elements.
Ideally, it should get full sun from mid-morning to mid-afternoon.
It can be set up as a permanent part of your garden, or it can be temporary in that it’s put away in springtime after use.
Before setting up a cold frame in a permanent place, it’s best to dig down 6-10” and amend the soil with compost and organic matter to help drainage if your soil is heavy clay like mine is. It’s important that your cold frames have adequate drainage to keep your plants healthy.
The key to success is to pay close attention to the temperature – it should be under 75 degrees in the cold frame for summer plants, and below 60 degrees for plants that typically grow in the spring or fall. If you need to achieve a cooler temperature, lift the lid to let air in.
Many gardeners follow this general rule: If the temperature outside is over 40 degrees, keep the lid open about six inches, when they top 50 degrees, take the lid off altogether, but be sure to replace it in late afternoon in order to trap enough heat in for cool nights.
On nights where temps dip below freezing, your plants in the cold frame will likely need some extra protection, you can do this by adding insulation on top of the lid – newspaper, straw, old blankets or whatever you happen to have handy should work.
Building the cold frame
While you could spend your hard earned cash on a cold frame, there’s really no reason to do so when you can put one together yourself fairly easily.
The walls can be made from just about any type of sturdy material, from plywood or concrete to my favorite, bales of hay. An old window makes a great lid, or you might use plastic sheeting tacked to a frame.
There are lots of great DIY ideas out there, including these:
Recycled window cold frame. This Old House offers this plan for a cold frame that uses a recycled window for the top, and is great if you’re looking for something more permanent as it’s placed on a foundation of bricks. Cinder blocks would also work well with this plan.
Better Homes and Gardens simple cold frame plan. This easy DIY plan from Better Homes and Gardens utilizes widely available materials like plywood and poly-sheeting and takes just a weekend or two to build.
Temporary straw bale cold frames are my go-to each fall for a few reasons. I don’t have any carpentry skills, so this is a project I can manage on my own. I can also set them up around existing raised beds, which gives me easy framework to use. I am also a huge fan of adding rotting straw to my garden beds for nutrients and mulch, and so I always have bales of hay sitting around decomposing anyway, so this gives them a job to do until I need them for something else.
Mother Earth News inexpensive cold frame. This cold frame plan by Mother Earth News uses inexpensive materials like polystyrene insulation board, scrap lath and molding, and fiberglass-reinforced plastic glazing or old storm windows.
What have your cold frame growing experiences been like? Share your successful tips or problems we might be able to help with in the comments so we can all learn together!
Written by Sow True Seed's Education Director, Angie Lavezzo.