Cold Frames: The Greenhouse Alternative You Wish You’d Known About Sooner
Gardening’s Best Kept Secret
You may have heard of cold frames, you may not have. For those that don’t know, a cold frame is like a miniature greenhouse that generally fits one level of seedling trays or other small potted plants. They are typically used as an alternative to a larger greenhouse in lieu of space or finances. If you’ve dreamt of a greenhouse but can’t make it make sense for you, let me give you some advantages to cold frames to think about.
Cost Efficiency
Don’t get me wrong, I still dream of the perfect greenhouse. However, it isn’t considered a necessary expense, even for this small scale flower farmer. While I could build an inexpensive greenhouse, I can build an even less expensive cold frame and get close to the same results. Something that is a great advantage to zone 3 gardeners and growers, cold frames provide a wider window for hardening off cool season hardy annual plants. Plus, if you really wanted to heat your frame, this can be done at just a fraction of the cost compared to heating an entire greenhouse.
Space Savings
Another advantage to cold frames is that they can be as big or as small as you have space for. They’re easier to build and can be created out of many different materials. I’ve seen wooden ones, cold frames dug into the ground and even frames made of straw bales. Typically they have glass paned windows for a cover but you could also use greenhouse plastic or panels. Some even add cold frame “panels” or “doors” to an existing raised garden bed.
Flexibility
Cold frames are more mobile than a greenhouse, so if you find that your growing conditions aren’t optimal, you can easily move it to a better location. For example, we built ours near the south facing side of our house which is painted a dark color. Our back yard also happens to have a slight slope that is south-facing which is perfect for capturing solar energy and heat. Our original cold frame was on a flatter spot and built out of strawbales and an old door. However it wasn’t sloped and didn’t properly capture solar energy so we built a more permanent, sloped cold frame closer to the house. This allowed for convenience and was close to electricity if we chose to use some sort of heating element.
Sustainability
While you can build a sustainable greenhouse, you’ll still need to secure a larger amount of building materials. Like I mentioned above, we had originally used straw bales. The cover was an old, outdoor glass door. Our second model was built from wood scraps and that same door. We also insulated the inside. If we had needed to heat it, we could have also sealed the inside and run a power cord through to add a heat lamp. Next season, I plan to add rock rather than the grass floor to help choke out weeds and decrease pest pressure. This will also help capture solar energy and create radiant heat when night temperatures drop. Basically, any reused materials that create a box with a clear (preferably glass) top that can be adjusted to slope will do just fine.
The Learning Curve
Full disclosure, we used our cold frame for a short period this past early spring and it was only to harden off my hardy annuals that were started inside. The thing is, in zone 3, whether I use a greenhouse or a cold frame, I’d spend a lot of money and energy to heat them at night when our temperatures drop to sub-zero temperatures. I get a much faster head start by starting seeds indoors (sometimes as early as February) and hardening them off starting in April. By this time, daytime temperatures tend to stay above zero degrees Fahrenheit and I can bring my seedlings inside at night. If I didn’t want to carry them in at night, I could find a heating solution and leave them out.
There is also an art to getting the correct angle towards the sun for optimal sun exposure. During the height of the day, if your cold frame captures solar energy efficiently, you may need to vent it to keep your delicate seedlings from frying. This may not be ideal for those that are not physically able to be home during the day to do this. If you want to get fancy, you could set up an automatic hinge that opens based on temperature fluctuations or on a timer. You could also vent the cold frame first thing in the morning and hope for the best!
Finding your heating elements may also take some time. Depending on the type of early spring you’re having, you’ll need more or less intervention in this area. In our first cold frame, we had tried to utilize a black plastic tarp to seal the inside from the wind and to better absorb solar heat. This may have worked well if we had angled our frame towards the sun better. However, the straw bales were not a good fit for the amount of early spring wind we had gotten last year. We then chose to build a more permanent structure out of scrap wood closer to our house. This allowed us to carry our seedlings a shorter distance as well as use some radiant heat from our dark-colored house. Next season, if I want to extend the early spring window even further, I can easily run electricity to this cold frame and add heat lamps. Overall, I’d like to avoid this as much as possible just to keep things simple and sustainable.
Reminder, starting seeds and plants too early will only create weak plants. It is better to wait until they can reasonably survive the weather at the time of planting. You’ll get better, faster results this way.
Scrap wood cold frame with repurposed outdoor glass door. Note the frost on the glass
Our simple venting system
What You’ll Need
The building materials are simple. You’ll need:
Walls
A Cover
Optionable Heating Elements
Walls
For the main structure walls, you can use any scrap wood you have lying around. Some people also use straw bales or dig them into the ground. If you don’t have any of these, it is preferable to purchase local or used materials.
Cover
For ours, we scored a free, outdoor glass door from Facebook Marketplace. Others use old windows but you could also stretch some greenhouse plastic across and weigh it down with rocks or logs.
Heating
The first way that you should be “heating” your cold frame is to angle it towards the sun to capture solar energy, the way a traditional greenhouse would. You can then get creative with how best to keep the heat in during the nighttime hours. Layering the bottom with dark stone pavers or rock will capture heat and slowly release it during the day. You can also insulate the inside as well as run a power cord attached to a heat lamp or other small heater. Just be cautious when using electrical heating elements if your main walls are made of straw as this could present a fire hazard!
Remember, heating your cold frame is optional. If you don’t have a need for early season crops, your frame can double as a really handy hardening off space after the weather warms up.
Cold Frame Crops
If you’ve gotten this far you may be thinking that cold frames sound great but you may not be sure what to use them for. As with any gardening, what you grow should reflect the types of crops you consume or need. For instance, broccoli is a good early spring crop as it prefers a cold start, but you wouldn’t want to use your cold frame’s real estate if you hate broccoli!
Below is a list of cold weather vegetables and flowers that may do well with an early start in a cold frame. This is not an all inclusive list.
Vegetables
Some cold hardy vegetables include broccoli, lettuce, spinach, carrots, radishes, onions, dill and garlic
Flowers
Some flowers that prefer an early start, that may harden off well or that you could start early for winter sowing include snapdragons, violas, pansies, delphinium, sweet peas, larkspur, dill, foxglove, echinacea and rudbeckia.
Side note, I plan to do an experiment with fall direct sowing cold hardy, early spring flower crops. Stay tuned for the results of this!
Baby seedlings hardening off on a sunny day
Snapdragons hardening off with the original frame’s straw bales now acting as a wind break
To Build or Not to Build
So there you have it! Let me know below if you already grow using a cold frame or if you plan to give it a try. Let me know if you’ve had really great success with any particular vegetables or flowers.
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