As the fall season draws to a close, gardeners turn to season extension techniques to maintain crops right through the winter. Cold frames are effective at insulating the winter garden against a killing frost.
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Always tend the smallest amount of land possible, but tend it exceptionally well.” Eliot Coleman, The Winter Harvest Handbook
Cold Frame: Mini Greenhouse
A cold frame is an unheated, temporary, mini-greenhouse that can sit on top of existing garden beds. It’s an affordable alternative to a permanent greenhouse for extending the harvest.
There are a few cold frames on the market, and there are also lots of DIY models.
Buy a Cold Frame
In 2010, we received a 3′ x 6′ cold frame from Gardner’s Supply Company as a Christmas gift. We were excited–one less thing to build! This model protects crops down to 24 degrees, and will protect them even farther when the crops are mulched well and covered with row cover.
Our cold frame has withstood the test of time, and is easily disassembled for storage when not in use. I highly recommend it, but unfortunately, the Gardeners’ Supply website indicates this model is no longer for sale.
So what’s a winter gardener to do?
Review of the New Cold Frame Model
I checked out the current cold frame being offered by Gardener’s Supply Company: the Cedar Cold Frame. I’m afraid I’m not very impressed. It’s a visually appealing design–I mean, who doesn’t like cedar? However, the size is a little odd – 2′ x 8′.
In addition, it doesn’t appear to fold for storage, so this model might present a problem when not in use. And finally, the price is set at a steep $225. If you’re short on time or building skills, but have the cash, this new cold frame might be worth the investment.
Alternatively, you can build your own.
Build Your Own Cold Frame
The best trait of a cold frame is its temporary nature, and its ability to be stored or repurposed when not in use. Erica at MomPrepares created a cold frame that is so easy and inexpensive, anyone can do it! Here’s how she built a cold frame using straw bales, an existing raised bed, and used windows.
Tips for Using a Cold Frame Successfully
1. Weigh It Down
The winter wind will pick your cold frame up and whisk it away if it’s not tacked down. My cold frame is made out of polyethylene greenhouse fabric, and is weighted down on all sides with heavy rocks and bricks.
The plywood ends of Erica’s straw bale cold frame above are screwed into the raised bed ends. If a storm was coming, I might look for a way to secure the windows or weigh them down as an additional precaution.
2. Choose the Right Over-Wintering Crops
Here are my favorite winter crops. These veggies should have no problem surviving the winter temps:
- Brussels sprouts
- collard greens
- green onion
3. When to Close and When to Open the Cold Frame Window
Cold frames can heat up quickly, and hot temperatures can kill your cold-loving vegetables.
32 degrees Fahrenheit is your guide:
- OPEN the cold frame when the temperature rises to 32 degrees.
- CLOSE the cold frame when temperatures drop below 32 degrees.
Excess moisture can encourage soil-borne fungal diseases that kill seedlings, so ventilation is important.
The straw bale cold frame mentioned above provides a breathable environment and will likely be good for ventilation.
Cold frame windows can be propped open when temperatures rise above 32 degrees.
Or consider an automatic vent opener, which opens when the temperatures rise above 55 degrees, as a fail-safe in case you forget or go out of town.
5. Additional Protection for Temperatures at 20 degrees and Below
When temperatures drop below 20 degrees, additional protection inside the cold frame will be necessary to protect your winter crops.
Alternatives to row cover:
- plastic milk jugs (bottom cut off, turned over plant, lid off. Add lid if temps are really cold.)
- baskets that lie flat
- terracotta flower pots turned upside down
- old sheets, blankets or rugs
- milk jugs filled with water and set among the crops (water stores heat)
*Don’t forget to remove additional covers when temperatures rise above 20 degrees!
6. Get a jump on Spring Crops
Experiment with sowing crops inside the cold frame in December. You never know, you might get lucky! Try spinach, Swiss chard, carrots, and Asian greens.
Other Season Extension Structures
There are a few other types of structures that can provide protection for winter crops.
A hoop house is a temporary structure that can be built over existing garden beds. It’s like a cold frame, just a different shape. It can usually accommodate taller plants than a cold frame. Lady Lee’s Home and Survival at Home both demonstrate how to build hoop houses.
A polytunnel is a portable greenhouse that you can walk into. Check out the polytunnel at The 104 Homestead.
A geodome greenhouse is a another take on a walk-in greenhouse that can withstand the weight of heavy snows, though it’s a more permanent structure. Check out the geodome greenhouse at the Northern Homestead.
How do you extend your harvest season?