A Complete Guide for using Cold frames for gardening
Colder temperatures are creeping into the air, but that doesn’t mean you can’t grow your own vegetables. A cold frame can help you do just that and this article will show you how to use cold frames for gardening.
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What is a Cold Frame?
Cold Frames are simple unheated “mini greenhouses” used to extend your growing season.
Most cold frames for gardening consist of a solid-walled, bottomless box with some type of glass top. The box can be made of wood, brick, or other materials. The box offers protection from wind, rain, snow, and freezing temperatures.
The lids (often called lights in gardening books) are usually made from some type of glass or plexiglass. The glass offers thermal protection from the elements, but also allows sunlight into the box. This creates a “greenhouse” like environment inside the cold frame offering a great place for hardy plants to grow in colder weather.
Cold frames for gardens are often designed for and placed on top of an existing garden raised bed or directly on your garden soil. They are normally put out either early in the spring or late in the fall and winter. Cold frames can be used to extend your gardening season.
By choosing the right vegetables and planting at the right times you can use cold frames to grow all through the winter even in areas that have VERY cold wintertime temperatures. Or they simply are used to add 45 days of additional growing time to either end of your gardening season.
How does a Cold Frame Work?
The wooden box of a cold frame protects the crops inside from cold weather, frost, and especially cold winds. The glass top adds protection but also allows the sun’s rays to enter the frame. This gives the plants inside plenty of light and the sunlight warms the soil and the air inside the cold frame.
On a typical cold night, you will find the temperature inside a cold frame to be 10 to 20 degrees (F) warmer than the outside temperatures. During the day that difference can jump to 30 or more degrees warmer depending on how much sunlight there is.
Where is the best spot for your Cold Frame?
The tops of cold frames are also usually angled front to back. Adding this angle to the glass allows more sunlight to enter the cold frame and can also cut down on wind resistance. Cold frames should be positioned in the sunniest part of your garden. They should be placed in areas where they can collect the most sun possible.
They will do even better if you can manage to put them up against a building or fence that is on the north side of the cold frame (in the northern hemisphere). Doing this offers protection from wind and added reflected light from the building or fence.
Pest Problems in Cold Frames
It is also a good idea to keep an eye out for pests in your cold frame. The nice warm environment inside is great for plants but also for pests like slugs, snails, aphids, and white flies. So keep an eye out for early damage and deal with the pests quickly!
Using a cold frame is like living in a warmer gardening zone!
Experts estimate that the growing environment inside a cold frame “moves” your garden 1 1/2 zones south! So for example if you live in Zone 5a, by adding a cold frame to your garden that garden bed will be more like a zone 6b garden.
And by adding the extra insulation of a heavy fabric row cover to the frames during the coldest months you get even more protection. We will discuss this idea further a little later on.
Advantages of Using a Cold Frame
There are many advantages of having a cold frame even if you are not interested in winter gardening.
1. Seeds get an Early start in Spring
In the spring cold frames offer the extra protection needed to get crops planted early. You can use cold frames to cover your garden beds. They will warm the soil and let you get your crops planted many weeks earlier than you could without protection.
Once a cold frame has been in place for 10 days the temperature in the top few inches of soil will rise dramatically. This will help seeds to germinate and then the cold frame will offer a nice warm environment for growth.
2. Hardening Seedlings
Cold frames are also great in the spring for hardening off seedlings. Cold frames are a great place to protect seedlings of plants like beans, cucumbers, squash, peppers, and tomatoes.
As these seedlings adapt to outside growing conditions, they will be protected from some of the cold temperatures that pop up in spring.
3. Season Extension
The protection a cold frame offers can add 45 days to each end of your gardening season. In the fall cold frames are great for extending your growing season.
Cool-season crops will grow well into November and even later with the protection of a simple cold frame!
4. Overwintering
Cold Frames for gardening are also great for overwintering crops for early spring harvest. Simply plant crops like lettuce, spinach, kale, and other greens about 2 weeks before your first frost.
These veggies won’t grow much before the cold weather sets in. If you protect the young plants over the winter inside a cold frame they will take off in the spring for an extra early harvest.
Should I buy or build a Cold Frame?
There are many different types of cold frames that you can buy or build. They vary depending on your needs and budget.
Buying a Cold Frame
If you don’t have the time or skills to build your own cold frame there are many different options available for you to buy. Most cold frames for gardening that you can buy will be made of polycarbonate sheeting. Many will have wooden boxes with polycarbonate tops, but I have also seen many made completely with polycarbonate sheets.
The only real disadvantage I see with these full polycarbonate frames is they are going to be very light and could easily blow away in the wind if they are not secured well.
Purchases can be made online at retailers like Gardeners Supply or they are even available on Amazon. Recognize that buying a cold frame isn’t cheap. Commercially available cold frames are very pricey, especially for their size.
How to build cold frames for gardens?
If you are a DIY person then building a cold frame is a great option. Building your own cold frame does require some woodworking skills and tools. At a minimum, you will need a circular saw and a drill, but a table saw and a chop saw can also be very helpful.
You will need lumber for the box and then you will need to decide on what you want to do for the lids. My cold frame lids are made from lumber and Plexi-glass, but you can also use old windows and build the boxes to fit the windows.
The nice thing about building your own DIY cold frames is that you can make much larger and more customized frames for less money. Materials for building a cold frame are still expensive, but you can get much more for your money if you build your own.
My cold frames are 4 x 8 feet and each cold frame has 4 lids. I have been very happy with this design that originally came from a design I saw in Eliot Coleman’s book, Four-Season Harvest. If you would like step-by-step instructions on how to build one you can check out this article I wrote here.
Improvised Cold Frames
You can always improvise a cold frame by using straw bales and old windows. Simply use 4 straw bales to complete the sides of your cold frame and then place the old window on top. This is really simple and works really well for protecting the crops inside!
Controlling Temperatures inside a Cold frame
Venting a Cold frame
One challenge of cold frames can be that they get too warm inside during mild weather. Remember that the inside temperature of a cold frame can be 30 or more degrees Fahrenheit higher than the outside temperatures. This can be a real problem on nice sunny days in the late fall or early spring.
When daytime temperatures are greater than 50 degrees the cold frame temperatures can reach 80+ which can be too high for many cool-season crops, and even tender warm-season seedlings. So you need to be sure to practice proper ventilation on warm days.
Installing simple vents in your cold frames is very important, or if you have a design like mine you can simply open one of the lids. If you are away from your garden during the day you can even install heat-sensitive automatic lid lifters that will vent your cold frames any time they get too warm.
Adding Heat to a Cold Frame
Most of the crops that you are going to be growing in a cold frame do not need much heat. That is part of the beauty of a simple cold frame for gardening. However, there are some ways to add heat to a cold frame if you would like some.
Some gardeners will add heat to a cold frame using some type of soil-warming heat cables. But the effect of these cables is negligible at best. They also cost money to buy and run.
A more time-proven method is to create a “hotbed” under your cold frame. This is done by digging out the soil in your cold frame down about 18 to 24 inches. This area is then filled with fresh manure. The manure is then turned and moistened for a couple of weeks until it settles.
Then you add 6 inches of soil on top of the manure and plant your vegetables. As the manure decomposes over the winter it puts off heat keeping the cold frame warm. (You can learn more about this process by reading this great article by the University of Missouri)
Fabric Row Covers add another layer of cold protection
Another trick we use in the coldest part of the winter is to use heavy fabric row covers to add another layer of protection inside the cold frames. These row covers add even more protection to your crops and help them survive the bitterly cold temperatures of December and January.
What vegetables to grow in a cold frame?
You will be surprised which vegetables you can plant! Since you are using a cold frame in the cool time of the year the crops you will be planting in them will be “cool-season” crops.
These crops will all last well into the winter months inside a cold frame. Here’s a list of what to plant in a cold frame:
- Mustard
- Carrots
- Beets
- Chard
- Arugula
- Kale
- Lettuce
- Spinach
- Radishes
You will find spinach, kale, chard, winter lettuces (like “winter density”) and carrots do especially well in the winter months, often lasting the entire winter. Taller crops like broccoli and cabbages will need specially designed cold frames that have headroom.
We have been growing in cold frames in our garden for well over a decade now and they have been a fun addition. We are now able to harvest crops 365 days a year in our cold zone 6b garden. I would highly encourage you to get your own cold frame, you will love being able to harvest fresh produce from your garden year-round!
How to Harvest Lettuce?
If I put a cold frame over a grow table that’s 3 feet off the ground and open underneath, would that still protect my plants? I’m afraid the roots will get too cold from underneath. Maybe plastic sheeting around the bottom? Thoughts?
It will not protect as well as if it were an on the ground bed. It would be helpful later in the spring but for winter it wouldn’t give you the protection you hope for because the soil will freeze from underneath.