The Cold Frame

A cold frame is a box covered with a transparent covering of some kind which captures the heat of the sun and insulates tender plants from the harshest of winter cold. It is also used to start seeds in cold weather. It’s usually between 10” to a foot high, constructed of sturdy material. It is placed so it faces south to capture the most heat of the sun. It can have the sides slanted so the covering more captures the sun but it can be four equal sized sides.

A finished cinder block cold frame with window covers.

A finished cinder block cold frame with window covers.

For those folks that have cold winters, like we usually have here in the NE area of Georgia, having a way to keep more tender plants alive without having to bring them inside or start seeds inside in limited space is a real help. This year we grew lemon grass which is supposed to be annual in our area because it’s a semi-tropical plant. So, I dug up the three big plants I had growing in one of my raspberry beds, and made a cold frame for them. It’s located on a south facing hill and is backed by the raised bed of the raspberries so it will catch heat from the sun.

So, if it’s supposed to keep things warm, why is it called a ‘cold’ frame? Because it’s really a little green house but you don’t heat it with an outside source of energy like you would a winter green house. With the price of green houses today, and what it costs to heat a big one, this is both energy and cost saving. However, you can set fairy lights across the top of the window coverings on a timer to provide extra lighting for the plants to extend the day for them.

The real value of a cold frame though is its use to start spring and summer plants early. Because it is warmer than the surrounding environment and keeps in the heat during the night you can start tomatoes, cucumbers, peppers, eggplant, beans, and even corn in a cold frame and have the plants transplanting size by March. I usually start those seeds in plastic seed cells or in seed soil blocks on trays around first to middle of February. By March they are 6” tall and once we get our last frost date, out they go into the beds.

Cold frames can be purchased in seed catalogues but they are very expensive. Grow Organic had them selling for between $118 and $140 for little ones. If you are at all handy, you can make them for a fraction of that. I’ve used 10” tall wood frames and 3.5 mil plastic sheeting that worked just fine. With some long screws set into pre-drilled holes in the lumber, you can make nice cold frames from 2” x 12” or 10” planks. Or you can use shorter screws and use “L” brackets on the corners. Either way, lumber works well. It should be treated lumber so it doesn’t break down. Or you can use raw lumber and paint it. It’s heavy though and not easy to move once you have it build. So choose the space to put it in carefully as it will tend to stay there for several years.  If you use plastic over the top, you just have to use something to prop up the plastic or it smothers the young seedlings. Bamboo poles or several milk jugs full of water which also hold in the heat work OK. If I had had windows last year then I would have preferred that over plastic sheeting as they hold their shape and don’t slump into the plants like sheeting does and they’re better at insulating.

This year I got a bunch of single and double celled cinder blocks from a friend and made the ‘box’, covered the bottom with cardboard (over grass to keep the grass from growing there)

Cardboard going down as the first layer.

Cardboard going down as the first layer.

 and black plastic (old construction grade trash bags I’ve used 20 or more times, holey and disreputable looking work just fine) over that.

Black plastic on the floor of the frame.

Black plastic on the floor of the frame.

I filled the cinder block cells with wood chips for added insulation which I already had on hand

Wood chips filling cinder block holes for added insulation.

Wood chips filling cinder block holes for added insulation.

It isn’t necessary but if you have the cold frame installed permanently, it is a help. If you’re just putting the cold frame there for a few months and plan on removing the cinder blocks later, don’t bother. You can also fill the holes (pointing up) with repurposed shipping plastic air bags or bubble wrap.

Over the box I then fit used window panes.

Windows fitted over single or double layers of cinder block.

Windows fitted over single or double layers of cinder block.

 My husband is a remodeling contractor and occasionally he is called upon to change out windows in older homes. He saves the windows for me. They usually have some exposed wood on the edges where they fit into the house window frame so I’ve had to spray paint that to protect the wood from breaking down. I have a number of sizes so I picked the size for the cold frame. I had to fit the frames into the areas of the garden where I had room. I made two long frames. I chose the sizes of the windows which best used the garden space and that determined the dimensions of the block boxes.

The block dimensions are just the right height for ordinary seed starting laying them with the holes up. Depending on the area you put your cold frame in, it can be the size of a single window pane or a line of them. You can also build them double high for taller things like lemon grass.

Double high frame wall with taller plant inside.

Double high frame wall with taller plant inside.

Or you can build them three sided with more than 2 courses of blocks and in the front and top put windows to have more southern exposure of light. You’re building a mini-green house then. You just have to secure the vertical one to the blocks so they don’t fall over in a wind. The top windows tend to hold themselves down without securing.

When it starts to get warmer end of February or beginning in March, the cold frame temperature will increase as we get longer and warmer days. Then you’ll want to prop up the lids (windows) with a stick or a brick to let out excess heat. But then return the cover to enclose the heat just as the sun is setting. That way the heat stays in all night but you don’t cook your tender babies.

You’ll want to make sure your seedlings receive just the right amount of moisture. Don’t put plastic over the bottom of the frame so moisture won’t run off. You want to let it drain so the babies don’t rot their little roots. But don’t let the plants dry out either. Check it every couple of days. Also, if it is going to rain and you wish to let that water your seed trays, you can remove the windows easily as they are not large nor heavy, then replace them when it stops raining.

It is important to have the window fit snugly against the box part to keep in the heat. So straighten out the ground you are laying your frame on so there are no gaps.

Flattening the ground first so the walls are straight and flat.

Flattening the ground first so the walls are straight and flat.

If there are some gaps, you can repurpose some bubble wrap around the top of the cinder blocks. The wood frames tend to fit without bubble wrap if they are the right size for the windows. If it really gets cold, you can lay an extra layer of plastic sheeting over it all and weigh down the sheeting with rocks or old bricks.

Plastic laid over window frames for added insulation.

Plastic laid over window frames for added insulation.

 I prefer clear plastic sheeting for this but white also works, just not as well. Rolls of 3.5 mil plastic sheeting are available at Home Depot for around $15 a small roll. It lasts several seasons and it’s generously long. You can also use some of the sheeting to cover winter crops over hoops or tomato cages laid on their sides to keep the plastic up off the plants.

Wire tomato cages laid on their side to act as plastic support over plants.

Wire tomato cages laid on their side to act as plastic support over plants.

I use a long roll of it to cover a row of containers also where I am growing greens and tender herbs.

Containers filled with soil and covered with tomato cages.

Containers filled with soil and covered with tomato cages.

Plastic sheeting is one of those materials that has multiple uses so I always have some. I fold it up in the summer and keep it out of the sun so it lasts several seasons. 

Containers covered with a long sheet of clear plastic.

Containers covered with a long sheet of clear plastic.

The only kinds of pests I have had to deal with are slugs. But we have had a new invasion of flat headed worms – which are a parasite, eat our good earth worms, and are a pest. They hang out under plastic so check on that now and then and remove them, killing them by crushing them into oblivion on concrete. Don’t flush them or just nip them in two as now you’ll have another one. They are thin and long, slimy looking, flat shaped, almost transparent, with a funny squared off head unlike our good round earth worms. The slugs will accumulate in the warmth and moisture of the frame, so look under your seed trays now and then as they love tiny seedlings. If you find your seedlings chopped off, look for slugs which hang out in the crevices of the bottom of seed cells or under the flooring plastic. Otherwise, the cold frame is a wonderful solution to getting your spring and summer garden started early, and protecting your tender plants over the winter.

By starting your seeds yourself you ensure the exact crops you want to eat or sell. And you are not dependent on the seedlings being sold in the nurseries which are limited in varieties offered. If you save your heirloom seeds, this is how you perpetuate our precious bio-diversity, it’s worth it to start your own.  The cold frame is the next best thing to a green house and so much less expensive.

 I have a limited number of extra windows for sale if you are interested, and very inexpensive used bubble wrap. Call me or email me. Cinder blocks are inexpensively purchased from a home improvement store like Home Depot or Lowes. They run around $1.25 a piece for the double holed size. Sometimes you can find them cheap when used. Check Craig’s list. You can paint them too for a little aesthetic interest. And they make nice raised growing beds as well. I even put soil in the holes around my raised beds and plant them with flowers, herbs and root vegetables which really like it there.

Painted cinder blocks filled with soil and planted with flowers, herbs and root vegetable seeds.

Painted cinder blocks filled with soil and planted with flowers, herbs and root vegetable seeds.

 I had a great crop of carrots last year from those holes in the cinder blocks. Pansies planted in them add color and interest.

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4 Responses to The Cold Frame

  1. Austin says:

    the cold frame looks great. I am considering doing something like this next year. For now I will focus on my hobby hydroponic gardening.

  2. Monica B says:

    What kind of paint did you use on the blocks?

    • didirks says:

      It came from Home Depot – one that handled all kinds of surfaces. My husband painted them for me so I don’t know exactly, surely exterior grade though. The cold frame wasn’t painted. And the door was a PVC frame so it wasn’t painted. Does that answer your query?

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