How to Make a Cold Frame by Utilising Your Brick BBQ
Simple and effective cold frame made with an aluminium frame and polycarbonate panels, measuring 40 inches square and 22 inches high. With the top that opens on both sides for access and ventilation when needed.
Multipurpose Your Brick BBQ for Gardening
When I think of cold frames I think of the brick built Victorian designs with brick sides and sloping framed glass tops, which of course if you have the space you can make quite easily yourself; just like making a brick BBQ. This article is a step by step guide to multipurpose your brick BBQ as a cold frame with tips and ideas of how to achieve it (illustrated with photos) on a shoestring budget by recycling scrap wood and salvaged reclaimed materials. All photos in this article were taken by me.
Below I describe my innovative solution into providing a cold frame for the spring and early summer without losing any valuable garden space, achieved by utilising our brick BBQ when its not needed for garden parties. And at a moment's notice, if one warm early summer evening, we decided to throw a BBQ party it'll take just a few moments to temporarily remove the glass coverings and plants; all of which can be put back just as quickly the day after the party when you're clearing up.
Harden off your plants ready for planting out
There's nothing worse than growing vegetables from seed in your greenhouse and then putting them out late spring only to die back from the shock of the change in environment. To be sure of a good healthy transplant from your greenhouse to the open ground in your vegetable plot you do need a transitional period where they're hardened off in a cold frame for a couple of weeks so that your plant can acclimatise to the harsher conditions outside.
If you don't fancy making your own cold frames, below is a selection of cold frames that may help you in successfully transfer your nurtured seedlings from greenhouse to open garden; and of course cold frames can be successfully used for the rest of the summer for growing other small vegetables such as lettuce and other salads.
I've always fancied a brick built cold frame as commonly used in Victorian vegetable gardens, but with the valuable space they would take up in a small garden I haven't built one yet. In past years when plants needed hardening off I used a couple of plastic (polythene) mini-greenhouses, which were quite useful and effective. The plastic mini-greenhouses were good value and lasted for a good few years but gradually the zip breaks and the plastic rips and you end up with just the plastic frame and wire shelving which I now use as secondary shelving in my greenhouse.
This year as I'm co-ordinating a communal vegetable gardening project with other keen gardeners wishing to share seeds, plants and knowledge it's important to harden off all the plants properly before passing them onto my friends as only one has a greenhouse. And rather than spend money on something that's only needed for a few weeks in the year and after a few years may start to fall to bits I considered building or making my own more substantial cold frame that would be cheaper to make than buy and would last much longer.
An aluminium frame with 4mm UV resistant double wall polycarbonate panels; 40 inches by 24 inches and 16 inches high.
Maximise on Space for Minimal Cost
Making your own cold frame is simple enough, whether it be brick or wood and glass; the challenge is finding the space to house it when in a small garden space is of a premium. So, in thinking out of the box, this is where I saw our brick built BBQ as a readymade cold frame; not that dissimilar to the Victorian designs. All that's needed is a glass top and a front. Therefore, grabbing a cup of coffee for a think, and then rooting around in my shed I found some old decking and part of an old front door that would make ideal raw material as a basis turning my brick built BBQ into a multipurpose brick utility that as a moment's notice could also be used as a cold frame when required.
The off-cuts of decking I found in my shed is an ideal wood to work with, being pressure treated it will last for years. To use the decking to make the frame for the glass top I just needed to cut it into 2-inch strips and with my router make a rebate to fit the glass, which I would need to buy; I bought a small piece of plate glass to fit from our local glazier for just a few pounds sterling.
We recently replaced our old front door due to substantial rot in the bottom half. I could have renovated it but we found a replacement in excellent condition at a local reclamation yard for just 20 so I cut off the bottom half of the old door and burnt it and kept the top half as its good solid wood that's still in excellent condition. It's this top half of our old front door that’s ideal for the front of the cold frame I was designing and for fitting a couple of hooks and eyes would do the job just nicely.
The beauty of transforming the BBQ into a cold frame in this way is that it makes it multipurpose; it uses space that is only otherwise required when we hold a BBQ, I get a functional cold frame that's cost me virtually nothing and I haven't needed to lose any valuable space in the garden. If we decide to have a BBQ when I'm still using it as a cold frame it's not going to take more than a few minutes to move the plants temporarily back into the greenhouse then put them back the following day once I've cleared out the BBQ.
This innovative solution means I can harden off a few plants at a time over the next couple of months as required for my garden and others and then just flat pack the top and front into a corner of my shed until it's needed again the following spring.
A large quality built cold frame measuring about 5 feet by 2.5 feet and over 18 inches high. A lot bigger than the one I've made and if you want to use a cold frame in earnest then this one does have plenty of capacity, versatility and potential.
A typical brick built BBQ will have two sides and a back of bricks with an open front. Metal prongs will be positioned about two thirds up to support the charcoal tray and other metal prongs (two on either side) near the top to support the wire trays for cooking; possibly set at different levels so that you can raise and lower the wire trays as appropriate. The lower two thirds of the brick BBQ is redundant space, even when using it as a BBQ. Previously we kept a potted tree in this space when not in use to make it more aesthetic, and when the space is needed for a BBQ we simply move the potted plant to a corner of the patio. Now, with a few simple modifications, and for little cost, I can make further use of the space when not needed for garden parties by utilising it as an effective cold frame.
A brick built BBQ generally is only used during warm weather; a common use of cold frames is to protect plants germinated in a greenhouse from frosts and cold nights while they are hardened off so that they become accustomed to the outside before planting them in their final position in the open garden.
The modification to transform your brick built BBQ is simple enough, the only elements required is a front and a top; the top should be clear to let light and heat from the sun during the day while keeping the warmth in and protecting the plants from frost and the cold at night. Ideally the front should also be glass although as the plants being hardened-off will be in the cold frame for just a week or two it's not so critical and a piece of exterior plywood would suffice.
Obviously you don't want a permanent cold frame structure; you'll want a simple construction that can be removed easily and quickly at a moment's notice in the event of deciding to throw a BBQ.
Simple and Effective
My solution was to:-
- Use the top of an old glass front door that was salvaged wood and glass that may otherwise never be used, and
- Make a wooden frame to hold a piece of glass for the top.
Whether you use a piece of exterior plywood or make your own simple wooden frame to fit glass for the front fitting it to the front of the BBQ simply, so that it can be easily removed for access and at a moment's notice if you decide to have a BBQ, couldn't be easier. I just screwed a hook on either side of the front piece near the top and using wall plugs screwed corresponding eyes into the brick sides of the BBQ.
For the top I knocked together a simple frame and bought a piece of plate glass to fit. I used bits of spare decking I had as the wood for the window frame, specifically because it's pressured treated and isn't going to rot quickly. I cut the decking into two inch strips, routed out a rebate for the glass; and once the frame had been knocked together and glazed held the glass in position with beading.
To obtain a slight slope to let in more light and allow rain runoff I used the same decking wood to make two long wedges; and to allow excess rain to drain away at the front I cut a couple of drainage channels in the front, one on either side.
As the top is a simple light weight window frame that isn't structural in anyway rather than fitting it using joints (as I would normally do) I just simply butt joined it and fixed it with exterior wood glue and decking screws.
For Ventilation and Warmth
As you will see from the photos I've utilised the metal prongs that at other times holds the charcoal tray as the support for the glazed top of the cold frame; this provides an open position that gives more ventilation during the day, and keeps the cold frame cooler at night as the final phase of hardening off the plants as you are getting them acclimatised to the outside environment before finally planting them out.
However, when you first transfer you plants from the greenhouse to the cold frame, and on cooler days, you need to keep the cold frame closed to keep the warmth in and the frost out. This I achieved by fixing more side brackets to the BBQ side walls at the same height as the top of my front piece; so that I can lower the glazed top into a closed position when required. I made the brackets myself from a spare bit of aluminium angle I had in my shed; I cut four pieces off the end of the aluminium angle as brackets and then drilled two holes in each so that I could securely fix them to the brickwork on the inside of the BBQ structure. Using wood to make the supports isn't an option because of the risk of the wood burning next time you have a BBQ.
Optionally, depending on the weather and on how hardy the plants are, during the day and on warm nights you also have the choice to keep the top glazed window in position and leaving off the front as part of acclimatising your plants to the outside.
Cold Frame Fittings - Simple is BestClick thumbnail to view full-size
Doubling your Usable Space
With this design (as demonstrated with the photos below) in early summer once the danger of frost has past, and dependant on how hardy your plants are, you have the option of placing a spare wire tray (normally used for barbequing your food) in the lower supports to create an extra shelf for plants and positioning the glazed framed window in the higher position for some protection from the elements. And if you wanted to add yet another wire shelf the glazed top will comfortably sit on the very top wire supports providing ample space for the third shelf underneath. Obviously extending its use in this way may mean making a slightly more elaborate front that’s adjustable in height e.g. two sliding glazed panels so that the top half could be raised or lowered and then easily be pinned in position to the desired height.
To adapt this idea further, rather than a sliding panel that was adjustable in height you could make a hinged flap (or flaps) to fit on the front that could be lifted up and hooked into place when needed or make a larger front piece from exterior plywood (or as a glazed window frame) that's tailored so that the whole of the front of the cold frame is enclosed.
However, in my case I’ve kept the front simple, as demonstrated in the photos, because in the spring and early summer when frost is likely I only need to harden-off those plants that should be in the garden early in the season such as broad beans and runner beans. When a few weeks later I bring on other plants, such as brassicas, the danger of frost has past and keeping the top part of the front open for ventilation day and night isn’t so critical if you’re transferring plants from an unheated greenhouse to the cold frame. Obviously if you’re moving plants from a heated greenhouse to acclimatise them in the cold frame to outside conditions then keeping the whole front closed up at night is going to be more critical.
Vegetables vs. Flowers
With an English Cottage Garden you can have it all, fruit, veg, herbs and flowers all grown together which looks attractive while also be functional.
However, since the separation of flower from veg in the Victorian Gardens most gardeners these days have a preference for food or beauty in their garden; putting greater emphases on one or the other.