I never expected that I would have a greenhouse. And I certainly didn’t intend to build one last year when I set out to replace my disintegrating cold frame.
The cold frame was a simple structure made of wood, with four short walls, the back being a foot or so higher than the front, topped with two very large and very heavy glass windows as a lid. My father built it for me about ten years ago and it was an extremely useful, if awkward to access, spot to harden off my tomato seedlings and protect other annual plants while they waited for the weather to warm up sufficiently. But it had reached the end of its life and needed to be replaced.
I decided that my fall project would be to build a new one. I started sourcing supplies online, and in looking for something lighter weight for the transparent lid, started researching polycarbonate panels which are a type of corrugated plastic that is lightweight, strong, and a good transmitter of sunlight. One thing led to another and I took a look, for the first time, at greenhouses, built from kits and with polycarbonate panels instead of glass. I was pleasantly surprised at the price, having always assumed that even a small greenhouse would be three times the price.
I let the idea sit for a few days and stared at the garden to figure out whether I really could make space for a 6′ x 8′ greenhouse. The old cold frame was about 6′ x 3′, and I realized that if I reconfigured the paths and some parts of the vegetable garden I could indeed fit a 6′ x 8′ greenhouse without giving up too much of the vegetable garden. Replacing the cold frame would cost close to $300 (all prices in Canadian dollars), the greenhouse kit about $1000. The greenhouse would offer me a longer growing season and way more space, hmm. The decision was made when (and this is going to seem unrelated, to bear with me) I switched car insurance companies and discovered that my new annual premium was almost exactly the same price as the greenhouse kit, including tax. It was surely a sign. I ordered the greenhouse kit.
Preparing the Foundation
After clearing out the remnants of the cold frame, I, along with my husband who was helping on this project, did a lot of shoveling to rejig the location of a path and relocate garden beds. The cold frame had sat in a depression, so the ground needed to be leveled out and then it was all topped with a thick layer, close to a cubic yard, of fine gravel.
On top of the gravel, I laid out 12″ x 12″ patio stones to form the floor of the greenhouse.
Then I used 4″ x 4″ pressure treated cedar lumber to build a frame on which to anchor the greenhouse.
Building the Greenhouse
Assembling the greenhouse from the kit was a lot like assembling a piece of IKEA furniture, just a lot bigger. The structure for the walls went up first.
Then polycarbonate panels were inserted into the wall slots, the roof pieces went on, and then the door was assembled and attached. Ta-da!
I hesitated to post this photo, because the foreground “features” my dead tomato plants. Not a good look for a garden blogger! My only defense is that it was late October and the tomato plants had reached the end of their lives. When I took this photo I was so excited that the greenhouse was assembled that I didn’t even notice until I looked at this photo much later.
Once I did clean up the dead vegetable plants the next photo I took was the greenhouse looking lovely in the snow:
Doesn’t it look cozy? I’ve discovered that once the sun comes out and it starts to warm up, this tidy little greenhouse self-cleans the snow off its roof:
Now if only I could figure out how to get the driveway to do the same thing.
If you’re wondering about how long it took to get the greenhouse put together, it was one weekend to get the site prepared and lay the stones, and another weekend to assemble the greenhouse itself.
I was concerned that the greenhouse would feel cramped inside, but it is fine. Because the frame is elevated on 4″ x 4″‘s there is good headroom inside.
Monitoring and Modulating Temperature
The greenhouse has been a lovely place to sit for a bit on sunny afternoons this winter while we’ve been in lockdown. When there is no sun, the temperature inside is exactly as cold as the temperature outside, but as soon as there’s a little bit of sun it heats right up. While that is fantastic now, it is something I am going to need to keep a close eye on in the spring.
To keep track of the temperature I purchased a digital thermometer that connects to an app on my phone.
I haven’t this thermometer* long enough to vouch for its accuracy, but I liked this model because it shows the highest and lowest temperatures since the last time the reset button was pressed. The app on my phone lets me collect and export all of the data, and enables me to check the greenhouse temperature from inside my house (note that this is through Bluetooth, which means the phone has to be within range of the thermometer. You can’t just log in and check it remotely. There is no Wifi connectivity). It is also a hygrometer to show humidity.
I think the most important piece of equipment I purchased for the greenhouse is a vent that opens automatically* when the greenhouse starts to get too warm. It would be so easy to forget to open the door on a warm day and fry everything inside.
As I understand it, the opening arm has oil in the shaft. When a certain temperature is reached, the oil becomes more liquid and releases the shaft, opening the window. When the weather cools down, the oil solidifies and the window closes.
This device was an optional add-on from the base greenhouse kit, but I think it’s essential. Keeping the greenhouse ventilated on hot spring days is going to be as important as keeping it above freezing on cold spring nights.
I do not intend to heat the greenhouse year round, but I do want to be able to put plants in it in early spring and into late fall, so I will be adding a heat source for cold nights. I have not quite worked out what to use, but I know that even a string of old incandescent Christmas lights can generate enough heat to keep a small space above freezing if that space is sealed up sufficiently.
The back and east side walls of the greenhouse are positioned against the wall of my shed and the wooden fence that marks the edge of my lot. Ideally, a greenhouse should have full sun exposure on three sides, but this was the only space I had available in the garden.
Since no light can come in those walls anyway, I have lined them with bubble wrap (a trick I’ve seen UK gardeners demonstrating on TV) as insulation, and also layered a reflective mylar emergency blanket* on top of the bubble wrap on the east wall to bounce more light and heat back in.
A greenhouse needs shelving of some sort to sit the plants on. I considered building shelves out of wood, but decided to use commercial baker’s racks* instead, because they were easy to assemble, cost-effective, and able to support a lot of weight (wet soil is heavy!)
I bought 6′ tall units that come with the support poles in two pieces. You are meant to screw the pole pieces together, but I just used them at their 3′ length and made two sets of shelves out of each kit. I bought one kit with 4′ x 18″ shelves, and another with 3′ x 18″ shelves, and that all fits just right within the greenhouse. The 3′ long unit came with green powder coating on the metal, while the 4′ kit did not.
I Still Can’t Believe I Have a Greenhouse!
I am quite excited at how the greenhouse is taking shape. I can’t wait to start moving plants into it as we move from winter to spring. There will be lots to learn about growing in this new environment, but I am looking forward to it!
If you are thinking about a greenhouse of your own and and wondering which one I purchased, it was the 6′ x 8′ Palram Mythos*.
*Disclosure: some of the links on this page are affiliate links, meaning, at no additional cost to you, I will earn a commission if you click through and make a purchase.