While it’s not that difficult to make a contraption that can serve as a rudimentary greenhouse, it’s quite another issue altogether to create a decent-sized growing area that won’t rip, tear, blow away or break under heavy snow conditions.
This season, I could not afford an actual greenhouse, but I was able to cobble together some parts that anyone can get a hold of and make a “greenhouse” that can grow about 30 kale plants, is separated from the ground, and can withstand heavy wind and snow.
The best greenhouse I’ve ever built for almost no money at all is a circular metal ring (cut from a grain hopper) that was 10 feet in diameter. I filled it with dirt as high as I could (nearly up to its 3-foot wall height) and then covered it with a tattered, old heavy plastic that was so beaten up, I had to double it. I supported it in the middle with a 2×4 plank hammered into the dirt, and secured it around the edges of the ring with a ratcheting strap.
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What is the Cheapest Way to Build a Greenhouse?
A very inexpensive version of a greenhouse can be built by stretching and securing at least one layer of medium-heavy translucent plastic sheeting over a frame of some kind typically made of plastic, metal or wood.
When a greenhouse is very small, it’s often called a cold frame and can be as small as a couple of square feet in area.
However, most people will not be totally satisfied with a cold frame given its limited growing space. However, it is quite possible to build multiple, larger cold frames to accommodate more growing scenarios.
Cold frames are relatively inexpensive, but not as inexpensive as my much larger “greenhouse”. I call it a greenhouse because it seems a bit big to classify it as a cold frame, and I can (and do) walk around in it.
Cold frames are most often built with glass or plexiglass and a frame to hold each panel of glass. This can take lots of time and skill to build, not to mention the cost of quality glass or hard plastic.
Another even less expensive way of making a greenhouse is to simply stretch some plastic (minimum of 4 mil but 6 mil is better) over a frame (most often bowed ribs of heavy wire or PVC plastic).
This method is typically used to cover a 4×6 or 4×8 foot raised garden bed. The problem with this option is that you will need to build a series of bases on each side of the garden and then engineer the arches to fit into those bases. You can also buy kits to help with the process, like the one below:
What’s the Easiest Way to Build a Greenhouse?
The easiest way to build a decent-sized greenhouse is to use a metal ring that is at least 8 feet in diameter. This would offer a growing area of 50 square feet which is the same as a rectangular garden of 5 x 10 feet. However, most large raised beds are no larger than 4 x 8 feet.
Even if you do use a rectangular bed, you’ll still need to find a way to suspend a plastic sheet over the growing area while offering enough room. You’ll also need to find a secure way of attaching the sheet to either the ground or the raised bed walls.
On the other hand, if you use a metal ring as I have, the advantages are numerous and significant when compared to rectangular raised beds.
Benefits of a Circular, Steel Raised-Bed
My circular metal raised bed offers me a few advantages that are not typically possible with any other inexpensive greenhouse build.
1 – If your ring is around 3′ tall (which is not difficult to procure) it will allow for a far greater variety of produce
Your raised ring bed is almost sure to be taller than a cedar square raised garden, and that means that you’ll be able to grow deep root vegetables like carrots and potatoes much more effectively than with a standard raised bed of 12 inches (which uses partially added soil and partially native soil beneath the bed).
2 – The ring (if taken from a re-purposed grain hopper – which is typical and recommended) will allow you to use a ratcheting strap to firmly secure the plastic sheet even in gale-force winds
A ratcheting strap is not a viable option for securing a plastic sheet around a square or rectangular garden (or any other shape other than round or perhaps oval).
3 – A round bed will allow you to use a simple, single support to keep the plastic off of your plants
Unlike a rectangular bed which necessarily needs a series of durable hoops to hold the plastic off the plants, a round bed only really needs one in the center.
It’s quite possible to build a more elaborate system of several crossing arches to support the plastic on a circular bed, but it’s a lot easier to do what I did.
I simply hammered a metal fence post into the center of the bed. Then I attached (with screws) a 4-foot long 2×4 plank to the metal post so that the 2×4 protruded higher than the metal post.
That was all I needed, but because we have very strong winds, I decided to use 3 more 2×4 scraps to support the center post by just jamming the into the side of the bed and against the center post.
I then capped the 2×4 with another section of 2×4 (about 6 inches long) and rounded all the edges so they would not tear the overlaid plastic.
Then, I simply stretched 2 layers of plastic (because each layer was so badly worn through with holes) over the upright 2×4 and secured the edges with the ratcheting strap.
4 – A steel bed will last a minimum of 25 years, but more likely – 40 – 100 years!
Cedar is the only other good choice for a raised bed, but its lifespan is closer to 10 years, while anything else like cinder blocks or Home Depot pine/fir framing lumber, will last only 1 – 3 years (at most) before rotting or cracking (especially in a cold, northern climate).
5 – Steel rings are already made – no construction required!
Having a ready-made raised bed can be challenging to transport (though it’s very possible with a 5×8 trailer)
6 – incredible growing area
In my 8-foot raised bed, I can plant over 300 cloves of garlic (assuming a growing area of about the size of a man’s fist). In Ontario money, that’s well over $1000 worth of garlic! Yes, my garlic is very large (heads are about 3″ – 3.5″ in diameter) and they sell for close to $5 a head in the local markets in Hamilton, Burlington and Oakville, Ontario. Talk about a return on investment!!!
7 – Awesome value if you have a big trailer and an angle grinder
I recently found a couple of fully-functional grain hoppers on Kijiji, and while the initial price of $500 seems steep, consider this; the hopper was 25 feet tall which means it can offer you 8 rings. Each ring would be very sturdy, last forever and it would be 11.5′ in diameter. That gives you 824 square feet of growing area.
You can do the cutting yourself with an angle grinder (with steel cutter blade) if the thickness of the steel is less than 1/4″ which it is on any grain hopper I’ve seen.
Consider also that just 1 of the 8 rings can yield enough produce to pay for the whole thing! Look at point #6 above where I was able to grow over $1K of garlic in a ring that was only 8 feet in diameter. Imagine what 8 rings that are 11.5 feet in diameter could produce!
Pitfalls of using a round, steel raised bed Greenhouse
Creating a raised bed round, steel greenhouse is not a perfect solution for either a raised bed garden or a greenhouse.
The biggest problem with this option is finding a round, metal ring. I was able to easily find one on Canada’s version of Craigslist (Kijiji) and on Facebook Marketplace. However, because it is a specialized item made from cut sections of a decommissioned grain hopper, it may not always be available for purchase secondhand.
The next issue is that because it is sure to be taller than any normal raised bed garden, you’ll need more soil to make it useable. While it’s not technically necessary to add more soil than a normal raised bed garden, it is recommended for the sake of easy use of the garden and for the ability to grow plants or vegetables with a large root system.
I can’t finish this section without pointing out that it’s quite possible you’ll pay more for your ring(s) if it’s new. I prefer used whenever possible because it will still outlast me!
Unless you devise an easy way to access the inside of the greenhouse, you’ll have to loosen the ratchet straps and peel back the plastic each time you want to harvest a few kale leaves!
As far as actual cost, it will vary based on a lot of factors, but consider my situation; I live in Ontario, Canada where everything is more expensive (other than health care out-of-pocket expenses) than it is in the U.S.
I paid $50 for 2 large steel rings that measured 8 feet in diameter. That cost INCLUDED DELIVERY from a farm located about 15 miles away. In just one ring, I grew over $1200 of garlic in one season. I’d say that’s a good return on investment eh?
Another simple and quick option (though much smaller) is a fire ring (pictured below). This one costs $70 but it’s 4 feet across. That’s not a bad deal and it won’t even be a full growing season in the Southern U.S. before the cost of the vegetables you grow yourself will exceed the price you paid for the ring and the dirt!
My DIY Greenhouse / Cold Frame on the cheap!
I filled my 3′ tall grain-hopper cut ring with about 2.5 feet of good growing soil. I mixed in vermiculite to add volume so it’s almost full to the top. By the way, I have a full garden of both lettuce and kale in January … IN CANADA!
Can a Greenhouse Stay Warm in Winter?
A greenhouse absolutely can stay significantly warmer in the Winter than the environment outside the greenhouse. In fact, even with my inexpensive, small, unheated greenhouse cover (with holes), I was able to keep it at least 15 degrees warmer but as much as 30 degrees warmer on warm, windless, sunny winter days.
Having said this, please note that in the dead of night with no sun shining and no wind blowing, it’s only a couple of degrees warmer inside than out.
However, I’m speaking just about my junky, cheap unit. If you get an actual small greenhouse, you’ll have it significantly warmer than outside, especially if you add a heater for a few hours a day – but that’s a whole other article!
Is An Unheated Greenhouse Worth it?
An unheated greenhouse can still keep the temperatures anywhere from 5-10 degrees (F) warmer than outside. However, if you take a large pail (or 3) with a lid and paint it black, and fill it about 3/4 full of water, you can increase temperatures up to 25 degrees warmer than outside in the Winter.
The black buckets absorb sunlight which slightly heats the water. The slightly warmer water then releases heat in the middle of the night.
I call this an “unheated” greenhouse since it does not use any costly energy-sucking heating device. So, yes, an “unheated” greenhouse CAN be a HUGE advantage over planting outdoors or just giving up growing altogether in the Winter.
One last thing; if you double the plastic on your DIY steel ring greenhouse, you can vastly increase efficiency or warmth retention. I did not have a good way of separating my 2 layers, but if I did, that would help even more!
Greenhouse on the Cheap – Main Takeaways
Do your very best to get a hold of a used grain hopper or 2′-4′ tall sections of a round grain hopper bin for raised bed gardens.
A simple post in the center of the garden (3′-4′ tall) will support the plastic cover(s) and a ratcheting strap will secure the plastic to the garden bed.
This scenario will allow for a 5-10 degree difference (and way more in the direct sun) in the Winter and allow for leafy green vegetables to grow all year long!
If you can throw in a few black, water-filled pails into the greenhouse interior, you’ll further raise temperatures for an even heartier crop.
The TOTAL COST for me? $50 per 8-foot diameter and 3′ tall raised bed with greenhouse cover – including extra soil for the deep bed and the garden itself – delivered to my yard!