Subtitle: So you want to build a greenhouse. Here’s what you need to know.

I built my first greenhouse in the mid 2000’s when we were living outside Durango Colorado. It was a DIY design based on using a wood frame, PVC pipe for the arches, and plastic sheeting to cover. It probably cost me less than $200 with new materials. I had wanted a greenhouse for almost 20 years at that point, but could not afford to buy a kit. When we got it up, it was early spring in Durango, an area that is almost 7,000 feet in elevation, and has a last frost date in early June! I had tomato seedlings started in my sunny laundry room, and was SO excited to now have a greenhouse I could transfer them to.

I lovingly put them all into the sunny warm greenhouse. The next morning, everything had frozen solid and all of my seedlings were very very dead.

The only picture I could find of the Colorado greenhouse (which was really just a hoop house). This was its second location. Our garden was fully enclosed, as we had LOTS of wind and LOTS of deer pressure.

I learned a super important lesson that day. I had assumed that a thin piece of plastic would somehow magically keep my greenhouse above freezing at night. I’m sure our night time temperatures were still in the low to mid 20’s. In reality, that thin piece of plastic did almost nothing, MAYBE giving me a 3 degrees above ambient temperature bump.

Our current 2014 Harbor Freight greenhouse, in April 2020. Weather station to the right, on the post.

So, if you want to build a greenhouse, here’s what I now know.

  • Build as big as you can afford. A larger trapped air mass inside the greenhouse means smaller temperature swings. Those 18″ x 36″ by 99″ covers that fit over a wire shelf system? That’s a TINY air space. It’s gonna be 120 degrees in there in about 5 minutes in full sun and basically the same temperature as outside at night. AND its going to blow over in the wind.
This is not a greenhouse. It’s mostly a waste of time.
  • Having mass helps. I keep three 55 gallon drums of water in my greenhouse as a “heat sink”. This water slowly absorbs heat during the day and slowly releases it at night.
  • Having trapped air in the covering helps. Using twinwall polycarbonate (basically a corrugated clear plastic) or a double layer of plastic with an air gap between can help quite a bit with temperature swings.
  • If possible, plan for electricity and water in your greenhouse before the build. Being able to put in a small space heater on cold nights can save you from moving a lot of flats to a more sheltered place during mercurial spring weather.
  • Its well worth investing in a min/max thermometer or an electronic temperature gauge that can send an alert to your phone when the temperature drops to a worrisome point. Knowing EXACTLY what the temperature actually is in the space over time helps a lot with planning.
  • If you have the space and equipment, a “pit” greenhouse built down into the earth can go a long way to mitigate extreme temperature fluctuations. I’ve wanted one for 20 years, but have yet to build it.
  • You’re gonna need air circulation and a way to cool off the space on sunny days. Your closed greenhouse can reach over 120 degrees surprisingly fast. The more vents and the larger the door the better. Automatic vents may seem expensive, but are worth their weight in gold over time.
  • Wind is your enemy. Build with your prevailing wind direction in mind, and do your very best to keep wind OUT of the inside of your greenhouse unless you can allow it to flow THROUGH the house. Otherwise, you’ve just built a giant kite, or the pressure on the panels will blow them off. Anchor the structure to the ground more than you think you’ll need.
  • Snow can take down a house quickly. Plan to build for strength and remove snow load regularly if you live in snowy conditions.

Our greenhouse is a Harbor Freight 10 x 12 ft greenhouse we erected in January 2014. It was a good compromise between the real high tunnel greenhouse I really want (which would be a minimum of $3,000) and the too small DIY greenhouse I’d already had. The Harbor Freight 10 x 12 runs about $850, but are often on sale in the spring and/or can be reduced in price with coupons. I think we paid about $600 for ours. Note: Harbor Freight makes this same greenhouse in a 6 x 8 ft version. Unless you are really short on space, spend the extra money and get the larger size, for the reasons I outlined above.

This greenhouse is decent. It’s MUCH better if you take the advice of this multi-part blog post and reinforce the bejesus out of it. Using about twice as many clips as normal on the panels can make a huge difference in wind (I ordered an extra bag). We probably spent an additional $200 on reinforcing supplies, including cross bracing. Other than needing to replace the roof panels this year (they were yellow and literally disintegrating from UV exposure), its held up well. I have two stacked benches on the north side and the east side, and a single bench on the south side. I can fit about fifty seven 1020 flats in there, though the lower back of the stacked benches are somewhat shaded from the plants above them. We designed the benches based on the length/width of a 1020 flat.

We replaced the roof panels this year. The original panels are 4 mm thick, and supposedly you can order replacements directly from Harbor Freight. But the replacement panels I ordered two years ago were the wrong size (they did allow me to return them). A friend with the same greenhouse has had better success and Harbor Freight had no explanation for why the panels they sent me were less than 23.5″ wide (ALL of the side and roof panels on the greenhouse are that same width). I suspect a manufacturing screw-up that no one noticed.

Before and after roof panel replacement

After that frustrating experience, we decided to just bite the financial bullet (which is NOT insignificant) and use this 6 mm poly from Home Depot instead. We’re hoping the thicker twinwall will hold up better and give us an insulation boost as well. Note: you can’t easily use the 6 mm in the window/vent panels, as the channel they fit in is only 4 mm thick. Our next project will be replacing the door (which is truly a frustrating and terrible design – it sticks and leaks air/wind) with a framed out storm door. And then we’ll likely slowly replace the wall panels as needed. But we expect to get many more years out of this small greenhouse.

© 2021 Miles Away Farm, where we’re miles away from having the greenhouse of our dreams (that’s at Frog Hollow Farm, lol) but are SO grateful for the season extension this greenhouse allows us in the spring. Check my online store (“Online Ordering” link under the header at the top of the page) for current available plant starts. I should have a good number of tomatoes and peppers toward the end of April.