Simple, Low-Tech Trick That Keeps Your Greenhouse Warm (and Cool)

Reader Contribution by David Goodman
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When I first moved to North Florida and hit winter for the first time, I realized I was no longer in the land of balmy beaches and 60-degree overnight lows in December. The first frosts hit us in November, then we had a couple of freezes (including one that dropped into the teens overnight) which made short work of the papaya, mangoes and other tropical plants I had lovingly planted in the yard the previous summer.

I know, “cry me a river,” you Northerners are thinking. “It’s 30 below here!” Sorry about that. No really. I feel terrible. In fact, there’s a tiny violin playing behind me.

This post may not help you all that much if you’re dealing with those kind of extremes, but the concept may still prove valuable with some tweaking.

If you live in about USDA zone 8 south, having a simple single-layer greenhouse can get you through the winter easily. Here’s the one I bought a couple of years ago. Simple and it works. I suppose I could have built one – but I’m pretty sure I couldn’t have done it for the price I paid for that model. Anyhow – back to my story.

When I set up the greenhouse for the first time, I was excited to feel how hot it got inside as soon as I zipped the doors and windows shut. I loaded it up with all my cold-sensitve plants, then put a thermometer inside so I could keep an eye on the temperature.

Within the next few days, there was a frost predicted. The night of the frost, I shut up the greenhouse and went inside. Late at night I went out to check the thermometer. It was almost as cold inside the greenhouse as it was outside! That wasn’t good. I quickly strung up a bunch of Christmas lights, plus a few flood lamps, then plugged it all in via an extension cord.

The next morning when I went outside after breakfast to open up the greenhouse, the sun was shining brightly and it was quite warm when I stepped inside. Too warm. I checked the thermometer – it was already reading in the 90s. Uh-oh. I needed to really stay on top of my venting or I was going to cook something! So I did, diligently shutting the greenhouse on chilly nights and quickly opening it again in the morning once things had warmed up.

It came to me then. I remembered reading that some folk in tough climates used walls of water-filled barrels to absorb the heat of the sun during the day. At night, the warmth would radiate out from the barrels and keep things moderate inside. I decided to try it in my greenhouse. A friend of mine had a source for 55-gallon plastic drums so I bought eight of them and placed them at regular spacings along both walls of the greenhouse, then filled them to the top with water and screwed the bungs closed. Once I did this, the temperature at night quit dropping as close to freezing … which is what I’d hoped for.

But something else strange happened as well. One Sunday not long after putting the drums in the greenhouse, I went to church and forgot to open the greenhouse before I left. After lunch, my family and I went to some friends’ house for lunch … and half-way through lunch I remembered the greenhouse. “Oh no! They’ll all be cooked!” I exclaimed. That got me some weird looks. Excusing ourselves, we raced home and I nervously opened the greenhouse, expected to be greeted by the odor of steamed papaya, blanched mangoes and roasted canna lilies. Instead, the temperature was in the 80s. Quite pleasant, despite the sun high above.

At that point I realized: the barrels worked both ways. I no longer needed to freak out over unzipping the greenhouse during the day … and I didn’t have to worry about it freezing overnight. If your freezes don’t last too long, try it. You’ll be amazed. When it comes to frost protection, thermal mass is your friend. (By the way: I recently did a little video on 4 different ways you can protect your plants from a frost: click here and check it out. I’m wearing a cool hat.) The main thing I like about the water barrel method? It’s totally low-tech. I hate dealing with electricity, propane, gadgets and wires. The simpler, the better. Plus I’m using a readily available source of free energy: the sun.

Of course, if you hit 30 below … you’re on your own.

For daily organic gardening inspiration and lots of tips on growing food in tough times, visit Also – if you’re interested in extreme composting, permaculture, biodiesel and other preparedness topics, be sure to sign up to hear David and other experts speak at the free online Survival Summit taking place January 20-26th.