Cheap and Easy Heating for Your Greenhouse

I tried all kinds of ways to cheaply heat my greenhouse, the simplest way turned out to be the best

A passive heater made from a trashcan filled with water

My greenhouse is a DIY job. Basically a 10ft x17ft wood frame enclosed with clear polycarbonate panels. For my Zone 9 climate it works pretty well. In the summer I cover it with 70% shade cloth to keep temperatures below 100° F. In the winter I remove the shade cloth and it stays at a comfortable 70°-80° F even on cold, cloudy winter days.

Where I run into problems is in the winter nights. The greenhouse sits near the bottom of the west side of a hill, so cold air tends to sink down the slope and collect there. On particularly cold mornings the temperature at dawn uphill where my house is might be in the low 40s, but down-slope at the greenhouse it’s in the upper 20s, which is too cold for all but a few of the plants and seedlings I keep in there.

Trial and Error in Search of Heat

At first I tried adding better insulation to the greenhouse, lining the whole thing on the inside with bubble wrap. Not only did that not help store any additional heat, but it also provided plenty of extra nooks and crannies for insects to hide in. So that was out.

Bubblewrap insulation doesn’t work that well

Then I searched for simple heating options, but they proved to be limited. Outside of a solar panel that provides enough power for seed mats, a circulation fan, and the chickencam, I don’t have any electricity down there, so an electric heater large enough to warm the space is out of the question. I tried a small camp-style gas heater, but running it all night proved to be expensive (propane is not cheap here in California).

I also tried the terra cotta pot heater technique I’ve seen in various places, but, even with really big pots, it simply doesn’t generate much heat more than a few inches away. (Not to mention that leaving an open flame burning unattended in a wood-framed structure seems to asking for problems.)

Eventually, I started investigating various forms of passive heating where you have something large that will absorb heat during the day and radiate it back at night. I tried all sorts of systems that use stone, brick, even old beer cans, but couldn’t find anything that would kick out heat all night without being as large as the greenhouse itself.

The Solution is in the Water

Then I came across an article about a passive solar greenhouse where they used a bank of 55 gallon drums filled with water to store heat during the day and kick it back out at night. It turns out that water has one of the highest heat holding capacities of any common material.

Heat capacity per Volume (btu/ft³/F)

Now, I don’t have the room in my greenhouse for one 55 gallon drum, never mind an entire wall of them, so that wasn’t an option. But what I did have was a couple of 30-gallon plastic trash cans with lids I was using to store extra soil and various greenhouse junk I didn’t want laying out.

These fit neatly in the spaces between the potting benches on sides of the greenhouse and the low shelf at the end, so they were near the plant trays, but they weren’t taking up floorspace. I emptied these bins of their contents, refilled them with water, snapped the lids on and waited to see what happened.

And, sure enough, it worked! It turns out that water is really good at holding heat – better than stone, rock, or most any other cheap and easily available material.

Now, a couple of trashcans filled with water doesn’t keep the greenhouse a balmy 70 degrees all night, but it does keep the areas within a couple of feet of them 4 to 5 degree warmer than the surrounding air, which is more than enough to protect seedlings and cold-sensitive plants from frost until the sun comes out in the morning.

As a bonus, using a cheap hand pump from the hardware store and some leftover hose, I also fashioned a simple gravity fed system that automatically refills the water barrels in the greenhouse from the rain barrels outside, so I’ve got an on-demand source of water as well as heat.

Black bag over the top helps the water heat

Water in the can (hand pump back left)

So, if you’re looking for a cheap and simple way to keep your greenhouse a little warmer at night, skip the expensive heating systems and go with a trashcan (or two) full of water. You’ll be glad you did!

And if you have tried this or have another simple, low cost solution for heating a greenhouse, make sure to share it in the comments below. ↓