Will a Greenhouse Protect My Plants in Winter? (how cold does my greenhouse get without a heater?)

This was THE biggest question that I had when I first started thinking about building a greenhouse, and it’s one of the most common questions

that I get asked now. We all want to know the what we can and can’t do with a greenhouse. What are the limits?

To get ready for my first winter with the greenhouse, I filled a black 55 gallon barrel with water and put it in the sunniest corner along with some high hopes. (I had read that letting the sun heat up a large barrel of water could help keep a greenhouse warm after the sun goes down.)

Now, I knew that I lived in a high-cloud cover area and that our winters are really gray, and that my greenhouse is on the north side of my house, buuut, I didn’t fully grasp how little sun reached the greenhouse in the middle of winter until that first year.

Every now and then I would take the lid off and reach down to touch the water. It was cold. No warmth at all. It was not getting near enough sun to warm it up.

The thing with gardening though, is that your experience might be different depending on your climate and how much sun you get.

When wondering how cold your greenhouse might get, it comes to down to this: when there is no sun, the greenhouse temperature will eventually match the outside temperature.

If you can prolong the greenhouse warmth for a few more hours after sunset, then that may give you some options with your plants. And that might be possible if you get consistent sun from day to day, or if you’re in a zone that doesn’t get too cold.

Gardening is an experiment, so if you’re looking to extend warmth in the greenhouse and slow the nighttime cooling, here are some things to try:

• Fill a large, black 55 gallon barrel with water; if the sun can warm the water enough, it will provide some warmth to the area right next to the barrel; painting it black helps it absorb and retain the more effectively

• Similarly, fill several 1 gallon jugs (painted black) with water; plan to have several so you get the advantage of mass (just 1 or 2 jugs will cool down too quickly to do anything)

• Insulate your greenhouse with bubblewrap all around the sides and ceiling; consider 2 layers!; this will keep the warmth inside for a little longer

• If you’re growing kale or other cold-weather crops, add another layer of bubble wrap immediately around their bed or pot

• If you’re trying to protect trees or other plants, wrap their pots and trunks in bubble wrap.

All of those ideas are for preserving existing heat. If you want to try generating some heat in the greenhouse, consider putting a compost bin in the sunniest corner!

If you’re feeling a bit disappointed at this information, and were hoping you could keep your greenhouse a little warmer, don’t forget about these awesome greenhouse benefits: it provides protection against harsh wind and protection from frost. These two things usually mean you can extend your Fall growing season a few weeks longer, and can start your Spring growing season a few weeks earlier.

If you’re looking to grow food during winter, some folks do grow kale and a few other cold-

hardy vegetables, even in colder zones. You’ll need multiple layers of insulation/protection and a decent amount of sun.

I’ll end this post by describing the absolute best thing about a winter greenhouse (at least for me!) On Sunny days, the greenhouse will warm up nicely during the day - often around 20-30 degrees. So when it’s 40F/4C outside, it can be 70F/21C inside! It feels amazing in the middle of winter.