I wish I had a heated greenhouse. I start my seeds in my office, which has decent light and can be heated at night to help heat-loving seeds germinate and keep tender seedlings from freezing.

But at some point, the plants have to go to the greenhouse or they’ll get hopelessly leggy. Besides, there’s not enough room in the office for all my seedlings, once I really get going in spring.

The greenhouse is great for raising daytime temperatures for the plants and for protecting them from harsh wind. It also protects the plants from light frosts, but sometimes the temperature dips below zero at night, and then the unheated greenhouse can’t protect my plants enough.

If I know the temperature will dive, I can haul all the plants back to my office just for the night, but it’s quite a job—several trips with the wheelbarrow—and always results in some plants getting damaged.

So I’ve gone for passive solar heating in the greenhouse. I had my daughter paint empty 3-litre juice bottles black, and I filled them with water and placed them around the greenhouse. During the day, the water in the bottles heats up, and at night, the bottles slowly release their heat.

Having only one greenhouse, I haven’t been able to scientifically test whether my hot water bottles help, but last year—the first year I deployed the bottles—I was impressed by how well the plants weathered cold nights in the greenhouse. I intend to expand the number of bottles this year, and would love to ring the entire outer edge of the greenhouse with water bottles. If all goes well, I’ll end up with my heated greenhouse, without actually heating my greenhouse.