I remember getting my first large greenhouse years ago when I was a new gardener. At the time I didn’t think much about heat. It was a Valentine’s Day present and I was anxious to get it put up so I could start using it. Once it was up I realized it was cold inside the greenhouse – too cold to put my indoor plants into – and I was so disappointed – so the greenhouse sat empty. Then we found a wood burner and installed it. That did the job as long as we kept it stocked with wood.
Then I heard about cold frames – and what is a large greenhouse really but a huge cold frame – or a high tunnel. It took years before I moved on and realized that I could use an unheated greenhouse for so much more than hardening off seedlings in the spring or keeping semi-tender plants in during late spring when there was still a risk of frost outdoors. Now I use my unheated greenhouse all the time – and have more than one.
I am sure most of you are familiar with winter seed starting. Well, guess what? A greenhouse works for that. You can use your milk jugs, pop containers or whatever you like or opt to use flats – just keep an eye on the soil moisture. I also use mine to protect hardy plants I did not get planted in the yard the previous season. I do not heat it.
During the frozen winter months, the seeds basically sit dormant just like they would in regular winter sowing containers. Once the temperature in the greenhouse begins to warm up those seeds sprout. Flats in a greenhouse do dry up fairly quickly. I generally check on mine two to three times daily once April arrives. The size of the growing medium really determines how quickly the seedlings can dry out. I use a lot of peat pots and peat based soils so this can be a problem for me. This is why I keep such a close watch on my greenhouses.
Some people install an automatic watering system in their unheated greenhouses once the temperature is above freezing but I do not do this. I use solid bottom flats and keep a little bit of water in the bottom so the soil blocks or peat pots do not dry out.
Another use is to grow a winter garden in them. I do this in one of mine. We set up raised beds on either side of it going the long direction. I then filled them up with good soil and plant in late summer. By fall I have a pretty good crop that I protect with a layer of frost cover and a layer of 6 mil plastic – making sure none of this touches my plants. I do have to water these plants during the winter – and that can be a pain – but the taste of fresh vegetables right from your own garden sure makes up for it.
What do you do with your unheated greenhouse during the winter months? Please comment below.