Hi,
I have. 6x8' greenhouse which is anchored to my deck. We live in Toronto, Ontario Canada and winters are extremely cold here. Just wondering if there's anything you can suggest to give it a bit of warmth for germinating my spring seeds so I don't have to use every south facing window in my house?
I don't want or have the ability to have solar heat set up so not sure how I can make it warmer so it isn't just a storage house all winter long.
Any advise would be greatly appreciated…..

There are a few things you can do but none of them are likely to get you the temperatures you will need. Try improving the insulation by adding additional layers of glass/polycarbonate and/or traditional insulation along the lower walls. You can also create thermal mass by creating a wall using bricks, stones, earth*. You might add pavers or bricks as a layer on the floor. Water barrels work well if it doesn't get too cold. You can install a heater, either electric or fossil fuel powered.

But I don't think you will be satisfied with the results. Given your climate zone, your greenhouse is unlikely to achieve or maintain the temperatures needed to germinate most seeds during the spring. You might be able to get those temperatures with a really big heater and a lot of energy but that's not a wise approach.

I'm in a similar climate zone as you are. I germinate my seeds indoors in a home-made seedling rack outfitted with lights and heating pads. I start most of my seeds in March and April. Once the seedlings have developed into small plants I walk them out to my hoophouse every morning and bring them inside in the afternoon. That way they are able to take advantage of the sunlight and warmth in the hoophouse but still survive the nights. As the season progresses I can leave them out for longer periods. I've usually got cold weather crops in the ground in the hoophouse in early May and by late-May I can usually leave most everything else out overnight.





*If you decide to add thermal mass you'll need to remove it once the weather gets warm or your greenhouse will be too hot

No matter how much insulation you add, without a heat source your greenhouse is really no more than a really big cold frame/summer house. Heat sinks will never get you to 90 degrees in the snow.

I completely agree with Henderman - your seedlings will do much better under fluorescent lights (or other grow lights) than they will on a windowsill; then when the weather starts to warm up a bit they will do great in your greenhouse. (And then when the weather warms up even more, you can plant them outside.)

Henderman gave you some great pointers. We used to have one rainbarrel cut the long way and it was heated with heater coils in the sand topping to start seeds. You can use bubble wrap on the inside walls for installation. We also
tarp ours to keep the heat in beside a rainbarrel. Small heaters with thermostat and one for the house so you can monitor the temps. Do you have a spare room where you can set up a place for seeds? We use LED lights in both our greenhouses. One we keep at 60 and the other 70 for the tropicals.

Agree that it's not a big deal to lightly heat a small space on various nights that dip below freezing in a Z8 setting, but months of temps that are well below freezing would require exponentially more expense, and efforts regarding insulation as mentioned. It will be warmer in there on sunny days, but at night, it's going to get as cold in there as if the greenhouse was not there. When I use my little shed as a greenhouse, I put an electric oil heater in there (looks like an old-school wall radiator, full of oil that is heated) and that works very well, but I am only needing to keep the temp about 10-15° above the outside temp on the nights when it is needed at all, and I am able to turn it off when I leave for work. Looks like it will be in service again this winter. I've gone and collected a whole new crop of thirsty tropicals to replace the ones I lost the past couple of summers from not watering often enough. Last winter I was able to squeeze the down-sized collection into the house.

I certainly wouldn't put water barrels in an exposed, unheated greenhouse in winter. And it might not be a great idea to pile a lot of extra weight on a deck - don't know what the support structure is.
One thing you can do cheaply to add a couple of weeks to your growing season is make a mini-greenhouse inside the greenhouse. Put a sheet of foam insulation in the bottom of the tent, install a light fixture and a heating pad. For most seeds to germinate, you do need the heating pad, at least for the night. If you're lucky, sun will give you enough heat in the daytime. Keep the seedlings well watered.


It doesn't need to be this big - it's my nursery. The hatchery is three bookshelves in the dining room.

If you organize the space well, you can add up to a month to the fall, as well. Keep some plants in portable containers on or near the deck and bring them into the greenhouse before September frost. You can rig up a temporary plastic tent for them, too.

Thanks to everyone who replied. I really don't mind starting all my seeds indoors just seems a waste to leave it empty during our 5 cold weather months. Last year I moved some herbs and a pot of strawberries and Egyptian walking onions in the greenhouse and covered everything with straw and row covers. In the spring everything was green and ready to go so perhaps I will add to this and try other plants

Sounds like a plan! I've got to get me some of those Egyptian walking onions - they look very cool. Any good for eating?

I'm not sure why Errands2000 bothered to move the walking onions to the greenhouse as they're hardy to zone 3.

They're top set onions - lots of bulbils grow where the flower should be. The stem falls over from the weight of the bulbils and they root where they land. They can cover a large area rather quickly if you let them. Eat the bulbils, any you don't pick will root and grow more plants. They have a garlic onion flavor.