I live in a very different zone than you. I don't think that greenhouse would keep your plants warm enough even on your porch in a zone 3. Could be wrong, but that's pretty cold;). You say the winters reach -30C sometimes -40C, let's say the greenhouse on your porch is 10 to 15 degrees warmer. Even 20! That's still too cold for succulents. Generally I see people overwinter their succulents no lower than 7C give or take a degree. Unfortunately I just don't think the greenhouse or your porch will provide enough warmth.

Keep in mind I'm a wimp and live in zone 9b;). I know nothing about greenhouses, wish I had a big one though;). I know nothing about lights either.

It also depends on what succulents you have. Some are cold hardy and may be able to handle being in that little greenhouse on your porch, but honestly I would say no it's just to darn cold;).

You could do a test run and put the greenhouse out on your porch and put a digital temp sensor in it. I wonder if putting a blanket over would help. Idk I'm just thinking out loud now;).

What succulents were you thinking about putting in there?

Without some kind of heat source inside the mini-greenhouse I expect it won't be any warmer than your enclosed porch. Assuming the sun would shine in to the greenhouse then it might hold a bit of warmer air than the porch after the sun goes down but I expect by early morning it will be just as cold as the porch temperature.

That GH will not be enough. Maybe if temps are just touch below 0*C, but it may not be enough for some plants even then. I remember someone in warmer zone (maybe z5 ?) saying they put bottles with hot water in to keep it bit warmer, and that was few weeks ago.

I think that my GH here in the Seattle area is heavier weight, I have had a green house like that. Can any of your plants survive 30C, none of mine can.

It's not going to work unless you heat it. IF you do end up trying this, at least get a remote thermometer than you can read from indoors, so you can see when temps start to get too low and hopefully will have time to do something about it.

Kara... what do you do in winter?

i am also in 9B with a small inclosed porch. I had the thought of doing the mini greenhouse but a) I need a lateral shelf for all the plants to maximize light availability and b) I didn't think it would keep them warm enough.

i think what I'm going to try is insulating the fence somehow, placing greenhouse plastic overtop, and putting a space heater out there with an automatic timer.

i really don't want to drop the $$$ for an indoor light system, plus my outside-space restriction is the only thing keeping me from draining my bank account to 0$ with new succulents.

Gabby- You won't need lighting or even a heater. I do what your idea is:). Basically make a mini greenhouse with plastic. You just have to remember to uncover so things can breathe. If you don't, the plastic covering will start a whole greenhouse effect, and begin to grow more unwanted things;).

Very important to pay attention to the weather forecast. So you can prepare. For example today I will be putting everything away for winter. Today's the day booo:(. Tomorrow- Sunday it's suppose to rain and then drop to the low 40's at night. So I will keep everything covered for those three days. Ok I just looked at the weather forecast again and it's only suppose to rain on Saturday. Hmm it's always changing. Anyways I'm still going to put everything away. The nights are getting cool. So I will cover on Saturday and uncover Sunday morning. Then just cover at night. So make something that will be easy to cover and uncover. You will be doing the succulent shuffle for a few months. Covering and uncovering, covering and uncovering;). Just worry about the rain and cold nights. There may be some days where you won't have to cover because it may be warm enough at night an no rains. For my plants I will cover if the nights drop to the mid to low 40's, but if the low is like 47F I don't cover. They'll be fine especially if dry. I know they could handle lower, but that's my cutoff;).

You have lots of Haws and I was always under the impression they could handle the cold. If kept dry.

If pots are too big like my Firestick I will cover with sturdy garbage bags and clothes pins.

Well I'm off to move things to their winter home;). I will be bringing my IPad out with me if I have any questions about a plants hardiness I can look online.

These are what I will be moving most my plants on to. Then covering with plastic by tacking to the fence.

I will also squirrel away things in corners so they're protected by the house and cover on rainy days and or chilly nights.

This gets covered too:).

Some things do come inside like my Desert Roses, Melocactus, some Euphorbias, and Dorstenia. If you have any questions about a plant's hardiness just look online. I keep these plants in a south facing window. Winter is a pain the you know what, but it's worth it.