Hi,
Tried to find the greenhouse you were looking at, but did't.
I'm also in ohio, zone 5, the coldest in the state.
I had an 'unheated' greenhouse years ago, but still at night if was going to get cool, has a small portable heater I would use to keep out the chill. I used it to start statice, which can take some coolness.
Can't speak for things like tomatos.
I think cool weather crops would be fine, and the hardier annuals and perennials.

These sorts of structures are very tricky to keep cool...they require vigilance as to ventilation...and will drop to essentially outside temp at night without extra heat.

I am zone 9 and our winters are for the most part frosts and rain probably like your spring. A greenhouse would definitely get you an earlier start but by how much I don't know. It is dropping to 30* here at night and the greenhouse warms up in the day and everyone grows slowly. My concern is that the temporary greenhouse would blow away in a spring storm. We impulsively bought our greenhouse and put it up. Then I started looking on gardenweb and was alarmed at the stories of storm destroyed gh. We took a lot of the advice and our gh is secure and a pleasure. If I were you I would consider a dug in cold frame. Good luck.

Are you planning to plant into the ground in this greenhouse, or on shelves?
If on shelves or in pots, forget it - it won't maintain any heat at night.

If you plant directly into the ground, you can pull it off but keep the following things in mind:
1. You can't beat more than a degree or two of frost. So, you can either stick to planting cool weather veggies very early, such as in March (btw, you can plant them outside in your area in April), or you can use floating row covers to assist in planting out warm-season crops earlier, like in early May.

2. This will overheat a lot during the day. Keep it vented on sunny days. If you are away during the day, you may have to err on the side of keeping it too vented and therefore too cool.

3. Good compromise for an early spring crop is to plant carrots, swiss chard, and collards. They can take some frost as well as some heat. Then later in spring (late April to early May), you can remove the greenhouse, put it somewhere else and plant out your warm-season veggies into it. Two birds with one stone. But if frost threatens any warm-season veggies, you have to protect with something, regardless of the greenhouse.

Looks like this GH is supported by forced air...looks like trouble to me.

I just posted a video of my new GH. In the background, you will see two cloches and a cold frame. Both of these would be a much better option. Especially the cold frame. There are many plans for both on the web and they would be much cheaper and much more fun (and satisfying) to construct.

Here is a link that might be useful: Watch this and look for the cold frame and cloches

For just getting a head start on veggies for a garden, I'd seriously consider just building some cold frames.

L.O.V.E. your greenhouse Lonmower!!!!

Dr. horticulture: what does this mean exactly?
"You can't beat more than a degree or two of frost."

If you're growing frost-tender crops (i.e. tomatoes, peppers etc) in such a greenhouse, you only have a margin of a couple of degrees. Once it drops to 30 F outside, it likely is 32 inside already. A layer of row cover is essential to derive any significant frost protection.

of course, if you're growing frost-hardy crops like spinach or brassicas, a greenhouse like this is very valuable in getting an early start in spring without having to watch the weather forecast all the time.

I'm asking because I live in Zone 5 (New england) and our lowest temp has been -11F this winter and i do not heat my GH. I have had NO frost in the soil (Some frost damage to most plants) and have successfully grown some more hardy plants through the winter. This has been a complete surprise to me when everyone continually seems to frown on winter crops.

While i can't tell you the temps in my GH (No thermometer) i can tell you what survived.

Spinnach - Suffered no freeze damage at all. Planted late november and harvested my first crop early January and a second crop early in february and it's ready for a third picking now.

Shallots. One planting in late november grew to 3" and has pretty much stayed that way with some new growth in the last week. Tips had some browning (1/2" or so) but no fatalities. Another crop sowed in mid december sprouted in late Jan. but stopped growing after reaching about 1". The cold really took hold in january and this is what stopped them from growing more. Just showing some signs of new growth now.

Brussel sprouts... sown in November and just sprouted in the past week.

Peas: Planted in late november. Reached about 18" in early january when the temps plumeted. They took some damage and most died back to about 1: above the soil. They were now sending out new strong growth, but i decided to resow and got rid of them.

Batchelor buttons: came up from seed in the soil (grew theses for my daughter's wedding in sept. and these were self sown. They grew all winter and are now about 12" and healthy.

Beets: grew well until early January when temps plummeted. I will replant as voles really did a number on the roots.

Lettuce: has done well from the november sowing and continues to grow (About 6" across now). took some root damage from the voles. Some damage due to temps... wilting and such. The remaining plants look fine.

Strawerries (ogala(?) everbearing): flowered all winter but berries would turn black.

I also tried some tropicals to see what would happen. Bananas were wiped out early december. Oranges and lemons have lost most of their leaves but they may come back. Hardy kiwi looks fine but regular kiwi may not have survived but the vine is still pliable so i haven't given up hope.

So you can see why i question what you mean when you say i may only have a margin of a couple of degrees. I wouldn't have thought ANY of this stuff would have survived at -9F if that was so.

My goal is to design a GH that i can grow crops all winter without supplemental heat. Everything i read says it can't be done, but what i see has me questioning that. To date i have not added any active heat storing features.

The one difference from a traditional GH is that mine is a double layered GH but rather than just having an inflated layer i have a permanant 2 - 3 feet of airspace between the envelopes. I expected just an extension of the seasons.

It also is a decent size: Interior is 48' x 22' while the extrerion is 50' x 26'.

Poppa

That is not a GH. It is a balloon kept inflated by forced air. You'll want to invest in a long cord to keep the noise far away from the house.

That balloon will not protect any frost-tender crops below, say, 30F if the power goes out. I don't see any venting in that balloon,. so a sunny day over 55F and you forget about opening it up and everything inside is toast. I'm not sure what the demographic is for that...um... product, but it is not a serious gardener.

If you are serious about a GH then that thing at Amazon is not the place to start. IMHO the place to start is a cheap real GH that you pick up used, or maybe a Harbor Freight on sale and a 20% off coupon. Maybe a high tunnel.

Then maybe if you are careful and insulate the north side and put an extra layer of GH film and bubble wrap and all that, you can grow cool season crops all winter if you can get enough light to the water that you use to retain heat.

I do it here in Denver area in coldframes, and I use water to retain heat, cover the coldrames at night, and have much more sun here than in NE.

I suggest going to the library and checking out a few books. I'll guarantee you zero books will have anything like this balloon thing in there, as if you are checking out a book, you are serious about learning about GHs and won't consider that balloon thing. What if the power goes out? Done. Get a real GH if you are serious. That balloon thing will turn you off and that would be a shame.

.02

Dan

Well, there's two things. First, I was only referring to frost-sensitive crops. Second, based on your description, you definitely have an excellent greenhouse, not a cheap plastic pop-up thingy from Amazon (to the OP: that's not necessarily a bad thing as long as you know its limitations). A 2-3 foot gap between double greenhouse layers adds a tremendous amount of insulation.

So, yes, gardening all winter is definitely possible in your area, with a proper greenhouse like yours. Unfortunately, I only have a single layered polycarbonate greenhouse which is very poor at retaining heat (I plan to move next year so I'm not building anything better). If it's 40 below outside, it's 35 below inside. So, I used a couple of layers of row cover over hoops, and now when it's 40 below outside, it's 0 F underneath the row cover. Still too cold for growing anything in my zone but it helps in getting an early start.

I know what you mean about folks naysaying winter gardening. Here, I get weird looks when I say I plant spinach in late March and harvest it into November (again, only possible with the row covers in my GH).