Keeping it at 55 may be pricey without additional measures like bubblewrap. But look for what’s called a milkhouse heater — a small electric space heater made for sheds, etc.

@nekobus thanks! I will look into bubblewrap solutions as well! Thank you so much!

No experience, but I think a thermocube could be used with that heater. Seems like a risk with a plastic structure, though, so I would suggest c9 Christmas lights, incandescent.

What are you using to insulate the floor? When the temps get in the 20's or teens it will take quite a bit of heat to keep it warm as the cold will come in through any opening.

I used a 22,000 BTU kerosene heater in my 14x18 GH and it could keep it toasty warm on a cold night but kerosene is often hard to find and was/is expensive. The walls were twinwall polycarbonate which helped insulate it some while the floor was gravel on top of the ground. I tried using a new kerosene heater in our 12x16 shed (not on the ground) to keep my bromeliad collection alive through winter. The kerosene heater often fizzled out after I got it lit (why I never figured out) and being above ground, I lost most of the collection due to these two facts. The GH on the other hand was often not heated my 3rd and last winter as I found out that plants could survive 30 to 32 degree interior temps as long as the wind was off of them.

If it were me, I'd not try to heat the whole greenhouse due to the high cost. Instead I'd get seed starting mats running off a thermostat, and I'd put plastic domes over them. This would not only allow you to precisely control the temperature, but you could maintain better humidity control with the domes in place. The equipment might be pricier than a small heater, but you would quickly come out better economically due the energy savings. I have four 20" X 48" mats in my greenhouse; they cost about $35 each and only use 100 watts. They are all plugged in to a power strip that plugs into a $40 thermostat that controls all of them. I bought a large piece of 3/4" styrofoam at a big box store and cut it to mat size to insulate the bottoms of the heating mats. I've got four 10" X 20' growing trays on each mat, with high plastic domes on each one. It probably cost about $20 for four trays and domes per mat. And in the summer you can use the setup to start seeds.

I don't see heating mats keeping a plastic greenhouse sitting on a deck in zone 6b above 55 degrees though. Starting seeds and keeping begonias alive are two different things. Maybe heating an aquarium with seed starting mats in the same greenhouse might work but it may take some giant aquariums to do that.

A very cool greenhouse is also a breeding ground for mildew on a lot of begonias. I learned this through the school of hard knocks.

The joys of a greenhouse are very rewarding but also there are some lessons to learn as you go along.

hcmcdole-

I agree that heating mats won’t keep the whole greenhouse warm, but that wasn’t my point. Trays with high domes sitting on bottom-insulated heating mats become mini greenhouses, which I think could work...presuming the begonias are of a side to fit into high dimes.

However, I just realized that the OP refers to the greenhouse as sitting on a deck, which might mean that, if the greenhouse is open on the bottom and the deck has 2X flooring, there will be a lot of cold air flowing that will defeat any heating method, including running an energy-greedy heating unit.

If the OP can post some photos or provide more specific details on the greenhouse’s physical construction, maybe we can provide better answers.

kudzu9,

Most begonias won't fit in a seed tray unless they are starter plants. Jiffy pellets make propagation easy but eventually you have to move them up and out to bigger pots.

mcdole-

You're not understanding. I chose my language very specifically. Planting trays are open trays that measure about 10” X 20”. They’re not seed trays, so they’re not subdivided. You can put seed starting trays inside them, but I was specifically referring to the basic tray that you can also set pots in. I often put eight 5” X 5” pots in a single planting tray. And you can set even bigger pots in them.

If you're not familiar with the term, this is what a planting tray looks like:

And this is a high dome:

Together, they form a mini-greenhouse and retain heat; the vents allow you to have some control over humidity.

kudz,

I completely understand. I've used the same thing for years but more for propagation than growing plants on a cold deck.

I didn't know I had to provide pictures to show you that I understand what you are saying. Maybe you should show pictures with full sized plants growing under them?

Here are some small begonias I started by leaf cutting.

As I recall I can get 18 three inch pots under a dome. I think it was 12 four inch pots and 6 six inch pots. It's been a while since I've done this.

What I am trying to say is begonias are not tiny plants (for the most part). These are ten gallon tanks and a Jiffy lid almost fits on top. There are 21 begonias between these two tanks and they were grown from seed earlier this year. They are in 2.75 to 3 inch pots. They will fill this tank before next spring.

This is begonia parviflora and is almost six foot tall.

B. platanifolia is a big leaf, tall grower too. The seed grown maculata next to it is nearly five feet tall.

another aluminum pan with Comte de Lesseps (the aluminum pans are about the size of the 10x20 tray you mentioned and you can buy them 15 to a pack for less than $10 at Sam's)

Escargot in a 12 inch squat pot

B. foliosa in a 10 inch pot, Ukigumo in a 8 inch pot, and Fireflush in a 10 inch pot.

Perhaps the OP can show the size of their plants so we can see what they are up against?

Thanks everyone for your replies. I learned so much about greenhouse and begonias!

This is my little plastic greenhouse for now. I am thinking maybe I should get a Harbor Freight one with polycarbonate panels.

And some Jasmines.

I should add I am not handy at all. If it’s too hard to keep it warm at night, I am willing to bring them in every night. My maculata has got many new leaves ever since I put it in the greenhouse.

paint girl-

I suggested heating mats, trays, and domes because it would save energy, but I see you have a number of plants that are too tall for most domes. I have a milder climate (Zone 8b) and I use an oil-filled electric heater to simply keep my greenhouse at about 40F so my sensitive plants don't freeze. These heaters are relatively cheap (about $50-70), and they typically operate at 1500 watts maximum. But I'm not sure that one of these heaters would put out enough heat to keep your greenhouse above 55F, especially since it looks like you would get a lot of cold air in from the bottom. To get an idea of what it costs to operate one of these heaters, I looked up the cost of electricity in your area, and it averages a little over 16 cents/kwh. That means that operating one of these 1500 watt heaters would cost about 25 cents/hour, or $6 per day if it was running continuously. And, again, I don't know if one of them would even be sufficient to keep the temperature where you want it.

The Harbor Freight greenhouse is not bad, and is a deal for the money. The polycarb panels would also make it more energy efficient than what you have. However, it takes a bit of skill to put it together, and sometimes not all the parts fit together easily. If you decide to pursue this option, there are a lot of helpful threads you can search for here on GardenWeb where people have discussed problems, solutions, and DIY improvements. Since you say that you are not very handy, hopefully you have a friend who could help you put one of these together.

A cheaper alternative is a shelf fitted with shop lights (LED or T5 or T8) indoors for winter.

Be warned that B. maculata is a mildew magnet when the temps are cool. I finally lost mine after 11 years. Be proactive on mildew - don't wait for it to take over the entire plant.

I run around 90 shop lights in the basement - mostly T8 and replace those with LED when the ballast burns out.