For future readers, the heater I linked earlier is just a piece of junk.

I went to Lowes to buy a new heater (very limited options), and when I got home it was 44 degrees in my greenhouse! That heater had been running on high all day long, but with a thermometer just 2' away from the heater, it was only about 2 degrees warmer than outside!

I bought a radiant heater from Lowes as a temporary replacement. It can't be permanent because it has to be reset every time the power turns off, but it will work for the next few days with 30 degree nights. Within about 2 hours, the temperature in the greenhouse went from 44 to 52.

Which still isn't warm enough, but it's better than before.

I guess my next option is a propane heater? I don't know, I'm super discouraged right now.

I've used a hot oil heater for several winters in a small shed and it did a great job, on wheels so easy to move. Looks like an old radiator. Laying a small sheet of metal on the top seems to help a lot so the heat spreads out a little before it goes up. I only turned it on at night, turned off when I left in the morning.

What are you trying to grow in your greenhouse? Many plants can weather colder temperatures in a greenhouse than outside, just because of the lack of wind.

Check out Eliot Coleman's two books for several practical tips, including using row covers inside a chilly greenhouse to give tender plants a few extra degrees.

What is your greenhouse built of? That can make a difference. What kind of heater are you using? It might not be up for the job of 1400 cu ft. I know sometimes heaters make the claim, but sometimes it really can't do it.

Around here propane is used a lot with greenhouses. I don't know what is feasable in your area, if it's propane, oil, electric, natural gas. If you can hit up a couple local greenhouses, or even better would be smaller growers with smaller houses... they might point you to the best kinds of setups for your area.

You could consider extenders. Things like using straw bales to insulate the base of your greenhouse, or large water butts painted black for thermal mass. If your greenhouse is simple plastic, you might want a double layer to help create an air pocket between layers. You can use row covers, cold frames- or even use them as hot frames if you can.

If you are into more off the track stuff. There are things like compost heaters, where you stack up your compost pile outside the greenhouse, but use piping and water to carry the heat of the pile into your greenhouse. You could build in a rocket mass heater, those take some planning and construction. Maybe even some solar can heaters could work in your area, that's where you take cans and build them into a box to work as a heat riser. A gravel or brick floor can help a bit with thermal mass retention, but mileage can v ary on that.

Something else to consider, but takes a lot of work. Dig down. Dig the floor of the greenhouse down a ways to be below your frost line to help the earth conserve your heat.

What are you doing with your greenhouse? Overwintering, propogating, seedlings, other? Depending on how you have your greenhouse set up, you could potentially have a divider in there with a cooler side that does not need as much heat, and a warmer side for your warmer needs.

What are you trying to grow in your greenhouse? Many plants can weather colder temperatures in a greenhouse than outside, just because of the lack of wind.

@HighColdDesert, I'm mostly wintering my plants that don't do well in this zone (cordyline, blue java banana trees, and several others), but I start hot peppers and a few other annuals in February.

The heaters I've tried are all WAY too small, letting the greenhouse get below 50 so I'm not sure if these plants will even survive :-( When it gets down to 18 later in the Winter, there's no chance.

What is your greenhouse built of? That can make a difference.

@beesneeds, this is the greenhouse I bought:

https://www.quictents.com/products/20-x-10-x-7-greenhouse

The material says, "Heavy-duty transparent PE with green leno". Whatever that means.

What kind of heater are you using? It might not be up for the job of 1400 cu ft. I know sometimes heaters make the claim, but sometimes it really can't do it.

Ain't that the truth!! LOL I've tried several, first the patio heater that I linked in the first post and then the biggest ones I could find at Lowes.

I've discovered that, like you said, the stats they give are just nonsense. About 25 years ago my first business was a car stereo store, and I often had to explain to customers that higher end amplifiers would list "wattage" in terms of "RMS", while cheap ones would use "Peak". This let the cheap amps say something like "1000 watts", which was "peak" and really more like 150 RMS. It was technically true, but misleading.

I've seen the same thing with heaters. When one says that it is 5000 watts but only pulls 12.5 amps? Yeah, that's not true.

Around here propane is used a lot with greenhouses.

Maybe you can give me some advise on that. I looked at this one:

https://www.amazon.com/Dura-Heat-Propane-Forced-Heater/dp/B001XW129C/

but it says, "Operates 7-14 hours on 20lb. propane(LP) gas cylinder".

So I'd have to buy a new tank every other day? That's $250 /month to operate!

Using a space heater raises my electrical rates through the roof at this time of the year, let alone in the dead of winter at it's coldest and having to run 3 at once at times.

Now with a 63% increase and the cost of heating I'm done with growing warm loving plants in mine.

Now it's a cold frame with the heater only keeping it above freezing or slightly more, 40's. I am finding out that just about everything I grow and even certain orchids do just fine in those temps.

Running one space heater last October cost me over 200 a month and 300 to 400 $ by January.

This month my bill was 64 dollars and what a relief.

I will keep mine set to only come on at 35 degrees and to keep anything from freezing and only into the 40's when it shuts off. The sun on any given day should warmer the house even further without the heater on.

I use if for my dormant figs, citrus trees, many perennials, some annuals, plants I know will do well in there that should grow in zone 6 and 7.

Love it.

If I should want a nice warm day in there just to sit and relax, I'll crank the heat then

That's all I ever tried to do with mine, just keep it above freezing. I've put most of my plants in there at one winter or another. None of them seemed to mind and I had a lot of succulent blooms happening.

I've arranged things to avoid using it this year because, like you said, heat is supposed to be a lot more expensive this year. Turned ours on for the first time yesterday morning. Usually that doesn't happen until well into November or later.

Tiffany, I heard it got cold down there all the way from Canada with the jet stream dipping way south. Crazy. I hear you

I heat with oil. The oil is at 549 a gallon and just a hundred gallon fill up is nuts. Let alone trying to keep the lights on at also a huge increase. Use to be a time when doing the xtras didn't cost and arm and a leg, but now only essentials is what I hope to keep going.

I kind of like all my warm loving plants in a sunny 4 seasons porch under l>E.D lights which is much cheaper. I don't have to shovel or trudge my way to the greenhouse during snow to enjoy them anymore.

I can smell, touch, water and look at all my plants in areas which I can enjoy 24/7. A good trade off I'd say.

I am thinking many will resort to using lights in their homes now that heaters and the such will bust their budgets.

We are considering not growing anything from December through mid March (zone 6b). We are extending our seasons rather than trying to go through winter.

I'm starting to regret a lot of things... buying a large greenhouse for one, buying all of these sensitive plants for another. Like you all said, when I did it the cost wasn't so bad, but income is down (way down) and expenses are up! Things are getting out of hand, the whole thing has become a money pit.

I've discovered that I really need about 20,000 BTUs:

https://www.pickhvac.com/calculator/heating-btu/

A true 1500W heater is going to put out closer to 5,000 BTUs. It would close to $2,000 (USD) to run 4 heaters and 4 thermostats, with 4 extension cords on 4 separate breakers, so that's not going to happen.

Another option is a 6000 BTU heater, which I'm only finding for 240V.

I just talked to an electrician about running a 240V panel. He hasn't given a price yet, but he acted like it wouldn't be too expensive because it's a relatively short distance and the wires will be easy to hide. If that's affordable then I might have to go that route. But now you guys have me worried about the electric bill on that :-/

What plants are you wanting to grow? What material is the greenhouse made of? How about pictures? That information is vital to know before you spend any more money and before people can advise you. Time spent in reconnaissance etc....

@floral_uk z.8/9 SW UK, I answered that a few posts above... I'll paste it here for you:

What are you trying to grow in your greenhouse? Many plants can weather colder temperatures in a greenhouse than outside, just because of the lack of wind.

@HighColdDesert, I'm mostly wintering my plants that don't do well in this zone (cordyline, blue java banana trees, and several others), but I start hot peppers and a few other annuals in February.

The heaters I've tried are all WAY too small, letting the greenhouse get below 50 so I'm not sure if these plants will even survive :-( When it gets down to 18 later in the Winter, there's no chance.

What is your greenhouse built of? That can make a difference.

@beesneeds, this is the greenhouse I bought:

https://www.quictents.com/products/20-x-10-x-7-greenhouse

The material says, "Heavy-duty transparent PE with green leno". Whatever that means.

Oh, that's a nice big one. I recognize the plastic, it's a lot like the kind on the little greenhouses I use as season extenders. I would bet it's a bit heavier plastic than mine are made of. They are great season extenders, but aren't always all that great at retaining heat overnight when you want them to.

Something you could try with that is to create an air pocket with a second layer of plastic. Use the super heavy window film meant for using outdoors. But use it on the inside of your greenhouse. Use greenhouse clips to clip the plastic to the pole framing. You can buy them pretty cheap online.

Using straw bales around the base of it might help a lot. Helps insulate and seal up the base from drafts. I get lots of leaves here, so I pack in leaves around mine instead of bales.

All that would help, but I don't think it would keep it above 50 without heaters. But if you only have a few plants, you might still be able to do hotframes inside the greenhouse if you have a hot compost pile handy.

Here are some pics:

The exterior, with me facing Northwest. It's in part sun, with morning and late evening shade but direct sun from around noon until dusk.

The interior. I lay down about 4" of mulch yesterday trying to insulate the ground and edges some.

My poor croton does NOT like to go below 50 degrees, I don't know if he'll survive.

My cordyline with asparagus fern. It can die back if necessary, but then next year wouldn't sprout until July so it would stay small. If I can keep it going over the Winter then it'll get bigger.

You can also see my Man de Villa in front of it.

My dracaena grass isn't doing too well.

Neither is my blue java banana tree pup:

You need way more insulation to have any heater work, and the greenhouse should be in full sun. It’s not the heaters fault. Do some more research on backyard greenhouses.

Ok, you don't have a lot of plants there. If you can, bring the most sensitive ones inside or otherwise kept above 50 for now till you resolve your greenhouse issues. Cluster the ones that can survive and do something smaller inside the greenhouse for them- you for sure don't need to heat all that space. When it comes to seedling time, you will also need to set up a smaller propagation house inside the greenhouse for that, and you for sure will need to heat it in Feb.

If you hang up a dividing wall inside, you can still take advantage of the season extenting in the spring. It will be cooler than your warmed cluster of plants and propagation area. But You can gain a couple to few weeks in the spring depending on the weather. And also, your pepper seedlings can appreciate the environment sooner than really being put outside.

I realize it might be a bit late in the year for this now, but I would strongly suggest moving your greenhouse to a fuller sun position. Both for the light requirements as seedling season kicks in, and also for the thermal increase. That thing needs as much heating time in the sun as you can give it.

Also... any heaters or other electrical. Get those things off the mulch pronto. A couple of cement bricks or level smooth stones or something underneath those. That's a fire hazard. And also an electrical shock risk if it gets too wet in there.

That's huge! Maybe put a smaller tent inside, but I'm already, as others said, worried if that's enough light. An inner structure, if not clear, would block more light.

Also agree that most of those would be fine winter houseplants, near an E/S/W window.

Find anything to keep the plants off of the ground, even if only raised 6". Upside-down pots if there is nothing else, though that is a top-heavy arrangement. The coldest air is at the ground, unless you have strong circulation. It looks like you already have one of the heaters I described, the hot oil radiator heater.

Thanks for all of the tips!

My old greenhouse was closer to 5' x 5', and I outgrew it pretty quickly:

https://www.amazon.com/gp/product/B01GDVVZY4/

I had it for 2 years and the plastic was starting to fall apart, which is why I went ahead and upgraded to the big one; not terribly expensive, and room to grow. I just didn't realize how much of an issue I'd have with heat!

Right now I'm thinking that I'll put the small greenhouse inside of the big one, though, and cluster the more cold-sensitive plants in there with a dedicated heater. The small one isn't air tight anymore, but it should be OK.

Also agree that most of those would be fine winter houseplants, near an E/S/W window.

A few of you said the same thing here, I'm just copying @Tiffany, purpleinopp Z8b Opp, AL because it's the most recent.

My issue there is that I only have 2 sun-facing windows that I can use, and both are already packed with plants :-O

It looks like you already have one of the heaters I described, the hot oil radiator heater.

Yes, I bought that based on your recommendation. I already had the hanging patio heater that did nothing, so I bought this oil radiator and another heater with a fan. The oil radiator works the best of all of them, but I've discovered that I just can't physically get more heat than that from a single 15A breaker. So anything bigger requires an electrician's help.