Google up "Hot Dawg" heaters. I've had a 36K BTU unit for three winters now, and it works great. Hang it as high as you can, and use some kind of fan system to keep the air moving around inside of the structure. The fan on the unit will blow hot air out to 25'. It also has an exhaust fan so you can run a horizontal chimney pipe. Whatever you do, be sure and ground your gas line to prevent lightning ground pulses. Mine is coupled to a 150 gallon propane tank. I keep my GH at 52 degrees, and in a cold Texas winter like last year, with many nights and days below freezing, I spent about $300 on propane, from mid-November to late March, or early April. The unit is 75 pounds and is a snap to install and hook up. Here's a pic. PM me if you want more info.

I live in north central Sask Canada. We put up a 16x24 double poly greenhouse two years ago and we get lots of snow. The fans are running all year, once in the winter, when the fans stopped because of power outages the snow accumulated between the hoops sagging the plastic because it did not all slide off. This put pressure not just on the ridges but on the plastic itself. You would be alright in rain, but in the event of freezing rain or the odd heavy snow it would help to have them running. Could you turn them on when snow is expected?

Really ? Even in my grow box? I've got 6X 4' T5s that I run 12 hours a day. You think I'd get better growth outside in a greenhouse? If that's the case then I'll most likely just go the greenhouse route. A little source heater shouldn't cost much more to run than the lights

I don't know anyone who does this exclusively or even mostly. It is not a good idea unless you are immensely wealthy and cost is no object. Perhaps some commercial growers do it, but I don't know any in my climate who do. The nurseries where I shop that have their own greenhouses and raise their own plants in our climate have a hard time just keeping their plants inside the greenhouses cool enough in spring, not to mention summer. There are some accommodations they can make---for example, building their greenhouses with sidewalls that crank up or down to allow better air flow, and they tend to use extensive and expensive cooling systems---far beyond the ventilation fans that most residential greenhouses might have. Even then, when shopping at those nurseries, I notice how very hot their greenhouses are by April and am careful to check their plants for spider mites before I buy them because those sort of warm greenhouse temperatures are spider mite magnets. I've walked away from tomato plants at my favorite local (local to the DFW area, not to my specific county in OK) nursery as early as earliest April, regretfully not buying those plants because they already are covered in spider mites because the greenhouse is getting too hot already during the daylight hours. These greenhouses have cooling systems and some shadecloth, but there's a limit to how much you can cool a huge greenhouse even with those measures in place. So, for the average resident like you and me, the challenges are even bigger since we aren't expecting any sort of financial return on money spent heating or cooling a backyard greenhouse. Greenhouses get extremely hot---it is, indeed, the fabled greenhouse effect. In order to grow exclusively in a greenhouse, you'd have to be willing to spend a lot of money cooling the greenhouse, and I mean for much of the year. Even in the wintertime, and with 50% aluminet shade cloth covering my greenhouse, I have to open both greenhouse doors and all 4 vents by 9 a.m. (8 a.m. is better) in the WINTER time or the greenhouse will hit 140 to 145 degrees by 10 a.m. on a sunny winter morning even if we were below freezing at sunrise. As you might surmise, even plants that need warmth and sunshine in winter will not thrive in a greenhouse allowed to get that hot---but all sorts of pests, including spider mites, will thrive in that heat. The spider mites are as big of a problem for the plants as the excessive heat itself. With shadecloth covering as much of the building as reasonably possible, open doors and proper venting in the winter time, you can keep a backyard greenhouse warm enough (with heaters at night and on cloudy, cold days) to keep plants alive but you will struggle with keeping it from getting too hot during sunny days unless you have a good ventilation system and a cooling system, like an evaporative cooler (as a minimum). My evaporative cooler (we called them swamp coolers when I was a kid) is large, is portable (on wheels) and cost us more than the greenhouse itself. A built-in system would have cost even more. You also need to be present during the sunlight hours to manage the greenhouse's environment---especially on a day when the weather is rapidly changing---and those days are extremely common here in autumn, winter and spring. While you can set up greenhouse heating and cooling systems on thermostats and timers (assuming cost is no object because the cost of all these systems add up quickly), and you can install vents that pop up on their own once a certain internal temperature is reached, if opening doorways and/or strategically placing ventilation fans in/near those doorways is part of your greenhouse temperature management plan, someone has to be at home to do those tasks every single day as needed and when needed. Then, depending on how much money you're willing to spend heating the greenhouse at night, someone needs to be home in the afternoon to close the doors and vents (if they are manually operated) while the sun is still pretty high in the sky in order to capture enough heat gain to help keep the greenhouse warm enough for warm season or tropical plants at night, which helps a little with how much money you'll have to expend on heating it overnight. You'll still have to run a heater or multiple heaters on a thermostat, but relying on heat gain from the sunlight can help you run the heaters a bit less at night. On most winter days, I need to close my greenhouse doors by 3 p.m. to get some heat build-up from the sunshine, but on cloudy days and very cool days, I might need to close it at 1 or 2 p.m. It varies every day depending on temperature, wind speed, etc. I love my greenhouse, but keeping the temperatures just right in the winter and spring takes up a lot more time than most people imagine. Without shade cloth I think it would be impossible where I live. Keep in mind that I'm a lot further south than most of y'all here on this forum and my weather is more like the weather in Dallas, TX, than in Oklahoma City, OK, but I imagine greenhouses in central OK can overheat on winter days the same as they can in southern OK. How hard could it be to keep greenhouse plants happy in a month like October? Well, way back in early October 2017, we had a day where the heat index OUTDOORS hit 106 here, and that is out in the open air, not inside a greenhouse with the greenhouse effect going on. One thing I learned quickly with my greenhouse in its early years is that I have to choose how to use it in winter. If I want to keep tomato and pepper plants alive and producing, I have to keep it warmer. If I want to grow lettuce, spinach, collards and other winter greens, I have to keep it cooler. I have to choose between the two---I cannot have both in the greenhouse at the same time in winter. The heat needed to keep tomato and pepper plants producing and enlarging fruit tends to cause lettuce and other greens to bolt, making them fairly short-lived as a winter crop in a warm greenhouse. If I try to keep the greenhouse cool enough to keep the lettuce from bolting (and it is a struggle to do so with our winter sun and sometimes hot winter days), then the tomatoes, peppers and citrus plants are too cold and growth almost stops. You'll still get fruit from the tomato and pepper plants but it will take them twice as long to ripen and they will not make much growth so you will get tiny fruit compared to what those same varieties produce in summer. This means you basically choose between having a good harvest of cool season crops and a poor to almost nonexistent harvest of the tomatoes and peppers you have in pots in the greenhouse, or you choose to have someone larger, somewhat normal sized tomatoes and peppers at the expense of the cool-season crops. In an idea situation, my greenhouse would be split in half, with one half serving as a cooler greenhouse for winter greens and the other half as a warmer greenhouse for warm-season plants, but it is one greenhouse, not two, so I have to choose how to use it. I'd like to split my greenhouse in two and use the two halves separately, but I think my husband wouldn't be in favor of tearing the greenhouse apart and moving half of it further away from the original location, nor would he really want the costs needed to keep two greenhouses both warm enough and cool enough on separate systems (and, honestly, neither do I because those costs add up) so one greenhouse it is likely to remain. Before I had a greenhouse, I assumed the biggest challenge would be keeping it warm in winter. I was 1000% wrong. The biggest challenge is keeping it cool enough any and every month of the year at peak sunlight hours. I normally only have plants in my greenhouse in autumn through mid-spring. I try to get all the plants out of the greenhouse before May begins. My greenhouse is mostly used to raise seedlings in late winter and early Spring, though sometimes I overwinter a few plants in it. (Sometimes I just move the overwintering plants into the sunroom on the west side of our house to overwinter there because it is well-insulated and stays much warmer at night than the greenhouse ever would stay.) The larger a greenhouse is, the easier it is to manage the heating and cooling. The smaller a greenhouse is, the more difficult it is to do the same thing. Small greenhouses, like those that most suburban and urban gardeners have the space for, can get very hot very quickly but also lack the mass to hold that heat at night so that they cool off very rapidly too. It is important to put a Min-Max thermometer in your greenhouse and track its minimum and maximum temperatures in every month so you'll learn how quickly and how high it heats up as well as how low the temperatures drop at night. Most greenhouse coverings, unless you use very expensive triple-ply walls or two layers of greenhouse plastic inflated with air between them (which involves keeping the inflator/blower running 24/7) will not hold heat well enough to keep the greenhouse temperatures much warmer than the outside air temperatures at night unless you're running a heater. Even with the better walls, you still have to heat at night and cool/ventilate during the day. You also have to tailor your usage of heat to what your plants need---tropical plants need a certain level of heat and humidity while succulents, for example, or standard vegetable plants have differing needs. Think about what sort of temperatures and cost are involved in keeping your residence warm in winter and cool in summer. Imagine extending that cost to a greenhouse structure that is not nearly as well-insulated as your home. That's the issue with a greenhouse. Why people imagine that a greenhouse with relatively thin walls will be able to maintain appropriate temperatures inside in winter without heating systems is beyond me--the thin walls do not hold heat well at night. They just don't. Think of our vehicles---how hot they can get relatively quickly in full sun and how cold they get at night. In that sense, a greenhouse--in terms of temperature control or lack of such---is more like a vehicle than a residence. It also is our specific climate here that makes this such a challenge. We are far enough south that we can have daytime highs in the 70s, 80s and (occasionally) even in the 90s. We don't get enough snow to benefit from its insulating effect like some folks in colder states who might have snow on the ground for months. We can see temperature swings from the teens and twenties at night (or even colder) to the 30s, 40s, 50s, 60s, 70s, or 80s during the day outdoors, and even higher inside a greenhouse because of the greenhouse effect. No two days necessarily are the same. It takes a lot of effort, at times, to keep the plants in a greenhouse happy, though it depends on what sort of plants you're keeping in that greenhouse. I haven't even addressed the summertime heat. My greenhouse, even with shade cloth, vents, open doors and a modest ventilation system is too hot for plants in May through September. I suppose if we had endless financial funds to spend on cooling it with a very expensive cooling system, we could use it in the summer months, but we'd spend more to cool the greenhouse than to cool our residence. There's no way it is cost effective for a regular homeowner to do this with a greenhouse of any size at all. Our greenhouse is a sauna in the warm months, and if we are having standard humidity levels, that sort of heat is a magnet for certain pests and diseases. My greenhouse sits empty most years from May through September and I don't even step foot in it. For a residential greenhouse it is not especially small---240 square feet---but one that is even smaller (many backyard greenhouses often are 8' x 10', 10' x 12' or 12' x 16') would be harder to manage because it would heat up and cool down more quickly due to the smaller size. So, can it be done? Perhaps, but not in an economically feasible way for the average resident's budget. Commercial growers can do it due to the economy of scale---it is just part of their cost of doing business and they figure in their heating/cooling costs as part of the cost of raising whatever they're selling. We didn't have shade cloth on our greenhouse its first growing season, and it really was too hot to use after March. Adding shade cloth before the next year bought us another 4-6 weeks of growing time in both spring and fall, but shade cloth is not cheap and it doesn't last forever. My greenhouse has very little in it right now. I think it is 8 years old and we need to remove the greenhouse plastic and replace it. That's a project (I hope) for very late December or early January. I don't see how we get around to doing it before then based on how busy our lives are at this time of the year. I am pretty sure that when we remove the shadecloth, which is one year younger than the greenhouse, it will crumble in our hands. Replacing it will cost more than replacing the greenhouse plastic. We needed to replace both last year but postponed it because of the cost. This year it simply must be done because the greenhouse plastic has ripped in two places. It is possible we will replace only the greenhouse plastic this winter and then add the shadecloth a bit later just to spread out the expense a bit more over time.