There are quite a few small models (about 6'x6'x6') of "pop-up" greenhouses that are pretty cheap ($120-180). The actual material of the walls/ceiling is polyethylene. (See the link below for one of these greenhouses.) I assume that this material does not insulate nearly as well as the thicker, more expensive polycarbonate (usually more than $700). I plan on using a space heater in my greenhouse to keep the temperature above 55 for my tropical plants. Will it just be running 24 hours to try to keep this thing warm on our cold Oklahoma winter nights?

Since I am going to be keeping tropicals in a greenhouse that has maybe a 2 full months with overnight lows in the 30s, do I really need to fork over the money for a polycarbonate? (http://www.allgreenhouses.com/Juliana-Greenhouses-B300ST-JU1087.html)

I would be interested in hearing from people who have either type of greenhouse - the cheapos or the nicer ones.

Thanks!

If you must keep things over 55 at night, you don't have to have polycarbnate, depending on the size of your greenhouse. If you're considering 6'x6', then a space heater will probably be sufficient with single ply polyethelene. If you go larger then that, you might have to run your heater most of the night. Get a good one, as the cheap ones will cut off pre-maturely.

The problem with a small greenhouse in OK is not keeping it warm, it's keeping it cool enough during the day. I've had a greenhouse here (Garvin & Murray counties) for 5-6 years now, and I have rarely ever had to heat mine during the day. They heat up very easily since we have so many sunny winter days, but they are tough to keep cool during the day without a good exhaust fan.

Build your greenhouse as big as you can afford. The first year DH built mine, he made it 12 X 20' and it was too small to hold all my plants. The next year he enlarged it to 12 X 35' and I still had a hard time fitting them all inside. So you really should go as large as you possibly can in the beginning, or you won't be satisfied.

One of the most affordable small greenhouses is the Harbor Freight 10 X 12'. I've got a couple of friends with them and they're pretty nice. They will need a lot of extra bracing to hold up to the winds we get here in OK though!

Good luck. Having a greenhouse sure makes winter more bearable!

* The plants inside a 6x6x6 gh will likely be too tight/dense. Not only will this prevent heat from your heater reaching all the leaves -- making them prone to cold damage through the plastic -- but the leaves closer to the heater may very well burn.

* In "tornado alley," a gh such as the one you linked may end up in the next county. Even if you staked it down with rebar, the structure itself is too flimsy to withstand forces twisting it any direction but downwards.

* I ran the calcuations for a gh your size, as well as the proposed covering. In order to maintain an inside temp of 55F when the outside dips to 20F, requires 6,486 BTUs. An electric heater maxes out at 5,120. Be advised that if you must run your heater on an extension cord (likely in a backyard), you will only get close to that output if you use a 12-gauge extension cord. Not only would it not be safe, but a "standard" 16-gauge cord, for example, would reduce your BTUs by at least 25%.

If you are reasonably handy, you could build a very sturdy, suitable structure, larger, and double-insulated, for maybe $200 more than your 6x6...using UV-resistant gh fabric.

BTW, your greatest difficulty will probably be keeping your plants from cooking in the sun, and not suitably warm when it's cold.

Not trying to beat your project down -- just want you to be successful and not disappointed!

-Bruce