I have an 8 X 8 Rion and it's great...I've been getting more into tropicals, well like palms and brugmansias and will be heating mine up a bit more this winter...I've had it for 4 yrs now and have never put any heat in it and it's been ok...I did loose a couple brugs over the yrs, but hopefully not now after putting just enough heat in it to keep the chill off...I really like the construction on my Rion...It's very sturdy...We poured a 6" concrete slab that it sits on.

See my other post over here: http://forums2.gardenweb.com/forums/load/strucs/msg1013360919516.html?12.

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.

Zach,
Don't go with one of those pop up greenhouses. You won't be happy with it and you'll wind up losing your plants. If you can't afford polycarbonate, go with two sheets of poly, with a fan blowing an airspace between them. Call or go by American Plants in OKC. They sell everything you need and are a good source of information.

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!

Here is a link that might be useful: American Plant Products & Supplies

I agree I have a 10X12 HFGH is Eatern Oklahoma and it has held up well.

Here is a link that might be useful: Greenhousing in Oklahoma

Allow me to play devil's advocate for you:

* 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

Have you thought of these Pit/Walipini/underground greenhouses?
I live in southeastern Oklahoma...and as you know we are always challenged when it comes to gardening and this year the heat was the major issue!..LOL Ive been exhaustively researching these ideas about 365 gardening and preserving while living off of the grid, so before I call out the earth movers I want to make sure i have all of my ducks in a row!.....I was very interested in this site... http://greenhouse.taroandti.com/2008/02/28/earth-sheltered-pit-greenhouse/ It seems they are doing a hybrid type at a low cost.
I would love to find others in my area that might have some experience with this and i also have a few questions for the forum?.....

1. I notice everyone is planting on the floor. Wouldn't it be better use of the space to do a terraced planting along the walls and this would act as a berm as well?

2. Although i am knee deep into self efficiency My moto is( purposeful can be pretty at the same time)... So has anyone added some of the more aesthetic details to their plans... ponds, decorative elements ect?

3. could i figure out a way to incorporate a tropical zone for figs and citrus .. a zone where i could let plants go dormant if need be... a cactus area... a fish pond/aquaponic area and a little mushroom area? or am i looking at digging several different houses?

Hey Zach,
This year I got my first greenhouse - a 6x8 Harbor Freight. I love it, I will post a pic showing the inside with the heater & Bubblewrap insulation, the little black clips holding the bubblewrap also double as plant hangers.
Have had a few nights with lows in the 20's and everything has done great, the only thing is with the heater being mounted on the north wall about half way up I can notice the temp is a little cooler toward the ground, but again no damage on anything yet. Went with Natural Gas due to our recent couple of crippling icestorms. One more draw back for me at the moment is I don't have any ventilation (no fans to move air) plus I haven't cut around the roof vents yet, I do have one plant that seems to be getting too hot due to the act that it is on a shelf right near the roof, but nothing major . I plan to get a shade cloth to cut down on the sunlight. I love my GH & I do have several tropicals I hope make it through the winter. Hope you get you one as well. Rick from McAlester OK. Also no increase in my gas bill for the moment, though we haven't really had any significant cold temps?? So I will have to see how that goes.

Kelly

the obvious benefit to digging a trench style greenhouse is being able to take advantage of the near constant ground temperature. The drawback I see to this is you need to go pretty deep and by doing so how much light are you losing because of the "walls". I guess if you make it wide enough the shadows wouldnt be to bad.

mike