I had a makeshift 8'x 10' greenhouse until it was destroyed in 2005.I used 6 mil plastic (from HD) to cover it. The plastic roll unfolded to 10' wide. I reinforced the edges of the plastic with duct tape and stapled it to the frame. The door was secured with velcro as you said. I used a cheap 1500 watt space heater and put a wireless thermometer inside the "greenshack" as I called it. It stayed above 40. Problems: Ventilation on those winter days when it gets above 85-90. UV rays destroyed the plastic in about 2 yrs. My new greenhouse, which I built in the fall of 2005, aside from having a sturdier frame, is covered with 6mm twin wall polycarbonate panels. Much better material, but more expensive ($60 / 8x10 sheet 5 years ago) I'm still using a 1500 watt heater in a 1000 cubic foot space and it will keep the temp at 50 on the coldest nights.
john

I also use the twin wall polycarbonate panel greenhouse. They work great because the twin walls provide the insulation. I bought the 10' X 12' greenhouse kit $799.99 from Harbor Freight and they have local stores in various cities in Florida. I bought it a few years back when it was selling for $649.99 and it is still standing strong with no problems. It has kept my tropical plants safe through the many winters with a 1500 watt space heater. I put it together in about 3 days and I'm in my 70's. I recommend it highly.

Lou

Here is a link that might be useful: Poly Carbonate Greenhouse

Thanks John and Lou. Sounds like a 1500 watt space heater will work then. My area is ~6' x 14'. Those ready made greenhouses at Harbor Freight are very tempting, but I don't have a place for a permanent greenhouse so I will have to go with a temp "greenshack". lol.

Lou, that's a good deal! I spent $800 just on the polycarbonate. Wish I had seen that. Judy, get a basic 1500 watt space heater. The fancier ones have one problem. When the power flickers in the middle of the night, they shut off and don't come back on.
john

Thanks John. What is the brand of heater you have? I sure don't want a permanent shutoff problem.

I have this greenhouse idea also, where I can put my Orchids and tomato-plants when it gets cold.... but I plan on using some old second-hand windows out back, I have 4 of them pretty wide and tall, and supplement them with the polycarbonate.... my son tore down several old sheds and the 2x4s etc. are all PT and still useable, I have some old HD 12" pavers for the floor, so it sounds like it might work and be low-cost..... now I just need to nag my son and get him crackin' on it.... if not this year maybe next..... dream on.... sally

Ask to test the heater at the store, plug it in.. get it running then unplug it for 5 seconds then plug it back in and see if it still runs without touching any knobs. That will make like the power went off then on again.

Lou

Thanks - Good idea Lou.

A very inexpensive way, also the way I am doing it, is to use pvc pipes. They are the ones you use for irrigation systems. There are tons of plans. just google it or check out youtube. the cheapest ones I have seen our semi circle ones.

I have 4 greenhouses,3 are from HarborFreight, good for the money. Two oof them are the 10X12 hooked together back to back. I just took out a dogwood tree so I can add another 10X12 so I will have 3 10X12 together as one.
Go to the Greenhouse and Garden structures and you will fine them.
Again good green house for the money.

Lowe's in the Villages on 441 also had a small greenhouse the last time I was there that could be taken down in the summer without much trouble.

With any GH you will need a fan also. It will get very hot in there durring the day. You need to keep the air moving in any season to prevent mold.