1) I wouldn't call a 6'x10' (assumedly single layer thin film) plastic greenhouse "inexpensive" at $250. You could get a 6'x8' Harbor Freight greenhouse for nearly that price, and it'll last far longer and be a far superior greenhouse (double-layer rigid polycarbonate panels).

2) You probably don't know what you're getting into here. Let's start with the basics. One, an unmodified single layer thin film greenhouses will get plant-burning hot during the day and lose all of its heat within an hour or so at night. Trust me on this one. You need heat storage and ventilation. Ventilation should be forced-air unless a large chunk of the roof comes up. During the year, depending on the design, you may be able to (*may*) get away with just keeping the door wide open. Heat storage is generally large buckets of water, painted black. A geothermal heat store is better. When I say "large buckets of water", I mean a *lot* of water. And it helps if you use forced air for heat exchange with the water. Don't underestimate how hard it is to store heat during the day that can keep the greenhouse warm through the night!

3) Single layer thin films lose heat very fast to the outdoors. They're also not very durable. If you want to insulate it further, I'd recommend a solar pool cover. Never use polyethylene construction plastic. It won't last a year. Bubble wrap is better, but still not ideal. The plastic your greenhouse came with is probably treated and will probably last a few years. As plastics age, they become brittle and less transparent. The best mix of durability, affordability, and transparency today is generally rigid double-layer polycarbonate panels. Commercial greenhouses often use air-blower inflated double-layer UV-treated polyethylene or PVC, which lasts a year or two for the PE and 2-3 years for the PVC. But, unless you want to do all that work over and over...

4) Use a reflective backdrop on the north side -- aluminized insulation is ideal (heavily insulate your north side, and optionally your east and west sides, since they don't provide that much sun in the winter -- the overwhelming majority of winter sun comes mostly from the south). Always align the long axis of your greenhouse east to west.

5) Even if you insulated it well and store heat well, expect to spend a lot keeping it warm in the winter! Heat can be either gas or electric. Gas heaters can be either vented or unvented. Unvented are slightly more efficient, are cheaper and provide CO2. The downsides are that there's more CO/fire risk and you can get ethylene buildup (you should have at least some venting in the winter anyway to reduce that). Electric heaters work well, but keep them away from water at all costs! Note that if you have a gas heater, it'll either need a gas line or propane tank that you regularly have to fill during the winter.

6) *Never* let your frame get wet. I made a big mistake on this front, and it cost me my greenhouse -- I installed an overhead sprinkler system. Steel frames, even painted steel frames, rust when wet. And it won't take long.

It's a lot of work and expense -- are you really sure you want to get into that?

As far as your existing greenhouse, there's no reason why you couldn't use it in a semi-sheltered area during the spring and summer. Because the materials are flamable, I wouldn't put a heater in it, and all due respect to the otherwise very good suggestions above, a greenhouse the size of yours couldn't hold enough buckets of solar heated water to keep it warm over night in winter.

Size-wise, 6'x10' is big enough to hold a lot of seedling flats. If you're wanting to start flowers and veggies put that baby into use as a greenhouse and leave the firewood out in the rain. Shelves made of planks and cinder blocks would make it easier to take care of them, but if you didn't have those, seedlings will sprout in flats sitting on the ground. Run an extension cord out there and put in an oscillating fan, set it on low and keep it going all the time. This will keep the air moving and help the seedlings off to a good healthy start. If it gets too hot you can buy shade cloth at the garden center that is UV protected and will help with the heat factor in summer.

I'd try to get my $250 worth from the greenhouse that you already have and use the time while it lasts to plan for something more permanent in the future. That's what I did and it was worth waiting. The GH that I have now should last 20-30 years-I hope that I last that long. Cheryl