What kind of plastic and how many layers? Last year I had a ghetto sort of setup with single layer 6-mil poly supported by wood over a veg. bed, giving me 4-6 degrees F of frost protection (more differential as the weather warmed into spring). As anney said, you definitely need to ventilate. I eventually abandoned that in favor of a 'real' greenhouse this spring as it was a pain to crawl in there and weed.If you can reach in there from the side it should be ok. I live north of you in Canada. I found the cold frame and other season extenders most useful in fall to get a second round of crops like radishes or bokchoy.In spring, it's abit more difficult because soil is too wet early on,and later (now), it can get too hot in there forcoolweather crops even if ventilated, and not yet out of danger zone for warmweather crops. sorrymy spacebar is actingup

You don't say what you plan to grow. Perhaps vegetables in rasied beds? Generally, I would ease myself into the new environment by having a reasonably sized garden at first with some soil ammendment. This would allow you to do some gardening while getting to know the local gardening community and the local methods. I am a frugal gardener and the thought of spending hundreds or perhaps thousands of dollars on raised beds and soil ammendments does not fit with my idea of gardening. I have some raised beds but those were made from salvaged lumber. Doing a little at a time is not a bad idea. It seems that in our "make over age" everything has to be done at once to look picture perfect. I do all my gardening with hand tools which includes a garden fork, a shovel and a crow bar. A little at a time gets the job done and saves my back.

The make up of a potting soil is actually less important than what plant nutrients are being provided. Almost no "soil mix" will be able to supply all of the plant nutrients needed for the entire crop cycle, unless it includes slow-release fertilizers. Commercial greenhouse vegetables are being grown in rice hulls, sand, rock-wool, bark shavings, peat, coir, water (hydroponic), etc. What matters the most for nutritious vegetable production is what nutrients are provided to the plants, aeration, and water availability. Hopefully the link below will be useful. Here is a link that might be useful: potting mixes This post was edited by cuestaroble on Sun, Jun 1, 14 at 19:39

I did some pricing on gutter cables last week. You can get up to 1,250 watts! If I were trying to protect tree roots, I'd bury the gutter cable and put alumized bubblewrap around the trunks of the trees to hold the heat in the soil during the winter... as well as find a thermostat to run the heating cable only when it's necessary.