Plastic mulch certainly helps - there's no doubt about that. You need to level the soil so that the plastic is touching, not floating above the soil, because the heat comes from the plastic radiating it, rather than from the 'greenhouse effect.' With a hoop covering, the sun heats the soil, and the plastic keeps the warm air in.

The good thing about plastic mulch is that it raises the soil a few degrees and that's it, so you know what you're getting. With a hoop house, on a bright sunny day you can burn up your plants if you don't ventilate it, so there's a lot of opening and closing. You get more heat, and you protect from cold nights, but it needs watching.

The only thing zone 6b tells anyone is about which plants will likely survive an average winter in your area. The Average Latest Date of Frost around here is May 30 (although the USDA is trying to tell us it is May 15) and with minimal protection I can put out frost tender plants a bit earlier. People I know that live in the Columbus, Ohio area are about a month earlier then we are here, so where you are should be about the same.
Elliot Coleman grows food crops year around in his garden in Maine, in hoop houses without supplemental heat and was spotlighted in an episode of Growing a Greener World that some may find of interest.
I have not found plastic, a material made from non renewable resources, to be much good in my garden and it is more harmful than not. That, however, may have as much to do with the cloud cover, which will limit solar gain, as anything. Plastic laid on soil restricts that soils ability to breath and can keep rain from reaching the soil unless holes are punched inthe plastic which then limits its effectiveness.

Here is a link that might be useful: GGW 301

Jon - thank for your insight into using hoop covers and plastic mulch.

Kim - thank you for your info and the video link. Interesting video regarding winter gardening and some ideas on how to make it work.

My plan is to use hoop covers for early crops... and plastic mulch and sporatic use of hoop covers, as necessary, for my heat loving crops. I think I can get a month's head start with both set of crops with these strategies.

I will post results as they are available.

Thanks again,
Tom

Plastic mulch is used by farmers, who depend on it to put a roof over their heads and feed their children. The fact that it works tells you all you need to know about the soil 'breathing.' You're welcome to garden any way you want, but please don't mislead people.

jonfrum, I believe that I stated quite clearly that "I have not found plastic, a material made from non renewable resources, to be much good in my garden and it is more harmful than not" which does not mislead anyone about anything. That is my opinion which I am entitled to and is something I found over years of gardening. The soils here that I covered with plastic developed putrid odors that told me they could not breath, anaerobic, in the absence of air.

Kimm: if I recall correctly, your soils are rather sandy, correct?

My soil is not "rather sandy" it is sand. I also have a high water table that can contribute to an anaerobic soil, except when we have a summer like 2012 when the water table dropped quite a bit.
However, I also found that the clay soils in Northwest Indiana, Southeast Ohio, Middle Oklahoma, and around Nashville, Tennessee when covered with plastic get that same putrid odor.