I think painters drop cloth is too thin. You need a heavier plastic but why you'd want a cold frame for hardening off plants beats me. I hope know you just couldn't put them out there and forget them. A cold frame takes a lot of work. It has to be opened and closed every day and plants tend to dry out in them if they are in pots. Even on a cloudy day heat from the sun builds up in a cold frame. Stick plants grown inside in a cold frame and you'd have fried plants

You can't put plants directly in the sun to harden them off. They need to be gradually introduced to the sun or the UV rays will burn them.

When I lived in Michigan the cold frames we used were made from old window frames with a box built to hold the window frame, using hinges. Before setting them out fresh manure was buried then covered with several inches of soil for bottom heat. And yes during sunny days they do need to be opened some depending on how warn the daytime was as well as how hot the sun rays were. Never found this to be that big of a chore in Michigan.

If you build it properly then it should work for you. When first starting with it you'll need to monitor the temperature outdoors and inside the tunnel. Keep in mind that your plants will be more at risk of burning than freezing. When in doubt make sure that it is ventilated during the day and closed at night. It's probably too early for things like tomatoes, peppers and basil to go into coldframes. Hardier things like spinach, onions, chives, lettuce, broccoli, mints, thyme, strawberries, cabbage etc are fine to be outside now.

You also need to harden plants off before moving them to cold frames. Not as much as a full time move to the great outdoors. I usually take about 4-5 days of increasing time outside. After being in my frames for about a week I consider them ready for transplanting though many will stay in the frames longer.

Below is a picture of my cold frames.

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This post was edited by gjcore on Tue, Mar 26, 13 at 14:01

A cold frame can be very useful.I've used 2 for years.A heavy plastic or glass is needed.There are many inexpensive ways to rig up something to harden off plants.Sometimes I put plants in our garden cart with plastic on a frame on top.A quilt with tarp over will keep things warm if it turns cool.
rose