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

One thing at a time. Don't transplant and stick them out in the sun on the same day :)

Noooo! :( Some of my peppers are wilting after last night and I don't know if they're going to make it... *sigh*, I hate hardening of plants.... I'm going to move them and a couple tomatoes that are showing stress-purple in their leaves back into my LED grow chamber. Which means they'll have to start over with hardening later, if they even survive :P My other plants don't look as burned, including other peppers and tomatoes -- I'm guessing that the location they were in was too exposed to the cold or wind or something.

Hardening off the plants is really just to get them used to the immense change going from Fluorescent lights to the intense light of the Sun. If you take a plant raised inside and put it directly out in the sun, you can literally watch the leaves wilt. It might be a surprise seeing how short of a time it takes to completely ruin the plant. We're talking minutes, not hours. I spend about 10 days to acclimate them to the morning sun. I don't give them any direct sunlight until it's been 5-7 days. I always plan it so that I can watch them very carefully the first 5 days. I typically start on Wednesday. I want to be able to keep them outside when I'm home from work that weekend. I have not kept good records of my hardening off scheme. I just make sure to peek at them frequently to make sure they're ok. At the same time, I want to get them used to the life outside as fast as I can. This should be pretty close to my hardening off schedule: * day 1: 1 hour before sundown (in shadow) * day 2 : 2 hours before the sundown (in shadow) * day 3: 3 hours before sundown (in shadow) * day 4: Saturday: Getting more aggressive. Keeping them in the shadow during the day. Plants are outside most of the day. I make sure to watch them pretty carefully since the intensity of daylight even in shadow is many times more powerful than the indoor lights. * day 5: Sunday: Shadow most of the day, start introducing evening sun. Checking plants frequently. * day 6-7: Monday-tuesday: Taking them out in the sun after coming home from work. Outside in shadow when I'm at work. * day 8-9: increasing the exposure to day-light sun. * day 10: Plants are able to tolerate morning sun as well as daylight sun. Putting the plants where they'll life the rest of the summer. I am sure that there's going to be various opinions on what's best and how fast. But in the end, the only important factor is to ensure the plants safety so they don't experience months of setback, or in worst case dies. The more time you can spend with them during this time, the better. I am pretty sure that the ones with the most plants in this forum, will take a week off their regular work to ensure the hardening off is done right.