Hardening Off is about conditioning plants for the total experience of being outside in nature. Wind, full strength UV radiation and cold nights. Some plants only need week to get with the program others take a little more. You'll just have to watch them to see which ones are handling it better. Plants that have never been in full sunshine or wind will wilt, so you want to start out in the shade (bright shade) and keep them watered. What I've done in the past was to trim leafy branches from weed trees and stuck them into the ground around my freshly planted seedlings. The umbrella of shade would last about a day (different trees handle this better than others). Right now a lot of trees haven't leafed out yet - but privet, eleagnus and ligustrum work well. If they wilt completely I will prune out some new ones and replace them each day. Sounds like a lot of work but my yard has tons of weed tree saplings and I was born with pruners in my hand so it isn't that much trouble. Anyway with twice a day sprinkling I could keep most of the saplings providing some shade for the transplants. After a few days I let things slide a bit and by the end of the week I yanked out the sticks and let them sunbathe all they wanted. If you keep your babies in flats (highly recommneded until mid April), you could just keep them in a shady part of the yard at first and then move them out near where you intend to plant them. If the weatherman says its gonna get cold (32 or below) then I would either cover them all up with newspapers and mulch or old bedsheets and if you're desperate - new bedsheets. Or simply move them up beside the house wall or near a door or window (an amazing amount of warm air leaks out of windows and doors). If I notice one of the plants is handling things without problems then I go ahead and plant it. Some take a while to adapt, others don't.

I was raised in Mississippi in the 40's about 30 miles South of Jackson. At that time, the area (Copiah County)was known as the Tomato growing capital of the US. Tomatos, Bell Peppers, beans, cabbage were the major crops grown on farms ranging from 5 to 100 acres. It was a common thing for these farms to have "hot beds" and Cold Frames to get the plants ready for the field. The hot beds were kept heated with kerosene lanterns and after the plants were moved to the cold frame, a thin white denim cloth commonly called "cold frame cloth" was used as the cover. The hot beds and cold frames were used for cabbage plants and later in the season for Tomato and Bell Pepper plants. In town, there was a steam powered "cold whistle" called the "Squeedunk" because of it's strange sound. The thing could be heard for close to 10 miles out. The local Western Union operator would get the weather report and relay it to the town fathers who would authorize blowing of the "Squeedunk" to warn the farmers of a frost or freeze that evening. If the Sqeedunk was blown during the day, school would let out so the kids could hurry home to help cover up the cold frames and get ready for the front that was about to pass through. Back in those days, weather reporting was about 3 hours in advance so there wasn't much notice of weather changes. Your idea of using a cold frame is a good one that has been tried and proven years back. In that part of the country, the time honored rule is to get tomatoes in the field before the first of April, but of course, for us here about 200 miles further North, that is a bit risky. Every year in Crystal Springs MS in July, there is a tomato festival that honors the time frame from the 20's through the mid 50's when the vegetables were grown. I have a friend from high school who after retiring is back into the tomato farming business to have a booth at the festival. He has about 1/2 acre of tomatos and his are always the prize winners. His recipe for tomato fertilizer is 5-20-20 and it is regularly available in bulk from the local Co-ops. For a side dressing later on the tomatos, he uses "sodie' as he calls it. It is not the old Nitrate of Soda (16%) but is a Sodium Caltrate that has lots of calcium along with the 15% nitrogen. He is adamant about keeping the Nitrogen low for tomatos. He says that the Phosporus and Potash will make the Tomatos "fruit and root" and the Nitrogen would just make vine. He grows them in the same spot every year and between seasons treats the ground with "Slag" and Lime. I am gradually pulling his secrets from him but it is a slow process. He picks tomatos when they are barely "pinking" and brings them in to an airconditioned double garage where they are sorted and layed out to ripen. This business that you see on TV with the upside down tomatos and vines covered with bright red tomatos is purely Hollywood stunt photography. Real vine ripened tomatos happen on the window sill of the kitchen and not outside on the vine waiting for Mr. Bird to come peck it the day before you were going to pick it. This guy knows how to market and sells his tomatos in boxes about the size of a Xerox paper box for $25 each or $1.50 per pound in peck size containers. Last year at the festival, he sold 125 of the full boxes and ran completely out of tomatos. The culled tomatos were sold from his farm (about 12 acres) to people looking for canners. Only the best shaped tomatos make it to the festival and he has people lined up at his booth. There will probably be about 10 booths of tomatos at other vendors but none have the quality and looks that his do. In addition to the ones sold at the festival, he probably sells an additional 100 to 150 boxes to people who come out to the farm. With the improved fertilizer and plants of today, he is probably making 20 times as much on his half acre of tomatos as his family did on the full 100 acres back in the 50's. He and I recently had a phone conversation about the old cold frames and the "Squeedunk" whistle. Now, he purchases his tomato plants from the Junior College in Raymond MS about 35 miles away. In January, he ordered 2500 plants from them, picked them up the last week in March and set out (by hand with a trowell- 2 days) just before the end of March. So far he has been lucky with the weather and hasn't had the hail that we have had here. With continued luck, he will have another banner year at the Tomato festival.

Ah, Steve, you're thinking like I'm thinking - but I'm thinking more about heat and drying winds than cooler temps. These last three years, I've been moving my tomato plants from shade-clothed greenhouse, where the humidity is 80-90%, temps are mid-80's, and maybe a slight breeze, out directly to the garden. And these last three years, within a week along come 20-30 mph winds, with the 85F temps, but with only 5% humidity. The plants just can't soak up enough moisture from the ground to compensate, and the plants never recover - stunted the rest of the summer. I'd thought maybe it was that old virus coming in early off the desert, but now I'm thinking more that its just the dry wind. So this year, I'm going to put them out on the east-facing front porch of the house - so they'll get strong morning sun, bit of a breeze to sturdy them up, and most importantly, be out in the dry air for 10 or so days before I plant them out. Peppers don't seem to be that bad. They go outside, face the same dryness and wind, the leaves get all wilty, but they recover. This post was edited by david52 on Sun, May 5, 13 at 12:13

Hardening Off Process: process takes 10-14 days Plants need to be toughened after a life of growing indoors 1) Decrease watering, without allowing wilting, a few days before beginning process  this will toughen plant 2) Put plant outdoors in a shade protected area for a few hours for first day 3) Over the next 10-14 days, gradually increase the outdoor time and sun exposure Using a circulating fan to toughen the plantÂs stem prior to beginning the process will allow it to go quicker. The plant should be out/exposed 10-12 hours before the final 2 days, prior to planting, of 24 hours outside. After successful hardening off, the stem will become thicker and the skin will show a waxy surface. Chloroplasts will rearrange such that they arenÂt stacked as much, preventing sunburn. As new leaves develop, theyÂll be smaller and better adapted for direct sunlight and life outdoors.