What were they in previously (under lights?). Were they placed into direct sunlight? How cold? Basil is extremely cold intolerant.

A typical hardening off schedule for me would be:

1. Late afternoon or evening place the plants outside (not in direct sunlight or if so only for maybe 30 minutes to an hour max).

2. If nighttime temps are warm enough I leave them outside that first night.

3. The next day I keep them in indirect light the entire day (under covered porch or under a tree is good). If on the colder side I cover with row cover which also helps shade them.

4. The next day if its overcast I will let them stay out all day in direct light. If sunny I will either cover with row cover/shade cloth or put them in a place that gets shaded for most of the day (allow them either either morning or late evening sun only).

5. After that they can pretty much handle it all - just need to keep an eye on nighttime temps.

I like to let them get acclimated to indirect sunlight gradually each day on prior to putting them out in the direct radiation. I start hundreds (thousand this year) plants so I cant really do the whole carrying them in and outside. So I need to find places that get the right amount of light. Row cover helps tremendously.

It's possible that they were put outside too early in the season. I have yet to put my tomatoes, peppers, and basil outside for any length of time because the temps have just been too cool.

When they do go out they will be on a screened porch (in the shade). Even here (being more southern then Ohio) that probably will not happen for a few more days due to the low temps being in the 40s, (too cool) especially for basil and tomatoes.

Tomato plants can have tissue damage below 50 degrees. Basil, too.

Just FYI, having a heating pad and a cover doesn't mean that it's above freezing in there. It just means it'll be somewhat warmer than the ambient temperature. So I'm assuming you had a thermometer in there to track the conditions. Of course, the plants shouldn't touch the cover itself.

Thanks. It got down to upper 20's. I had a good, thick, plastic cover and on north end black wood to adsorb heat. My point is during the day it warmed a lot and cold frame trapped heat. Nights below 35 I also put blankets on top. I doubt it ever got below freezing anywhere - but close.

Plants were under lights. First day I saw damage on plants - worst on tomatoes. Peppers did the best. Basil, parsley also had damage - on edge of leaves and sometimes spots on leaves at random.

For sure it went below 50, I did not know this would cause damage to tomatoes and basil - likely that is it.

What died: tomatoes, parsley, basil. Not all but some.

"Low Temperatures

Although tomato plants can survive temperatures down to 33 degrees Fahrenheit, they show problems when temperatures drop below 50 degrees F, according to the U.S. Department of Agriculture Research Service. Cool nighttime temperatures interfere with the tomato plants' ability to convert sunlight into sugars through the process of photosynthesis. Low temperatures also reduce pollen production, resulting in fewer fruits or even fruit deformation. Cat-facing, which is puckers, scars and cracks on the blossom end of the fruit, occurs when nighttime temperatures drop below 60 degrees F."

http://homeguides.sfgate.com/night-temperature-tomatoes-76197.html

If it was in the 20's I'd consider yourself lucky to have what survived. I agree with lucky that it was probably too much sun given that the parsley keeled.

One thing - if you don't have any other options, try to continue growing the plants with a little more care (get them back into more ideal conditions). You might be surprised at what continues growing.

Most are going to make it and several tomatoe plants did FAR better the others - good info for the future. Cherokee purple tomatoes had zero issue. Roman and cherry had lots of issues.

I also think it was the sun.

More info: in the past my hardening was nowhere near recommendations. I leave out less time and less days BUT they go into a cold frame with a heating pad - along with other seeds I am just starting. In the past 2 year I had no issues at all. The cold frame definitely minimizes the amount of hardening required.

Yes, that's true that tomato *fruiting* can be affected with temps below 50F. Inferior pollination, cat-faced fruit, etc. But foliage can withstand temps down to just above freezing. I routinely subject tomato seedlings to temps in the 30s, and they do fine. They don't grow at those temps, but they don't keel over. They're a long ways from doing any fruiting.

Get a thermometer.

I had a thermometer I took out of my fridge. It shows a range of below zero to 90 degrees. Immediately it went to no reading and never came back until in fridge! Odd.

Well, that clearly was indicating that there was no temperature. I guess you can't worry about temperature that much if there is none.

It's way too early to put tomatoes and most vegetables started indoors outside in zone 5. Wait until the last frost date (mid May) and then start hardening them off, 15 minutes of sun the first day, gradually increasing the time over the next week or 2.

Your plants are burned and cold. Way too early, too much too soon. Lesson learned!