Forgive me if this has been already covered in depth, but to type in any of these words into the search feature...

I think I’ve got a handle on the fact or idea that soil that is too cold at putting out tomato sets in the garden will stunt and otherwise cause serious problems for tomato plants. Not talking about seeds and germination, but what is what you might consider the lower end of soil temperature for tomato plants to remain healthy?

Is it ~60 degrees F, 15 C, is it lower or higher than that? Anyone have experience with more or less nailing down any numbers on this? I’m looking to put in a few tomato plants as early as possible, as a gamble on the weather and any sort of late frost ruining my plans, but I know it will be a futile and counterproductive effort if the soil temperature is going to more or less automatically doom the plants to stunted growth and being diseased prone. I might get lucky on the weather, but since soil temperatures don’t move up and down quite so rapidly as the air temperatures, I’m just wondering what number temperaturewise might be give a reasonable shot at healthy plants.

Typically here, late frost seems to be more of a limiting factor on getting in tomato plants into the garden than soil temperatures. By the time the reasonable danger of frost is past, the soil is already warm enough. But if we somehow get a mild late winter and the frost never comes, I want to just see where the soil needs to be to have any shot at all.

So long as there is sun hitting the soil, it can get warm pretty quickly and hold onto that heat well enough. For short periods of time, 55°F wouldn't kill the plant. I wouldn't grow it at that soil temp for long, but if you have to get the plants into the ground and your unable to wait (Been there) then this will be ok. Around my area, before the last frost date the soil is ready and waiting. Also, wet soil retains heat better. So if you notice a cold front coming, cover your plants and moisten the ground before hand.

The lower the soil temp, the more difficult it is for beneficial aerobic bacteria to grow, and this problem is compounded by wet weather and high levels of organic matter in the soil, leading to damping off and root rot.

Build a little hoophouse, would be my suggestion. Plastic is a proven method. I get tomatoes about a month earlier in a high tunnel.

I can't answer your question.
I will share with you what happened to me last year. Draw your own conclusions as to what you might want to do. My ground planting target date was the 1st of May. On the first of May, the extended forecast said-- chance of frost on the 7th. My plants were already root bound in 2 inch square deep 4 pack containers. I switched them to 16 ounce individual containers. I didn't have enough stuff to do all of them. Therefore 13 plants had no home to go too. I took those plants out, and stuck um in the ground. I had enough 5 gallon buckets for cover. The tray plants were carried inside the garage each night. My garage is unheated, but nothing has ever froze in it during winter. The frost barley missed me. But frost was forecasted on the 10th once again. Rains came the day after that. There was 16 days difference between the plants I carried in every night. And the ones set out. Those I carried in was almost unmanageable. They were over knee high. The warmer garage nights really made um grow. The ones left outside at night to fend for themselves-- were 10 inches shorter. This occurred in just 16 days. The ones outside did produce, but were never the plants of the ones I sheltered in the garage.  PS-- drilled holes in the bottom of 16 ounce plastic cups can work, if you are larger potting squares poor. .

My target is 65F at 4-6" depth.