I usually plant those out when average soil temps. are in the upper 40s, which is also about the time the soil is dry enough and severe freezes (

From TAMU:

Correct temperature readings need to be observed on three consecutive mornings. Readings should be taken at a depth of 1 - 2 inches for seeds and 4 - 6 inches for transplants.

Use the following guide for minimum soil temperatures for seeds and transplants:

60 F - tomatoes, cucumbers, snap beans
65 F - sweet corn, lima beans, mustard greens
70 F - peppers, watermelons, squash, southern peas
75 F - okra, cantaloupe, sweet potatoes

Then there is the graph linked below. Scroll down to the chart on field transplanting.

Dave

Here is a link that might be useful: TEMPERATURES AND TIMES REQUIRED FOR GROWING PLANTS FOR FIELD TRANSPLANTING

That chart on field transplanting would seem to be air temps.

Onions are extremely hardy. As soon as the soil can be worked. Mine are already in the ground in nebraska. My direct down radishes are also up already. I won't put my brassicas out for another week or two. I have had broccoli plants survive all winter though. So I imagine they could out soon. Lettuces aren't as hardy. Spinach and carrots are also fine to seed as soon as the ground can be worked.

I didn't take it that way buy yeah I guess it could be read as air temps.

Found another that said "sit in the garden bed for 60 seconds, pants off. If you can stay there, comfortably, go ahead and plant." :)

Otherwise most of the info seems to be broken down by specific vegetable - best soil temp for tomatoes, best soil temp for peppers, best soil temp for planting potatoes, for cole crops, for herbs, etc.

When you think about it there aren't too many other things that get get planted as transplants rather than seed.

Dave

General rule;(IMO) if a seed can germinate in soil of X degrees, then it can grow in it or even in 5-10F cooler. Peppers need about 5 -10F higher temps than tomatoes. So do eggplants.

But It does not mean that if you plant your tomatoes in 50F soil, they will die. To my experience, they will just sit there pretty for a while and eventually get used to it. That is natural acclimation. I often take chances and plant early. This year I will warm up the soil around my plants by plastics. I also think that the plants' top will tolerate temperatures as low as your refrigerator. But the roots like warmer temperature in order to get adequate amount of nutrients. MAYBE you can compensate for cool soil by folriar spraying. MAYBE !

I just go mostly by the calendar....Set out onions March 30th, lettuce and spinach April 10th, Glads [first setting] April 5th, peas April 5th, potatoes April 15th, Broccoli April 14th, Tomatoes and peppers May 10th, watermelons and cantaloupes May 18th, etc.

These days, the calendar method is becoming less reliable. We're running 10 degrees below normal this month.

I'm particularly concerned with brassicas, the possibility of failure due to unseasonably cold conditions at setting out.

particularly concerned with brassicas, the possibility of failure due to unseasonably cold conditions at setting out.

They are a difficult balancing act - out soon enough to beat the heat but not so soon the cold gets them. But the threat to them is more air temps than soil temps. So the best solution is out early as possible but under row cover for protection.

Dave

PS: I agree - the calendar is the least reliable timing method.