For productive root growth, these are guides from several textbooks and professors in the agriculture department at Pierce College.

Acceptable range: 50 - 80 F
Optimal range: 65 - 75 F

Soil temp is only relevant to germination. After germination it is air temps that are important for growing on.

Ambient temps of 60-65 degrees. You can go a bit cooler but try to avoid going much warmer than that. For optimal growth when growing young seedlings in the greenhouse we work very hard to keep it in the 60-65 degree range and to avoid anything above 70.

Dave

I agree with Dave. Over 65 will increase internode length.

Thanks everyone for the helpful feedback. I should not have any problem holding 60 to 65.

Larry

I have my temperatures in the 65 range and my tomato and pepper plants have purple stems and veins within the leaves. How do I correct that problem while keeping the temperature within the range?

Based on my experience and readings, purple stems and veins are not a problem at this stage but rather a normal physiological phenomenon. I have never started seedlings that were not purple in some part or other and I raise nice plants both hybrid and open pollinated. Don't worry.
Victor

Yeah trying to correct that will only create other, more serious problems.

Dave

Awesome. Thank you. Would cold soils cause stunted growth as well or is that a different department all together. I'm feeding weekly with diluted fish fertilizer. 8-6-6 25% strength. Thoughts?