I agree with mandolls....fluorescents don't put out much heat compared to incandescent, but it still puts out noticeable heat which surprised me. I am also a newbie and need to invest in a soil thermometer, but I can tell by experience so far that the shelf I added immediately above my top tier 4' 2 bulb fluorescent light fixtures stays a nice comfortable "warm" and has been perfect for germinating all manner of vegetables so far. You could even box in (or just tent it with plastic) the area above the light and it would become even warmer but that's not necessary for my setup inside the house where the air temp is usually 70-75 anyway. With a cooler basement setup you may want to build a sort of grow box to trap more of the heat rising from the light fixture as your germination station. Now I've just planted daylily seeds on the rack to germinate so we'll see how they do but I expect good results. Good luck with your seed starting!

I was curious about the heating pad v seed warmer difference too but mine is mostly for some plants that seem on the edge of unhappiness from the poorly regulated temperatures. I reckon if I can help them through to longer days than more light will help them too. "Water risk" is balony, IMO. My partner's (not so bright) ex made what was essentially a 6' heating element wired into an extension cord. I had to pack her stuff (yup) and when I saw that I was alarmed. Fortunately she's a dilettante and had forgotten about her (messed up interpretation weed filled) fling with what she called permaculture. So apparently some 15 years ago you could buy bulk heating element but I strongly encourage you to follow at least basic safety precautions. If you plug it in and the naked element scorches you, that's not good.

Heat is needed only for germination and not recommended for growing on. You'll find several discussions here about this vital point. Standard recommendation is once the seeds germinate they should be removed from the heat and provided with cooler growing temps to avoid cooking the young roots (among other things). Any covers should also be removed at the same time. Heated soil not only risks the roots but encourages leggy growth and soil fungus problems. 45-65 or 70 is considered the ideal growing range. Cooler requires added heat, warmer requires active cooling. Dave

I was hoping busy1 would see your question and respond because if anyone knew if it could be done, it would be him. I start all my vegetable seeds indoors on a homemade light shelf that holds 20 flats (if I cram them in) or that easily holds 15 flats. Once the plants are a couple of inches tall, I carry them outside to the screened-in porch where they happily spend winter days in bright light and coolish temps (or warm ones if it is a warm, sunny day). At night, if I think the back porch temps will stay above freezing, I leave them out there. If the temps are likely to go slightly below freezing, I just throw a Reemay frost blanket over them. However, if it is going to be very cold, I bring them back inside. Cold weather can kill some small seedlings very quickly. I've never lost seedlings left out overnight on the back porch, but it has insulated walls and an insulated ceiling and it still gets very cold. I cannot imagine leaving small seedlings in an unheated greenhouse, and even if you heat it, as Busy1 noted, you will have to be sure there's good ventilation to prevent damping off and other disease issues. I'd suggest you get a Min/Max thermometer if you don't already have one, and keep a daily log of your greenhouse highs and lows, beginning as soon as possible. Also keep a record of your outdoor lows and highs so you can compare outside air temps with your greenhouse temps. By tracking the temperatures that way, you'll be able to determine how much warmer the greenhouse stays at night (if it stays warmer at all) and also how hot it gets during the day. That will tell you if it is warm enough to start seedlings in it. The biggest misconception I think most people have about greenhouses is that they think the greenhouses stay nice and warm at night, which isn't really true. Without a heating system, they get just about as cold as the ambient outside temps. If you do as Busy1 suggests and use a heating mat and reflective insulation, that will help quite a lot. I still think you'd have to throw frost blankets over seedlings on very cold nights. It only takes one night of too-low temps and you could lose every seedling in the greenhouse. In addition, there's the issue of seedlings underperforming later on because they were too cold early in their life. That's a particular problem with pepper plants, which need to stay consistently warm and cannot tolerate very cool nights the way tomato plants do. If pepper plants are subjected to very cool yet above-freezing temperatures early in their life, they can remain stunted and unproductive for months, and sometimes for the whole growing season. Dawn