If I'm growing seedlings indoors for weeks, I prefer to use the unconditioned basement rather than conditioned space to grow seedlings. The warmer conditioned spaces encourage the seedlings to grow faster, which is a no-no without LOTS of light, which is difficult to provide.

So growing under lights in cooler temps (like an unconditioned basement) typically lead to sturdier transplants than growing warm under lights.

Before I got my 480 sq ft hoophouse put in I used a very sturdy low cold frame that I put about fifteen steps out my back door. Mine looks very similar to this:

The outdoor mini cold frame was *EXTREMELY* useful in hardening off plants prior to transplanting them out. I also had a small heat mat out there to put the more cold-sensitive plants on top of if it was just too cold.

I think that the short cold frames retain heat in the right places a little better than the tall ones with tops that are too high off the ground.

My frame is about 4 feet by 8 feet and was totally indispensable until I got my big hoophouse. I think this year I'll get the little cold frame in operation again because it's a shorter trip than making the way to the big hoophouse for setting out and bringing in transplants.

I guess what I'm really trying to say is that I like to use a combination of a small indoor growing space (at cool temps and under fluorescent shop lights) and a small outdoor cold frame (with a little heat mat if needed).

You will always need to harden off plants prior to transplanting them out permanently, and a well-made polycarbonate cold frame allows you to do so at much lower risk to your plants!