Re: When To Transplant Spinach Seedlings?

I believe you'll have best success if the seedlings develop some
before you set them out.  They need good light - if you don't have
good light, you need to provide decent artificial light.  I'd wait at
least until seedlings develop true leaves.  You won't have to
fertilize until you have bigger plants - the seed has enough nutrients
for small seedlings.

When setting out, be sure to do it gradually.  Expose plants to direct
sun only for a few hours at first - best if the day is shady, cool,
and calm (no wind to speak of).  Protect from wind if you have a cool
cloudy day but it's windy!