How deep do your roots grow?

Seeds planted 1/8in

I wish there was a timeline with pictures somewhere out in the virtual world where I could compare my growth with the "normal" development.  I planted the brassicas on Feb 23rd in a Burpee 25 cell netted pellet eco greenhouse (plastic is plant based and the netted pellets are coconut coir).  They almost fully sprouted by the 26th and were under lights since then.

Seedlings emerging

After about a week all the seedlings looked great with cotyledons and about an inch worth of vertical growth.  I had them <2 inches under lights 16 hrs a day.  I kept them on the dry side and watered them from below.  I also misted them with chamomile tea/water to help avoid damping off.

1 week old

Things seem to be going well.  I even see some buds to suggest that the true leaves are about to show themselves.  I had read that one shouldn't pot up into soil until the true leaves are out since the seedling feeds off of the cotyledon.  So I wait and wait and wait.  And about 2 weeks out the cotyledons look a little dusky.  I'm not patient so I lift up one of the pellets and I see that the roots are emerging from the sides and dangling long out the bottom.

2 weeks old 

See all the roots hanging down?

This seems to suggest to me that the roots are going deep looking for food.  The coconut coir has no sustenance per se so do I pot up now or do I use a water based fertilizer.  I sent out cries of help to SFG foundation forums and Dave's Garden forums.  It seems as others have had problems with the roots coming out of the bottoms.  I also was warned that I might "burn" the roots by feeding directly.

After much hemming and hawing I decided to pot up on March 11.  I used cowpots (biodegradable and made from cow manure) and an organic potting mix.  Clearly the pellet netting did not stop the roots from penetrating the sides so I planted the entire undisturbed pellet into the soil.  I watered with a new seedling organic fertilizer (PHC).  Hopefully, the roots will now have a place to go and the seedling will be well fed until they are ready for transplant in the garden mid-April.

Now again it will be wait and see.  Fortunately, if they all die from being pot up there is still time to start anew for the season.