Why did my indoor seedlings stop growing at ~ 2-3" tall just a few weeks after sprouting...
Started seedlings in basement boiler room, generally humid, ran a fan Kept florescent lighting at about 6" above the soil then seedlings for 16 hours/day (researched & bought recommended lighting -- can't remember the specs) Not sure what the daytime temp was, but I'm sure it dropped below 70 at times, definitely cooler at night Used MiracleGro seed starter (never fertilizer or pesticides) in cardboard toilet paper rolls with newspaper stuffed in the bottom to hold the dirt in (disintegrated quickly after being transferred to the garden) Kept them on cookie sheets and watered from the bottom only when the top soil became dry Sprouted quickly using a grow mat and turned it off once seedlings popped up Green beans grew like gangbusters; cucumbers & melons eventually started growing again weeks after initial growth spurt; and the peppers and tomatoes didn't really start growing again until they got outside. Most of the seeds were from Gurney's; approximately 95% germination rate Thank you!!!
Based on your (very thorough) info we still can't say for certain, however here are some thoughts:
#1: We'd recommend keeping the grow light about 1" above the seedlings.
Since they had almost no root at first, be sure the soil wasn't getting dryer farther down than anticipated before you watered.
Peppers are normally very slow. Not surprising that growth lagged there.
Take a look at some of the videos about growing vegetable transplants in our Grow It Eat It section on our website. Something may ring a bell: http://extension.umd.edu/growit/food-gardening-101/starting-vegetable-seeds-indoors
I feel your pain. It is so frustrating to research, execute, then underperform. I've grown thousands of seedlings inside and out over the yrs, and while there are so many variables, my suggestion is that you may be overly concerned with watering from above, as well as constructing that funky cardboard cell. I have had good results from clean plastic 4-9 cell containers watered from top. I do use 2L soda bottles with 3 nail holes in lid to apply at soil level to avoid stress on leaves. I've never used heat mats, and think that lamp heat is usually sufficient (but put a thermometer where you are growing in bsmt.
If your seeds germinated then it seems unlikely that the seeds are at fault. Presumably seedlings are most likely to die from damping off or insufficient water (or inadequate light). The former is diagnosed by brown rot then collapse at the base of the stem. The latter causes wilt then collapse. My best guess is that bottom-watered cell did not transmit adequate water through your newspaper base into the soil up to the plant, or perhaps to get the top of soil moist, there was too much water and roots rotted. Regardless, why not try something different this yr for all or some of your seeds? Good luck from an amateur gardener in A.A.Co.