I use only seedstarting/potting soil mix. I put them under lights immediately as they come up. I water moderately. I take them outside for HOURS as soon as possible. I feed them well with an all purpose plant/flower fertilizer. I transplant into the garden at 27 days from sowing...excellent results always.

How warm is warm? Brassicas grow better in cool temps, even while still indoors under lights. Mine are in the basement where it sometimes goes down to 11C overnight.

Are you potting them on at any point? I've found that brassicas need to be moved and replanted deeper than they were where they sprouted in order for them to grow sturdy.

Definitely go back to the lighter potting mixes, too.

It's about 65-68F in the house where the plants are. I found that they would get stunted in cooler temps for me. Then again they seem to do that anyway...
I tried moving them to bigger pots but their root systems are almost nonexistent and they wouldn't grow at all once in bigger pots. Just wilt and die even if I was extra careful. I even added a little rooting hormone (when they were about 2 weeks old) to the water hoping they'd perk up but it didn't have any effect. I've tried seed from a number of different sources so it's got to be something I'm doing wrong.
Thanks for the suggestions!

I tried brassicas for the first time this year and they were the easiest plants i have ever started from seed. my experience sounds similar to Wayne_5's above.

i planted cabbage, cauliflower, broccoli and kale. I got the seed from seed savers.

I won't be much help to you though because it sounds like i followed the same steps you did and had the opposite experience: they grew too fast and rooted too much. next year i will have to start them only 4 weeks before planting out rather than six (or put them in bigger pots).

the only things that i might have done differently than you is after they got up and going i watered them with a weak/diluted fertilizer solution almost every time i watered. it was a balanced fertilizer mix with some micronutrients too. i hear that brassicas are heavy feeders.

the second thing that might have been different for me is that i sprouted them at around 70 degrees but after they were up they went into my basement which is cooler. 50s at night and maybe 10 degrees warmer in the day. i had heard somewhere that they sprout better at warmer temps but then like to grow at cooler ones.

I used standard t8 shop lights with natural light bulbs just an inch or less above the leaves.

one last note is that brassicas are supposed to prefer slightly alkaline soils. i just used plain potting mix but you might try a different potting mix maybe? I dunno. I think my compost is more acidic than my soil, but i live in an alkaline area.

sorry for the long and not very helpful reply. i'd be very interested in hearing an experts reply as well.

Seems like some kind of fungal problem. Increase air circulation and light.

BTW, brassicas are superb self-sowers. Depending on how demanding one is, one can rather easily never have to start them again.

Hmm. Yeah, I really don't know what else to suggest. Definitely abandon all the mediums you've tried except the seedling mix; they all sound too heavy (to me anyway). I make my own mix out of two parts peat, one part perlite and one part vermiculite. I've never adjusted the pH for brassicas, but I do use a balanced synthetic fertilizer at each (bottom) watering. Do try setting a fan on them, as suggested. Are you re-using pots? If so, try giving them a bleaching (and thorough rinse) before using. Short of watching you actually handling them to see if there's anything there, I'm not sure what else I can think of. Sorry!

I have a friend who got seeds from one source that didn't seem to perform well for her.

Other than the suggestions that others have given you, that's my only thought.

On the other hand, I can't grow a successful radish for anything. Kale, no problem. Way too much sage, ditto. Enough tomatoes that my kids refused to pick another one. But no leeks either but garlic, shallots and chives are fine.

Something that no one has mentioned yet is the light. What kind of light are you using? If you're using sodium vapor or incandescent lights, that explains your problem. Seedlings need blue light, such as T12 or T8 shop lights as suggested by Jonathan, or metal halide lights, or blue/white LEDs.
The fact that they don't grow at all and don't exhibit root growth suggests to me that you are giving them the wrong type of light.
Many of the "grow lights" on the market are designed for flowering plants when supplemented with sunshine, not for starting seedlings. Can you provide a picture, description or link to the type of grow light you have?

This post was edited by Slimy_Okra on Sat, Mar 29, 14 at 20:31

Lights are blue or white LEDs (I'll have to look for specifics in the morning). I typically keep the lights very close to the plants once they start to come up. The containers I use are 4-pack cells bought from the local garden center.
I think I'll just stick to direct sowing or using volunteers for my brassicas from now on. It's too much work and space to have these things keep dying.

Good idea. Hard to go wrong with direct sowing. Brassica lends itself very well to winter-sowing.

Why waste energy on lights when you can pickle and store fresh stuff in a cellar?