I've been trying to start cauliflower, broccoli, and cabbage seeds for the past few years. Every year it ends the same. Seedlings start to pop up a few days after being seeded and look great for a few weeks. After that (shortly after the first true leaves appear) they get leggy and start falling over before shriveling up completely.
I only water them when the trays start to get light and the only fertilizer I add is a touch of fish emulsion. For growing medium I've used straight peat moss, potting mix, seed starting mix, composted horse/cow manure/wood shavings, and composted wood chips. Nothing works. I keep the seedlings inside where it's warm and keep grow lights on them (couple inches above the seedlings) constantly until the first true leaves start to appear. Once the true leaves show up I decrease the light to about 12 hours and start putting them outside on warm, overcast days for short periods of time.
For those of you who grow your brassicas from seed, how do you do it? Obviously I'm doing something wrong.