The biggest issue when growing seeds indoors is not getting enough light to the seedlings. As they strain to find it they will grow very tall. This is called legginess. Here are my broccoli seedlings after 4 days.
They look so tall and proud at first glance, but underneath there is a weakness in them that will not be able to support the coming growth. As the plants develop more leaves the stems will begin to keel over. Compare them to ones planted straight in the garden last year.
Firstly, I wanted to try to prevent legginess from happening to the seeds still coming up so I added another light to each box. Most of the time the lid is open as shown, to allow air to circulate. I’ll probably add more air holes before using them again.
I also raised up the trays by overturning some spare trays and placing them underneath. Ideally the light should only be one or two inches above the plants. Mine had been about five. As the seedlings grew I removed the layer of spare trays, as needed.
Secondly, I had to try to fix the legginess problem (in the broccoli, cauliflower, cabbage, arugula and kale). I very carefully chose the least leggy seedling from each cell and replanted them. I had no idea if they would survive, or whether the long stem, now buried, would rot in the ground. Two days later and all seems to be well.