sprouts stopped growing

I set some sprout seed in a seed tray in an unheated greenhouse a month ago. They were away like lightning, but after about three weeks, they seemed to stop putting any timber on. They are about 3/4" high and look fit and well.

I was wondering if the seed tray was deep enough for them Matt? Would I be better putting them outside?

i sow mine in cells/trays and then transplant them in their own individual pots then later plant down at the allotment.

Plant them deeper when transplanting.

Find it helps avoid more of the bugs down at the site and is another bed that is easier to keep weed free before putting in their final positions. I used to suffer couch grass too so it made the plants stronger and able to withstrand problems better