After the trays have sat stacked for a few days, it is time to uncover them and put them under the lights. I don’t wait a specific number of days. Every morning I check all my stacked trays and uncover any that are ready for light. There is some wiggle room for how many days you wait. However, if you unstack too early the newly germinated seeds dry out and die. If you wait too long the soil ends up molding because there isn’t any airflow between the trays.
You will notice in both of these examples the leaves are yellow. Because they have been stacked they haven’t gotten any light – after a day in the light they look significantly different and the leaves are green. Once I uncover the tray, I put it under my lights and give it a good watering. For most varieties, this is the only time I water from the top. These are newly germinated seeds, and as such they dry out easily. I give them a good saturation making sure to water the entire tray lightly.
I grow with two trays – the soil and seeds are in a tray with holes which sits in a tray without holes. Every morning I rotate all the trays on the racks, then check in between the two trays to see if they need watered. If I pick up the top tray and there is some water in the bottom tray, it waits until the next day. If not I put enough water water in to almost cover the channels in the bottom of the tray.
That’s about it for watering. Some varieties that grow super fast, like radish and sunflower, I water from the top occasionally because they dry out easily. The rest I only water from the bottom because I found out the hard way they get damping off when watered from the top.