Here I list my tips for seedling care. Be ready, because sprouts can appear as soon as three days from planting, especially of you are using a bottom heat source. Otherwise they can take up to a week.
Provide light. When you see a sprout, it's time to turn on the lights! You can put the tray in a window for light, however that may result in too-tall, bending, spindly plants. The best route is to rig up an incandescent light source that can be hung within 2 inches of the tops of the sprouts and that can be raised as they grow. This prevents them from growing too tall, too fast which produces leggy, weak plant stems.
Remove dome and bottom heat once the majority of plant cells or containers have at least one sprout in them.
Also--if you are using the Gardener's Supply APS, now is the time to install the wicking parts according to the directions provided with the system.
Lights off at night. Make sure plants have light for 18 hours--no more, no less. You can get an inexpensive timer device that will turn your lights on and off. Not necessary but is helpful if you forget things easily like I do. Keep in mind that too much light can be as bad as too much water or too much fertilizer and the opposite is true with not enough light.
Provide a breeze. Proper air flow can enhance seedling care. Place a small fan set on the lowest setting in front of the plants. The breeze will help the plant stems grow stronger while providing nice air flow around them. The fan can be on the timer too--I usually just leave the fan going all the time.
Add water to the tray so roots can absorb it from the bottom. Keep seedling mix evenly moist but not soaking.
Notice new leaves. You will notice that the second pair of leaves to emerge are different than the first leaves that appeared upon sprouting. The first leaves are called, "cotelydon leaves" and the second pair are called,“true leaves”.
Remove extra seedlings. Remember that you planted two seeds in each plant cell or container? You now should have at least one but more likely two seedlings per container or cell growing together. When you have "true leaves", you must remove the extra seedling in each cell. I know it sounds brutal, but it is necessary--two seedlings sharing one cell is not healthy. The best way to do this is use a sharp scissors and carefully snip the extra seedling at the soil level. I try to remove the weakest, leggiest and/or most undeveloped looking of the two.
Provide nutrients. Also, with the onset of the “true leaves”, begin fertilizing the seedlings once every two weeks or so. The experts say to use ½ strength fish emulsion in water. I’m not usually organized enough to have a measuring device handy so I just put a few drops of the liquid fish emulsion into my small water can and fill it up the rest of the way with water and water as usual. I do this every other week if I remember.