I planted my petunia seeds about 2-3 weeks ago. A few days after they came up, they are starting to die. What am I doing wrong? I have them on a heating mat and under a light during the day and shut them off at night. I mist them once or twice a day and sometimes pour water in the tray. The soil doesn't look or feel too wet.
This sounds like dampening off, which is a fungal disease that will kill off your seedlings.
Make sure you use a very light soilless mix to start the seeds.
Planting medium should be evenly moist.
Air circulation is important; I like to use a light fan for a few hours each day. You can even use your hand to gently brush across the seedlings several times a day.