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When starting petunias from seed, do you sow shallow? What is the best temperature for the heat mat for the best results, and when do you start using the grow light? Before the seeds sprout or once they sprout?
With petunias you'll want to just scatter the seed across the surface of the soil and not cover them at all. My heat mat only has an 'on' and an 'off' switch so I can't tell you how warm to set yours, but I'll bet the spread sheet has that information. And as far as light it's best to set your tray of seeds under the light right away. The seeds require light to germinate so the sooner the better.
I saw you have another thread asking about light. Some people will disagree with this, but I don't have mine on a timer so in the beginning they get light 24/7. After awhile I run out of light space so they get 12 hours on then 12 hours off so that I can rotate all the seedlings underneath them. And you'll want to keep the seedlings as close to the light as possible...a couple of inches away is ideal.
If the seeds are pelleted, just press them in. If it is iddy biddy seed (not pelleted), damped the soil 1st & scatter the seeds. A fine mist of water will settle the seed for good soil contact, but not too strong that will push the seeds out. Cover with plastic dome and place under lights for about 16 hrs a day. The seeds need light to germinate.
The heat mat I have will heat my tray to about 21ºC or 70ºF. It really depends on the ambient room temperature. I have the shelves in my basement utility room. Cement floor and very small windows, so it stays very cool in the room. The lights will create heat, so need to watch that. I have a little digital thermometer that is inside/outside. The little outside sensor is hardwired to the base. So I have ambient temp beside the trays under the lights and the little sensor inside the water tray which is a good place for soil temp. I don't think you want it too high. If I'm home, then the furnace is usually turned up, so I shut the mats off. They got to 29ºC, 85ºF which I think is too warm. We turn the house heat down at night, so I put the heat mats on.