I'm using Sungro 3 organic seedling mix from Steuber's Distributing in Snohomish as the medium for planting seeds.
So I'll remember next year, here's the procedure:
I have two propagation stations so I can germinate seeds at two different temperatures, most seeds at 70 degrees, the seeds for heat-loving plants (tomato, pepper and egg plant) at a warmer temperature, around 80 degrees.
Vegetable seeds don't need light for propagation, but when days are cool I keep the grow lights on as an extra heat source. The heat mats are limited output and keep the cells at a maximum of 15-20 degrees higher than the ambient temperature.
The propagation heat mats are of two sizes:
With this setup I can have 8 trays propagating at a time.
I set a heat mat controller for a desired temperature for the related seeds, from 65 degrees to 80 degrees.
A heat mat controller turns off power to a mat when its sensor registers higher than the set temperature.
I have two temperature controllers. Depending on the number of plants I have at each temperature, I may hook the large mat and a small one to one controller and the other small mat to the other controller. Or I may have two equal areas at the two different temperatures. I like the flexibility.
After the seedlings are up, I remove the clear covers and I leave the grow lights on for at least 10 hours a day. I have the lights about 7 inches above the tops of the seedlings.
It's also at this time that I have an oscillating fan blowing air. This is a prelude to hardening off as well as a way to preclude development of bad fungi and bacteria.
I let them grow, looking for the first set of true leaves.
For details about propagating cool-weather plants see Cool-weather plants.
For details about propagating heat-loving plants see Heat-loving plants.