I would be careful of the temperature of the heat mat, which can cause the mix to become too hot, so I would use some kind of buffer to raise the seed tray and inch or so above the mat.

For me I have an AeroGarden with LED lights and I put my seed tray 2 inch below it. My record is 1.5days to germinate from seeds that was recently fermented and stored, which is very fast. Though on average, for healthy seeds it usually takes 3-4 days if the temperature and moisture level is right. I like keeping it at around 80*F and not too wet. If the top of the soil mix gets a bit dry from the grow lights I spray it with water, rather than watering the whole thing and causing the whole mix to get too wet.

I understand the excitement completely!!! I have started some tomatoes way ahead of schedule. I am going to try setting them in pots in my window well in about 6 weeks. :)

Here is my setup with grow lights. I put the soil in a cup that is clear so I can see when they need to have water. I tend to drown my plants... :) I put a hole in the bottom of the cup before I add the soil. Then I make sure the soil is wet before planting. I plant VERY shallow because those little seeds can get lost. Then I spritz the dirt with a spray of water and put a baggie on top. It takes about 3 days and they are up. As you can see from the photo through the plastic, the little seed heads are still attached. I keep the plastic on until those are shed. Otherwise, I have problems with them staying attached and the plant dies. Just my way to do things to avoid the problems I have run into over the years. I just use the heat of the lights to germinate my seeds. It works for me.

What kind of grow lights? I have LED grow light and in 1 month they were 2 ft high, which was about a month and a half early before plant out. Lesson learned, now I don't start until about mid March and plant out early May after the hardening off process.

They are T5 fixtures w fluorescent bulbs, two blue spectrum and two red spectrum in each fixture. I think the blue are about 6500 and the red are less. I started some perennial Calamintha nepeta - Marvelette Blue on Jan 2 that are supposed to bloom in abt 12 weeks and two tiny plants are already setting flower buds and one plant has an open flower with another ready to open (in about 5 weeks). They may be bloomed out before the growing season begins! :-) The indoor lights really make a difference. I'm also trying Alpine Strawberries this year and they're also doing great. Probably shouldn't have started all of these so early but for the little strawberries, I was told if started early enough, I'd see a few berries this year. I can't wait to see the little tomato plants!

Mary

I started my seeds last Saturday (2/6). Five to 10 seeds per pot. They sit on a heat mat (temp is set, no option to change) on top of the garden shed fridge.

On Wednesday (2/10) I had germination in one of the pots. Over the past 2 days I've had more. They get moved to the cold frame as soon as they germinate. When they get their first set of true leaves I start potting up.

As Dave said there are few factors. Number one , IMO, is temperature. I think 80 to 85F is optimum for fastest germination in about 3 to 4 days.

At 70F (my room temperature it takes about 7 days. At 70F room temp, no mat is really needed unless you are in hurry and just can't wait. I have found extra heat is helpful with germinating pepper more than tomatoes.

Sey

Before we remodeled the kitchen and got a built-in fridge I would always start my tomatoes & peppers on top of the fridge ... without a heat mat. When we put a free-standing fridge in the garden shed I wasn't sure there would be enough heat, so I bought a heat mat. Can honestly say germination rates seem to be the same ... with or without the mat. The only real difference was room temp ... heated house vs. unheated shed.

There are charts that show germination at various temperature for different garden vegetables . Here is a chart.

At 58F it will take 14 days, at 68F >>> 8 days.

So between room temperature ( 68F) and higher temp (86F) there is just a couple days difference.

Old, dried out seeds will take longer to absorb moisture and soften.

Read, study various methods, read and study some more. What works for some may not work for you. The basics are talked about over and over. Light, air flow, moist but not wet...sterile seed staring mix, not potting soil...if not sure, bake a tray in the oven.

After understanding the basics, adjust to what you have...i have a dampish basement workroom, no air flow so i run a small fan 24/7. 55-60 but i use a heat mat for a few days, then the next tray gets some heat...for me 2 seeds per cell is fine. I get 95% germination now with a good system...i used to put three tom seeds in each cell.

Don't try too many seeds at once. Experiment and expect some failures that i call insurance. Try another 6 cell the next weekend and see what works. (from the same packet)...seeds are cheap unless you blow it all first try, : ) like i used to do....

I like stuff simple....my home which is usually 60-70 degrees inside, works great for germinating seeds. Sometimes I use my non-adjustable heating mat....sometimes not. It may help a little. My cheap shop lights have worked great at growing my seedlings (no special bulbs....just white light). I am sure you could throw a lot of money at this project....but I have found it to be unnecessary. PS my seeds seem to germinate at around 95% or so. It can take 4-10 days normally to see their little heads popping up.