These little beauties are marigolds that we planted 3 days ago in our walipini/greenhouse. They've been on a heat mat which keeps the soil temperature at 72° F.
This tray was not placed on a heat mat and nothing has emerged despite being planted the same day.
Conclusion: heat mats work.
I have some of my seeds on top of the radiators around my house lol! Hopefully that helps them sprout soon!
It should as long as they don't get too hot.