I have a 300-something watt waterbed heater that works great. I thought it might pack a bit TOO much punch to use as a heat mat for seed starting, but in a deeper container might be perfect if I pair with a thermostat.

Here's the question (merits of using this notwithstanding) - is it safe to water plants sitting on the mat? Or would I need to move them off the mat every time I water? I'm not talking about submerging it, but just the "runoff" from watering hitting the mat and running off as the water drains from the containers.