I bought a Jump Start Heat Mat Thermostat and love it! I use the old heating pads for starting seeds indoors that don't have the auto shut off and was using a darkroom thermometer to keep tabs on the soil temperature. I found it to be something of a chore because I'd have to keep checking the temperature every hour or so.

With this, you just plug your heat mat into it, set the temp you want your soil to be then place the probe in the soil and plug it in. So far works awesomely! I'll probably end up getting another one for next year.

Here is a link for it:

https://www.amazon.com/dp/B000NZZG3S/

I planted this tray 2 days ago .. it's all hibiscus and the hardy ones are already coming up! No more worry about the soil temperature.

HydroFarm is a good outfit, too. I have a bunch of their small heat mats and a couple of the large ones. When I first bought the small ones, there was a defective run of them, so several of mine died after the first year. When I contacted HydroFarm, they replaced them all, and without asking for a receipt.

How cool is that!