Hello, Is it possible to use Home-Assistant to monitor a Seed germination heat mat with a Ds18b20 and control the temp with a relay?
I am intending to do something similar with a Sonoff TH10 and its ds18b20 cable.
Is it what you have in mind?
That looks like a great idea, I can find the TH16 on Amazon would that work the same way and do they work with Home Assistant?
I am waiting for my TH10 but the TH16 is similar. I am already using some other Sonoff switches with Home Assistant and I never had any problem (you will need to install Tasmota).
Although I have not tried it, there is a Sonoff integration you could try first:
Hello again, would it be possible to use the ESPHome devices like a NodeMCU to monitor the temp of the mat and turn on and off a WEMO plug or other device with an automation?
A little late, but hey… 
I’m currently planning something similar, as I need something to monitor and control temp, humidity and a heat mat for my filament for 3D printing. 
So to answer your question, yes that should be possible. ESPHome is great for such things, I run a few different devices with it: ESP8266 (as NodeMCU), ESP32 (NodeMCU as well) and Shellys.
Do you already have your heat mat? If so, how is it controlled? Or does it only switch on/off?
My plan is to use a DHT22 inside a box to measure temp and humidity. The NodeMCU should then turn on/off the heat mat to get the right temp. That’s the theory, right now I’m still looking for a good heat mat.