I want to keep the roots warmer on several larger trees that I collected this fall to help preserve them. I live in zone 5-6. I got some "VIVOSUN" 20" X 20" matts and the digital thermostats to go with them. I set the temp at 40' F, which is as low as it will go, put the probe down in the soil of the trees about 6-8" and plugged them in. The thermostats always light up with what the temps are in the probe area. One reads 34F and the other two read 43-47F. The thermostats are designed to heat to the temps they are set at and then shut off. I have been in communication with the factory (foreign country) via email and not had much luck. I know there is controversy with using heat matts and the like, but if someone has had better luck with another brand/product, I would appreciate hearing about it. Thank you!