Don't get one that is for heating pipe like Frost king.
These have little buttons on them that make them automatically come on right before freezing.
You tape the button to the pipe so it will detect the temperature of the pipe.
I use this for my outside pipe and keep it plugged in all winter.
It only comes on if there is a freeze.
A better choice would be something that is automated and for plants and soil.
In the link below is some nice stuff you can regulate to the temperature you want.
Here it is.
Worth
http://www.google.com/aclk?sa=l&ai=C...3Dheat%2Bcable