Take the battery out and take it in the house...yes it's a pain in the a$$, but if the tractor is plugged in and the battery is nice and warm, chances are it will start.
A second block heater won't hurt either
Taking the battery out is less trouble than trying to get the thing started in the cold.
Like Fred said, cover it up, keep it out of the wind, or leave it running.
My dad used to go out an hour before bedtime and start his truck. He would leave it run an hour, shut it off and go to bed, and when the nightly nature call happened, he would go out and start it again, sit and have a smoke and a milk and go shut it off. It only sat maybe 3-4 hours at a time...not long enough to freeze up and not start. (Not making it to work wasn't an option with him)