I include a matrix show a time when items are going to be done. They know it is average billing.
I also say we do not mow in the rain and services are provided based on growing and weather patterns not a calendar.
The best way to make this successful is do as much as you can in the otherwise slow periods.
Take off a month from December 20th to Jan 20th. Show up at these accounts there is almost always something to do. If their home does not have beds to be weeded or flowers to be dead headed then they are not a candidate for these services. I do not believe flat monthly accounts are suitable for just mow accounts.