damn thats handy never knew about that flag! By the way the proper flag is /region flag __global__ grass-growth
Dang too late thought i was the pro who knew how to do this..... -_- tip to everyone wanting something simple to stop from happening look at this list: http://wiki.sk89q.com/wiki/WorldGuard/Regions/Flags
There is also an option in the WorldGuard configuration for it, if you want it globally across all worlds easily.