Nice one.
Another Q: any proven ways of keeping cats out of a planter? Tried netting and a 2L bottle half full of water but they seem to like digging past them.
Why are they going into the planter?
If there are mice in it, there's probably nothing you can do.
If they are using it as a toilet, you can try making it more uncomfortable, and offering them a better alternative nearby. They tend to like dry, loose ground for their toilet business, wet and hard they don't like.
If they just like sitting there, you could also try making it more uncomfortable. Wet soil, or plants that smell of lemon could help.