Posted: 08/02/2016 at 07:43
Hi Stoney and welcome
My guess is that if you don't have bulbs in your planter, then the squirrels are digging to bury their nuts, either from bird feeders or nuts they've harvested from trees - acorns, chestnuts etc.
I used to watch this happen from my desk at work - we fixed smallish gauge chicken wire over the planters and the trouble was fixed - the plants grew through the wire and it was soon hidden.
The squirrels started burying their nuts in the lawn after that - it was great fun to watch and amazing how they frequently returned to the right spot to dig up their hidden treasure.