My dog digs in the flower bed only at night while were sleeping? How can I stop him, the red pepper doesn't work well.

18 month old Siberian Husky. Listens mostly when we are around like NO, SIT, LEAVE IT etc... but seems to dig mostly 90% of the time at night when were asleep.

Also whines allot when he doesn't see one of us.
He is not tethered but left to roam the backyard and is not allowed in the house.

He is exercised daily a 30 minute dog walk in the early am and 90 minutes of play in the evening at the dog park.

Not sure if I'm doing something wrong...