electric fence. or collar that shocks them.
Sprinkle dried coffee grounds around the plants, good for the plants, but dogs seem to want to travel elsewhere for digging
.410 double-barrel loaded with rock salt.
Could it depend on the breed of dog ? My front yard -chain linked- and electrified- has been “landscaped ” by two Great Pyrenees for 8 years. They are of the ones who dig fox holes summer and winter, sometimes yo will only see a nose or ears sticking out of the snow !
Electric fencing works just great also-the dogs learn very quickly where not to touch. For breeds such as the Prys ,who use double dew-claws to climb even 6 ft. fencing
it is essential.
Chicken wire may do the trick. Of course they may be big enough that they can topple the chicken wire. Try building a box around your flowers that’s tall enough to keep them out.