You turn around and your dog is chewing bark from a flowerbed. The exact mulch matters. Wood pieces can splinter or lodge in the gut, and cocoa-bean mulch is especially dangerous for dogs. Other garden products may contain chemicals. If your dog has swallowed mulch, call your veterinarian or an animal poison service promptly with the product details and estimated amount. Vomiting, abdominal pain, drooling, or unusual tiredness need urgent veterinary advice.
Protect the garden and the dog
Block access to the bed or supervise the dog on a secure lead until you can replace or contain the material. Keep the packaging or take a photo of the product label for your veterinarian. Do not try to induce vomiting unless instructed by a professional. A dog who repeatedly eats non-food materials should have a veterinary assessment rather than being written off as merely curious.
Teach "leave it" with easy objects away from the garden and reward moving with you. Offer safe chewing and sniffing outlets and check whether your dog is left in the yard with little to do. Don't test the cue by letting them repeatedly reach the mulch, management does the safety work while training catches up.
The best garden may be one where the dog cannot reach the mulch at all. There is no reason to wait for a successful training session before changing the landscaping.
Sources: Hudson Veterinary Hospital: Is It Bad for My Dog to Eat Mulch?. Cornell University College of Veterinary Medicine: Gastrointestinal foreign body obstruction in dogs.