You really need to look at the dogs whole day if they are being left or destructive in the garden.
Dogs with energy and no outlet will go self employed and do things that they enjoy but may be incompatible with the humans they live with.
If your dog is getting enough stimulation and exercise they should not need to be left unattended in the garden. They should be happily chilling or sleeping maybe near you or at times on their own.
As they grow used to relaxing they will then be able to transfer this relaxation out to the garden where they can chill, smell the roses and listen to the birds. They will then potter link Nell - (but even she chews sticks
)
So after a long walk when your dog is tired and relaxed, take you coffee outside have your dog on a lead and get him to settle on a mat whilst at intervals you drop treats onto the mat. If he gets up ignore him but pop a treat back onto the mat. he will return to the mat. You are encouraging a calm state. Build on this indoors and outdoors.
However if he is chucked out into the garden for example when he is full of energy and raring to go he will very likely carry on with destroying the plants as he has nowhere else to put his energy.
Re teaching him to like chews and kongs put in a small amount of kibble and play with the kong. Make it really easy to get out. Wipe cheese on the outside of the kong so it smells nice, pretend to keep it to yourself and not let him have it , that will make it even more appealing to him. Do not just dump it down and walk away, make it fun.