All dog breeds love to chew and eat things. This is simply one way for them to learn about their environment by using their sense of taste. So it's only natural for your Cocker Spaniel or German Shepherd to be curious about how that houseplant of yours might taste. You can however break him of this habit.
You really need to keep an eye on your dog when you have a garden and houseplants. As soon as he looks as if he is going to go for one of your plants you should immediately give the command “No.” and then make sure that as soon as he moves away from the plant to give him praise. You can then offer him one of his toys instead of the plant. Do this consistently and this should help him to turn to a chew toy instead of the plant.
You can also try and use some bitter tasting sprays. One would be bitter apple and the other would be bitter herbs. These are especially made to ward off pets. You just mist your plants with this formula and it should keep them away. If you don't want to buy that you can make your own by using vinegar and water and put that into a spray bottle and use this instead.
Another thing you can do is get a spray bottle and fill it with water and put the nozzle on stream and not mist and whenever he goes for your plants spray him in the face. Of course if he ends up thinking you're playing with him you will then have to add one to two teaspoons of ammonia in the water. That will definitely end playtime.
You can also get special dog repellent that comes in granules that you can sprinkle around your outdoor garden plants to help keep him away from those plants. Or you can go ahead and spray them with your vinegar water too. Whichever you use make sure to redo apply every few days especially after there is a rain storm.
Always make sure you are consistent in your training of your dog when it comes to anything even with the plant training. You need to continue to use whatever technique that seems to work until your dog simply no longer will go near your plants for any reason.
As much as it is bad for your plants and upsets you that they are being destroyed by your dog, the more important issue here is that not all plants are good for your dogs. Some houseplants and garden plants can make your dog very ill and even some of them can kill your dog. So it's important that they learn to leave all plants alone.