Cats need to eat grass from time to time. Some substance in the grass causes them to vomit a while later, which cleanses hairballs from their stomach that would otherwise cause digestive issues. More info can be found in this related question.
So cats have a natural instinct to eat fresh green plants once in a while. If they don't have access to grass, they will chose other plants to chew on.
The easiest way to save your plants is to give your cat some grass to chew on. You can buy little flower pots of grass in some florist shops or you can simply make your own:
- Fill a flower pot with fresh soil and water the soil.
- Sprinkle some seeds of either lawn grass, wheat, rye, barley or oats on top of the soil. Rice might work as well, but I've never tried it.
- Sprinkle a light cover of soil on top of the seeds.
- Keep the flower pot in a nice place for a few days until the seeds germinated. If you live in a cold climate, keep it in a warm place. If you live in a warm climate, keep it out of direct sunlight.
- Don't forget to water it if the soil looks dry.
- Put the pot next to the plants your cat likes to chew.
As mentioned above, eating grass can cause vomiting. That's normal and healthy for a cat.