Eatingplants is an exceedingly dangerous habit as many household plants are poisonous. If your cat is developing a habit of eating plants, you must separate the two until the problem is solved. Either remove the plants or confine your cat to an area with no plants. Do not delay. One good meal of the common houseplant Dieffenbachia is usually the last meal your cat will eat. Consult with your local nursery and do not keep poisonous plants in your house.

http://www.perfectpaws.com/plant.html
Just make sure you don't have any poisonous plants around.