There are a number of reasons why your cat may decide to invade your house plants--and none of them are good ones. Whether it's that Kitty sees the plants as an all-night salad bar or a convenient place to use the bathroom, it's never a good thing for the plant owner. Keeping cats out of your plants requires pure ingenuity on your part. Cats are clever creatures and the battle for the plants may all boil down to a game of wits.
Try to Uncover the Reason
There may be a reason that your cat has opted to eat your plant rather than or in addition to the gourmet cat food you purchased. Some cats love having something green to eat. If your cat is an indoor cat that can't get to the green grass outside, the plants may seem like their only option. Cat grass is a great alternative to house plants if you make it available to them. You can purchase it a local pet store and it only takes days to grow. Once the grass is a few inches tall, set it out somewhere for Kitty to find it. This may curb the urge to go after your plants.
Check the litter box. If your cat is using your plants' soil as a restroom facility, it may behoove you to check the status of Kitty's bathroom. Cats don't like to use dirty litter and it may be as simple as keeping a cleaner litter box. Cat boxes should be scooped daily--twice daily depending on how many cats you have and their individual bathroom habits--and changed at least once a week.
Make the Plant less Appealing
If the plant tastes disgusting to your cat, they may be less inclined to take a bite. Dust the leaves with cayenne pepper. Your cat should smell it before tasting and will suddenly lose the desire to chow down. Tabasco sauce and Bitter Apple will produce the same results without harming the plant or the cat. Cats don't care for vinegar either; however, it can alter the pH level in the soil and damage the plant if you aren't careful. Research your plant before using vinegar on it; while some plants thrive in acidic soil, others don't and it may kill them faster than your cat can.
To avoid Kitty using your plant as a restroom, try putting decorative rocks on top of the soil. Kitty may be less apt to scratch around in the rocks as they don't feel good on the paws. You can also line the soil with a decorative paper to keep the cat out. Paper may not look as nice, but it may keep the plant safe.
If all else fails you can keep a spray bottle filled with water handy. Spray the cat every time you catch them in the plant. You have to be quick; if your cat gets away and you spray them after the fact, they won't associate the punishment with disturbing the plant. After awhile, Kitty may come to link the connection of bothering the plant with a blast of water from the spray bottle. A last resort may be to keep the plants in a room that your cat can't access.