I sprinkle crushed red pepper in my house plants to keep my one cat out of them. He only tries to use them when the litter box is full (hardly ever since they go outside) and it's been raining hard for a long time (yard flooded).

This helped for sure when we first got him and we kept him in all the time. My other cat doesn't want to use the plants, but he's also not near as sensitive to smells as this cat.

Karen