Keeping Cats Out Of Houseplants

I recently adopted two male cats from a shelter. They are about two years old and are well behaved in every aspect except one started using my large potted houseplants as a litter box. After researching various remedies on line, I bought a bag of "rough" decorative gravel/rocks from Home Depot for about $5 and put them on the surface of the soil and problem solved! I watched my cat stand on his hind legs, paw at the rocks just once, and then went directly to the litter box. There are different kinds of rocks in different colors. I got mountain granite and it's a tan color. An added benefit is that it helps retain soil moisture.