I'm assuming that the plants are quite large? If possible, I would hang them from the ceiling. If the plants are too big, however, I've had success with cutting out a circle of cardboard to fit around the plant to the edge of the pot. You make holes to surround the trunks and then make another small hole that you can fit the watering can spout through.
Well, first you need to know *why* he is peeing in the plants. The most common reason is that the litterbox is too dirty. Try dumping the litter, cleaning the box thoroughly with soap and water, and putting in fresh litter. You should also have your vet check him for a urinary tract infection, which is the second most common reason. Other possibilities are explored in the litterbox secrets articles at http://www.littlebigcat.com/index.php?action=library.
Once other reasons are ruled out, my favorite trick is to "mulch" the top of the dirt with that white river rock that people sometimes put in the garden. The pieces are a little bit sharp and cats don't like to walk on them. They are a lot more attractive than tin foil! I have them in all my big pots to keep the cats from standing in the pots to chew on the leaves. Of course, now they just stand on the edges of the pot, but it did make it less of an easy trick!