This article is from the Problem Behaviors in Cats FAQ, posted to rec.pets.cats newsgroup. Maintained by Cindy Tittle Moore with numerous contributions by others.
Between digging and eating in your plants, cats can do considerable damage to a garden. There are a number of ways to keep cats from digging in, chewing on, or eliminating in your garden.
Some people have successfully used the "diversionary" tactic by planting catnip in another corner of the garden entirely, confining the destruction to one spot.
If you have not yet started your garden, put chicken wire down and plant between the wire. Cats dislike walking on the chicken wire and most plants (unless they grow too big) do just fine growing between the wire.
Other people have reported success with different sprays, gels, and products specifically formulated to keep animals out of your yard. Check your local pet store.
Lemon peels, soap slivers (use biodegradeable soap) dipped in cayenne pepper and other organic materials have also been reportedly successful.
Cats hate water: surprising them with a squirt gun (or turning your sprinklers on) can discourage specific cats from returning.
One reader reported success in putting up cast-iron cut-out cats with marble eyes in strategic places in her yard. The decorative cats were apparently real enough to cats that they kept clear of her yard. Try looking around hardware stores or gardening stores for these. Lifesize cat statues might work as well.