Outside and feral cats can cause trouble in a garden, as they like to lay and dig in tilled dirt. Cats might also use your garden as an outdoor litter box. Since cats don't like the smell of citrus, a homemade citrus spray can help keep them out of your garden.
Supplies
It seems cats don't like lemon or orange. You can purchase orange oil from natural food or organic gardening supply stores. Lemon oil can be found at a natural food store or in the furniture-care section of some hardware stores. Since you are going to want to dilute the oil, you will need a clean spray bottle you can use solely for your citrus spray. The oils will not dilute in water, so you will need some grain alcohol.
Homemade Citrus Oil
If you can't find or don't want to buy lemon or orange oil, you can easily make it at home. You will need orange or lemon peels (dried and ground up), grain alcohol, a mason jar with a two-piece lid, a coffee filter, a bowl and a piece of fabric a little larger than the mason jar lid. Put the peels into the mason jar and cover them with alcohol. Seal the mason jar lid so it's air-tight. Shake the jar a few times per day for about three days. On the fourth day, pour the mixture through a coffee filter and into a bowl. Pour the filtered mixture back into the mason jar. This time, cover the opening of the jar with the fabric and only use the rim of the lid. After a few days, the alcohol will evaporate and leave only oil.
Using a Citrus Spray
You only need a few drops of orange or lemon oil per quart of alcohol. You can figure out what mixture works best through some trial and error. Shake the spray bottle to make sure the oil is dissolved into the alcohol. Spray the ground, fencing and anywhere else you would like to discourage cats. If your flowers are sensitive to alcohol, you can use water instead, but the oil will not dissolve into the water. You will have to shake the mixture almost constantly.
Additional Information
After you go through all the trouble of finding or making your own orange or lemon oil, you might as well use it for all it is worth. Both oils have multiple uses around the house, so here are a few tips. Orange oil has also been shown to kill insects, like termites, and repel mosquitoes, ants and flies. Both lemon and orange oil can be used as effective household cleaners. You can create a very inexpensive room freshener by filling a mason jar halfway with baking soda and adding a few drops of lemon or orange oil, then capping the jar with fabric. It is important to note that orange oil, as well as dried orange peels, are very flammable, so keep your them away from open flames.