Cats have a strong reaction to smells, which makes using certain scents as cat deterrents as easy as leaving a piece or lemon where you don't want them to go.
But just because a scent works to keep cats away, it doesn't mean the source of that scent is safe -- even if the source grows naturally in your garden.
Cats and Smells
Cats are, essentially a walking nose. Their sense of smell is 14 times that of a person's. Olfactory receptors in a cat extend throughout their whole head.
Cats use this superpower to sense danger, food and mates. Accordingly, they have strong olfactory likes and dislikes, due mainly to various evolutionary impulses.
Cats Hate Citrus
Cats hate the smell of lemons, limes, oranges and grapefruits, and with good reason. Lemon in particular is toxic to cats if ingested. Cats like to put as much distance between themselves and citrus fruits as possible. Using fresh pieces of citrus fruit or peels should keep cats away from your garden or your Christmas tree.
Cats Hate Spicy Scents
A sandwich with a nice zingy mustard may smell great to you, but cats detest the smell of spicy plants and foods like mustard. Clove is another great cat repellent, as is cayenne pepper. Like citrus oil, hot peppers contain capsaicin, which cats instinctively avoid because it's toxic to them.
Cayenne pepper also sends cats running, which compels some people to sprinkle it in their garden as a repellent. But this can be harmful, even cruel to cats.
Herbal Scents Cats Hate
Cats being pure carnivores have a strong dislike of most plant smells. Lavender, rue, geranium, absinthe and lemon thyme are especially unpleasant to them.
Cats also hate eucalyptus oil and oil of wintergreen. Soak pieces of cloth or cotton balls in one of these and cats will keep their distance.
Use Chemicals Cautiously
Many commercial sprays contain perfumes and extracts that cats find repellent . Most are based in natural scents, but some contain artificial chemicals that can be dangerous.
Another highly effective chemical cat repellent is mothballs. The problem, however, is that mothballs are extremely poisonous to cats and other small animals.
Mothballs are pesticides that slowly release a gas vapor to kill and repel moths (and their larvae) and other insects. Mothballs are also used to repel snakes, mice and other animals, though this use is not recommended and can be harmful to pets, children and the environment.
Keep in mind, of course, that cats are individuals. Some will tolerate unpleasant smells better than others and some will disregard otherwise repellent smells entirely. If one course of action doesn't work, try another, you're likely to find a scent that works well at keeping cats away from where you don't want them to be.
And if you have any doubts, check with your vet for the safest and most effective repellents.