Stray or unwanted cats often damage gardens, cause allergies, litter on landscaping, fight with household pets or tussle with other strays. Even household cats sometimes wander into places they should not. Much in the same way catnip affects felines, certain spices have an adverse effect on cats. Common spices are a safe, harmless and natural way to keep cats out of the places they should not roam into.
A specially designed organ called Jacobson's organ governs a cat's sense of smell. This organ is finely tuned, unique to felines, and hard-wired to cats' sexual and aggressive impulses. Due to this sensitivity, cats are greatly affected by the smell of many (if not most) spices. Common mild spices offensive to cats include lavender, rue, absinthe, lemon-thyme, rosemary, ginger and citronella. As they grow in the garden, fresh varieties of these spices repel outdoor cats, while sprinkled dried varieties ward felines away from thresholds or trouble spots within the house. For bigger problems, whole dried leaves of these spices tied in bunches do the trick, as do common plants such as geraniums and eucalyptus. Oil extracts of any of these plants or their pulp mixed with water create a sprayable cat repellent.
When mild spices aren't enough, stronger spices keep away persistent unwelcome cats. As a general rule, the spicier the scent, the more a cat's sensitive nose dislikes it. Black pepper is a very common spicy cat repellent when used in its powder form. Cayenne pepper, however, is the heaviest-duty item in the homemade cat repellent arsenal; cats absolutely despise its smell. For a particularly potent cocktail, mix two parts cayenne pepper, three parts dry mustard and five parts flour to create a cat repellent dust---spread this anywhere to keep out unwanted felines. Keep in mind the stronger spices, especially cayenne pepper, make cats ill when ingested. Pepper and strong spices can also hurt cats' eyes or elicit aggressive behaviour, so caution must be employed when using these anti-cat scents.
Though most humans find the scent enjoyable, felines generally loathe the smell of citrus; elements of citrus fruits used in the same fashion as spices serve the same repellent purpose. Peels of grapefruits, limes, lemons and oranges spread around off-limits areas keep unwanted feline company at bay. Similarly used, the odour of banana peels deters many cats. In a strange testament to their peculiar olfactory tastes, most cats seem to enjoy the scent of the citrus-tinged spice lemon grass.