Thyme is one of the scents that fleas avoid. Essential oils can be diffused into the rooms to repel fleas, or the plants can be added to the garden. Any cultivar from the Mentha plant family—peppermint, spearmint, pineapple mint, and more—contains oils that produce smells that fleas hate. Yarrow plants, while lovely with feathery foliage and colorful blooms, do not have a particularly pleasant scent to humans or fleas. Myrtle oil has a sweet, fresh, green, and slightly camphorous scent fleas tend to avoid. Eucalyptus oil has a fresh, minty, woodsy, slightly sweet note that fleas hate. The herbal scent of basil oil is not pleasant to fleas. Lemongrass oil contains citral and geraniol notes that repel both fleas and other insects. The pungent, piney smell of rosemary is a natural insect repellent. Pine oil is a good choice for cleaning products because fleas hate the smell. Scientific studies show that the scent of thyme, mint, yarrow, and myrtle are effective flea repellents.