Mentha pulegium – Pennyroyal is a native of Europe and is best not used as a culinary herb, as it is too strong to be ingested by pregnant women. That said, it is an excellent cat repellent and can be planted where ants are wreaking havoc. It is also an aggressive spreader, so you may want to contain its vigorous root system. Grows 4-16″ high by 2’+ wide. Hardy to zone 5.
Lavandula stoechas ‘Boysenberry Ruffles’ – This cultivar of Spanish (or French) Lavender is a part of the Ruffles & Lace Series and features caterpillar-like lilac-purple flowers with white to pale pink bracts on top. It blooms from late spring through summer. This species of lavender has a higher camphor content which repels cats. Grows 24″ high and wide. Hardy to zone 8.
Plectranthus caninus – Scaredy Cat Plant is also marketed as Coleus canina, Piss Off Plant and Dog’s Gone – it is a herb native to east Africa and Asia. It has fleshy leaves that emit a pungent scent when bruised and will produce lavender-pink terminal blooms. Plant after danger of frost has passed in a full sun exposure. Grows 12-18″ high by 24-36″ wide. Zone 9.
Thymus x citriodorus ‘Aureus’ – This golden variegated lemon thyme has large leaves with a strong citrus flavour for chicken and fish dishes (or use as a cat repellent). It is an attractive evergreen perennial that does not look out of place in rockeries or containers. Prefers a part to full sun exposure with excellent soil drainage. Grows 6-10″ high by 10-12″ wide. Hardy to zone 5.
Ruta graveolens – Rue is an evergreen herb native to the Balkans that is primarily grown as an ornamental plant now. It features bitterly aromatic finely-cut bluish-green foliage (which repels cats) and pale yellow flowers in summer. Many people will experience blistered skin when exposed to it in the sun, so use gloves when handling. Grows 3′ high by 30″ wide. Zone 5.