Answer: The Roma philosopher Pliny in his Natural History advised placing bunches of rue (Ruta graveolens) around domestic dove-cots to keep cats away from them. Since it appears that cats dislike the plant and will avoid it, it might be worth planting it to see what effect it will have.
An unpleasant smelling substance, e.g. one containing oil of citronella, smeared on plants and shrubs may prove a deterrent. And covering the area with fine wire mesh or nylon netting a few inches above the ground is often effective, though unsightly. Growing seeds can be protected by surrounding or covering the area with prickly braches or leaves, e.g. of gorse, rose or holly, though obtaining sufficient of them may present problems. Another suggestion is to lightly dab the cat’s gums with lemon juice or vinegar and then to spray the leaves of the plants in the area with a dilute solution of the same liquid.
Several balloons, firmly pegged down, can be used to provide aversion therapy: the noise as they burst discourages further invasions. Alternatively, a loud eclectic bell or horn placed in the area and sounded by remote control when the cat approaches can use it to avoid the spot.
