Answer: I am not aware of any plants that specifically repel cats, but some gardeners report success using garlic and citrus scents (such as orange peels) to deter them. There are also repellent sprays available such as Liquid Fence. Some gardeners report that sprinkling thorny trimmings around will deter them, some suggest poking short sticks into the ground to make walking difficult for them. Also, sometimes changing mulch materials or making sure the mulch is wetted down can make the area less attractive. If you have a sand box, cover that as well. Another approach would be to ask the cat owners to keep their pets indoors (it is safer for the cats, no danger from dogs or cars.) To some extent, the cats in gardens problem seems to me to be seasonal so it may decrease in a few weeks once they are accustomed to the warm weather and have worked out their territories for the season. I hope this gives you some ideas.
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