There are many repellents available that are claimed to deter cats including orange or lemon scents which cats dislike the smell of. The home remedy of scattering citrus peel about your garden is based on this idea but has varying results and looks rather unsightly. You could try growing Coleus canina which has a pungent smell that cats dislike.
I have found the most effective method with my neighbourhood cats is to put temporary netting around young plants until they are big enough to tolerate some disturbance (although I find once they are established cats tend to stay away anyway). I also keep the soil damp as cats prefer loose, dry earth. You could try pegging chicken wire across the soil surface, cutting holes to allow for the new plants. This will certainly prevent the cats from digging but can prove inconvenient when weeding or tending plants.
There are a few more technological options such as motion activated water sprinklers or motion activated ultrasound devices available on the market although these can be expensive. A well timed soaking with hosepipe or water pistol might be just as effective if you spot the culprit in time!
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