you can buy some pellets in the garden centre that will discourage them,worked for us ...oh and Tiggy (no2 yorkie) barks like hell and chases her when she sees the cat in our garden ....dont think Tiggs would know what to do if she caught her as shes a really scardie dog!
I bought a large container of pepper - a cheap one of course (at Costco) - and scattered it all round the edge of the garden.It doesn't hurt the cats - might make them sneeze - and as it is vegetable based it is biodegradable so doesn't hurt the garden either. It did the trick but if it rains you have to put some more down.
As mentioned you can buy repellent from garden centres. But another tip is to half fill big plastic coke bottles with water and leave them in the garden. Something to do with light reflecting through the water, and the cats don't like it.
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