Composting is best, but you can bury them in the soil to decompose. Smashed, they can be a good slug deterrent, like broken sea shells, sharp gravel etc. I wouldn't recommend leaving them whole on the surface, unless you want to encourage the various insects that will use them as a temporary home.
Thanks! I wasn't sure about composting because the shells might contain juglone?
Around the gardens as slug deterrent is a great idea, so I will try that 👍
If they do, (expect so), I would imagine that the quantities are so low it doesn't really matter. There is some question about whether Juglone really affects plants anyway....
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Around the gardens as slug deterrent is a great idea, so I will try that 👍
https://www.gardenmyths.com/walnuts-juglone-allelopathy/