Janet wrote:
A wide barrier-ring of dry woodash works well, slugs and snails hate
crawling across it even when it's wet. Or, old dry dead bracken, mashed up
a bit.

Isn't bracken supposed to be carcinogenic?

Only if you ingest it, and not to courgettes. It's a great compost
material and soil conditioner.

Fair enough. I thought there was also an issue with it giving off some kind of
spores that weren't good to breathe. I only have a vague memory of a discussion
about this, though, so I may have dreamt up the whole thing.