I have railway sleepers in my garden, sort of a wall holding my raised garden

They are full of pitch and tar (to save them rotting) they are several years old

There are gaps in them, I am wondering if I can put plants in the gaps and would they grow ?

If so, what kind of plants

I was also thinking of drilling holes to make a kind of bug hotel

Any ideas or recommendations really appreciated

I share your pain mine have been decimated too.
Unfortunately I think the very wet weather has tipped the scales in favour of the slugs.
I think it is rather unlikely those plants will recover so I suggest that you sow some more at home and then plant them out under plastic bottles as protection. Having said that I would also leave the existing plants in situ till your replacement plants are ready just in case they beat the odds and recover.

I hope everything will be ok, good luck to you!

The slugs have taken off the top of my new dwarf beans will this plant grow or will it die?

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