As seen during our appearance on BBC Gardeners World we use garlic wash as a slug deterrent on the nursery. Please find below the recipe that we use and find very effective. The quantities aren't exact and you can always alter them as you wish.

Store outside in a cool dry place or in the fridge. Can be used throughout the season, make a fresh batch every year.
Open a window because it's going to get garlicky!
Attention Customer
Many varieties will now be cut back as we head towards autumn. We are doing this slightly earlier this season due to the extreme heat that we have had.
Sienna Hosta Team