The weather has a massive effect on the gravity of a slug invasion. The ground is wet and a rainy night is sure to invite the unwanted pests for a midnight chomp.
Slugs prefer cool, dark and moist conditions and thrive all year round. They hide under decomposing leaves and organic matter. The predators of slugs do keep numbers down but in recent years these numbers have diminished from the lack of biodiversity. A milder climate means they survive the hibernation period more often than not. The use of pesticides may keep weeds or surplus growth in abeyance but also kills off predatory species that would reduce the reproduction. The uniformity of gardens plays an important factor, some gardens can be too clear of debris thereby decreasing the number of predatory species.
With this in mind, plants can unnecessarily be decimated without biological control or organic deterrent. A sustainable and ethical method may be to plant species that repel the slug community. A slug is very effective at breaking down organic matter. They play a role in the soil ecosystem but not welcome when it affects your border. Hostas and Marigolds are favourites with these gastropods, the texture of the leaf is quite significant in the attractiveness. Often leaves that are bitter, more rigid and less soft can deter them. Equally anything with bristles or small hairs. Woody plants usually are low risk, it is usually the soft, immature leaves that they prey on. Ornamental grasses are often a good choice Carex, Miscanthus, Phormiums and alike. Euphorbias (Spurge) are poisonous by their sap and for slugs they have a bitter taste. Hellebores are equally effective but for a different reason. Their waxy leaves means they get left alone. Stachys byzantina (Lambs ears) too but for their fur. Nepeta (Catmint) emits a strong scent that slugs stay clear of and this can be said for other strong smelling herbs. Rosemary, Lavender, Sage and Oregano to mention a few. The scent in smaller plants seems to be a deciding factor when discouraging slugs. Lily of the valley, Alliums and for their scent. Begonias too for their waxy leaves.
The maturity of the plant can be relevant. Often in their early years Delphiniums, Primroses and Dahlias can be tempting and as they grow older they lose the attraction. Often this can be achieved by transplanting in a cloche or coveted areas and harden of what has been propagated. This is good practice for hardiness in temperature but also in the battle against slug destruction. The leaves as they become older are less palatable.