Salt is a proven remedy for killing slugs.
If your deck is visited by an abundance of slugs during the evening hours, there are a few remedies to get rid of these pests before they ruin the vegetables and flowers in your garden. Slugs prefer a cool, moist and dark environment; and while you can't do anything about the dark or cool weather, you can eliminate the moisture on your deck with items you may already have at home.
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If you notice that slugs are a significant problem in your yard, take a flashlight onto the deck in the evening and hand-pick them before they make it to your garden. Drop the slugs in a bucket filled with a solution of liquid dish soap and water to dehydrate and drown them before disposal.
Salt is a tried-and-true remedy for eliminating slugs and is safe to use in your deck area. The salt must be applied directly to the slugs, which isn't a lot of work if you only see a few slugs on the deck each night. However, for an infestation of the pests, it's best to take a pail filled with salt water onto the deck, pick the slugs up with tweezers, drop them into the bucket and let the salt do its work.
If you have potted plants on the deck that you want to protect from the slugs, border the pots or plant trays with strips of copper that you can purchase from your local garden supply store. The rough consistency of the copper doesn't agree with the soft, slimy body of a slug and will trap the pest so that you can hand-pick it away or kill the slug after a few minutes of contact.
Washington State University suggests making your deck appealing to animals that eat slugs by adding a birdhouse. This will stop you from having to be on slug patrol every evening since slugs can be a year-round problem in the garden. A birdbath on the deck may prove to be an effective slug elimination remedy as birds will come to the bath and see their next meal crawling across the deck. Washington State University also recommends constructing a man-made pond with a waterfall near the deck so that frogs will be more likely to visit your yard and eat the slugs that are abundant on the deck and in the garden.