The Index → Avoid
✕Avoid
Eggshells and coffee grounds as a slug barrier
Tested directly. Slugs cross both without apparent difficulty.
What it is
Ringing plants with crushed eggshells, coffee grounds, sand, or similar sharp or bitter material to stop slugs.
The numbers
- Direct trials — including a widely reported RHS study — found no significant reduction in slug damage from eggshells, coffee grounds, sharp grit, pine bark mulch, or copper tape.
- Beer traps did show an effect in the same work, and are nearly free.
- Nematode biocontrol (*Phasmarhabditis*) is the one treatment with consistent results, where it's available.
Why it's here
It's the most widely repeated garden tip that has actually been tested head-to-head and failed — which makes it a useful calibration point for everything else you read.
The catch
Eggshells and coffee grounds are fine things to add to compost. They just aren't a barrier. For slugs: beer traps, hand-picking after dark with a flashlight (unglamorous and extremely effective), ducks if you have them, iron-phosphate bait if you don't, and keeping the mulch pulled back from seedling stems while they're small.