Eggshells as a slug barrier

The claim

A ring of crushed eggshells around a plant creates a sharp barrier slugs will not cross, and the shells add calcium to the soil.

What the evidence actually shows

The Royal Horticultural Society ran the obvious experiment: 108 lettuces in nine pots and nine raised beds, ringed with crushed eggshells, pine bark mulch, copper tape, sharp horticultural grit or wool pellets, grown for six weeks and measured with a leaf-area meter. There was no difference in damage between protected and unprotected plants. Not a small difference - none.

The calcium claim fails separately. University of Minnesota Extension notes eggshells do not prevent blossom end rot, because that is a calcium-transport problem inside the plant rather than a soil-calcium shortage, and in any case eggshells decompose far too slowly to supply calcium on a useful timescale.

Why it is so persistent

It is intuitive. Sharp things ought to deter soft-bodied animals, and the failure mode is invisible - a gardener who rings a plant and sees damage assumes they needed more eggshells rather than that the method does not work. Slugs also travel on a mucus layer that handles rough surfaces far better than intuition suggests.

What to do instead

Iron phosphate baits have real evidence. Trapping and hand removal in the evening works. Reducing daytime shelter - boards, dense low foliage, debris - addresses the population rather than the plant. If a barrier is wanted, the one material that performed in controlled testing was sodium silicate (waterglass), not any of the popular options.