Do eggshells stop slugs or add calcium?
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Neither, in testing. A Royal Horticultural Society experiment ringed 108 lettuces with crushed eggshells and other barriers for six weeks and found no difference in damage between protected and unprotected plants. Eggshells also decompose far too slowly to supply useful calcium, and they do not prevent blossom end rot, which is a calcium-transport problem inside the plant.
What do people believe about eggshells?
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 does the evidence show?
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 does the eggshell myth persist?
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 works 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.
Key takeaways
- An RHS trial of 108 lettuces over six weeks found no difference in slug damage with eggshell rings.
- Pine bark, copper tape, horticultural grit and wool pellets also failed in the same trial.
- Eggshells do not prevent blossom end rot, a calcium-transport problem inside the plant.
- Slugs travel on a mucus layer that handles rough surfaces well.
- Sodium silicate (waterglass) was the one barrier that performed in controlled testing.
References
Frequently asked questions
Do crushed eggshells prevent blossom end rot?
No. University of Minnesota Extension notes that blossom end rot is a calcium-transport problem inside the plant rather than a shortage of soil calcium. Eggshells also decompose too slowly to supply calcium on a useful timescale.
How long do eggshells take to break down?
Too long to matter for calcium. University of Minnesota Extension notes eggshells decompose far too slowly to supply calcium on a useful timescale.
Why do gardeners think eggshells stop slugs?
Sharp things seem as if they should deter soft-bodied animals, and the failure is invisible: a gardener who sees damage assumes more shells were needed. Slugs travel on a mucus layer that handles rough surfaces far better than intuition suggests.
What does stop slugs?
Iron phosphate baits have real evidence, and evening trapping and hand removal work. Reducing daytime shelter such as boards, dense low foliage and debris addresses the population. If a barrier is wanted, sodium silicate (waterglass) performed in controlled testing.
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