For as long as I have been gardening, there has been a tip I have come across at least once a week: the idea that a ring of crushed eggshells placed around plants is an effective, natural barrier to slugs and snails. With more and more gardeners keen to avoid the use of pesticides, could this age-old advice be ever more relevant to home growers? Let鈥檚 take a look at the science.
Here is how it is supposed to work: the sharp, rough edges of the crushed eggshells are thought to be irritating to the soft, smooth bodies of slugs and snails, so sprinkling a ring of these or other rough-textured aggregates like grit, sand or chipped pine bark around plants creates a sort of impassable barrier.
The surprising thing to me is that have consistently that rough aggregates are ineffective impediments to the movement of land molluscs. A 2018 study found that lettuce plants encircled by a range of these materials didn鈥檛 fare any better against slugs and snails than control plants with zero barriers. It is thought this is because the thick mucus these creatures produce to slide along creates a smooth, slippery trail across even very sharp surfaces.
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In fact, other trials have shown that slugs are by the scent of yeast, so it isn鈥檛 unreasonable to propose that trace remnants of decomposing raw egg found in these shells could actually act as a lure to the very creature you are trying to avoid.
However, other natural barriers might be worth considering. have been shown to be effective at reducing the keeness of slugs to munch on young plants in at least one trial. The problem is the caffeine responsible for their effectiveness is also toxic to plants, meaning the grounds need to hit the sweet spot where their caffeine levels are low enough to be safe to use on plants, but still high enough to work on slugs. Not exactly easy to do at home.
So far, the best evidence seems to be for a solution of , which trials have shown promising results for. Its active ingredient, allicin, is, however, the same compound that makes garlic give you terrible breath. And I can personally attest to the looks even your most polite neighbours give you when you have been out spraying garlic on your plants.
But what if the problem was really of our own creation? In reality, only a few species of slugs and snails actually eat live plants. Many actually help break down dead plant matter, unlocking their nutrients, resulting in healthier soil and better plant growth. They also form an important part of the wider ecosystem as food for the songbirds many gardeners are trying to attract. So, aside from very small seedlings and a few particularly vulnerable plants, such as dahlias, getting rid of slugs and snails may even be doing more harm than good.
James Wong is a botanist and science writer, with a particular interest in food crops, conservation and the environment. Trained at the Royal Botanic Gardens, Kew, in London, he shares his tiny flat with more than 500 houseplants. You can follow him on X and Instagram @botanygeek
For other projects visit newscientist.com/maker
