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’s take a look at the science.

Here is how it is supposed to work: the sharp, rough edges of the crushed eggshells…