RHS may have found the secret to getting slugs and snails to leave your lettuces alone

A slug - Nobuo Matsumura / Alamy Stock Photo/Alamy
A slug - Nobuo Matsumura / Alamy Stock Photo/Alamy

The RHS may have found the secret to getting slugs and snails to leave your lettuces alone - but it comes with a catch.

A study by the gardening charity testing popular calcium deterrents found that unfertilised lettuce was much less palatable to the molluscs, but also less delicious for humans.

Summer student Emma Thornton studied two brands of calcium deterrent, designed to make crops less attractive to slugs and snails, in the first independent study to test whether the products work.

Calcium products, which often come in liquid or spray form, are a type of treatment added to the plants every two weeks to deter garden visitors from having them for dinner.

Lettuce treated with these products and a fertiliser was tested alongside lettuce fed just with fertiliser, and lettuce left alone and fed with no fertiliser or treatment at all.

Leaves were left in boxes with grey field slugs, brown soil slugs, threeband slugs and the common garden snail, to see which type they preferred.

Calcium sprays are designed to be a humane, eco-friendly alternative to lethal methods that kill the slugs and can also harm birds and other wildlife.

But the study found no difference between plants doused with the calcium feed or a normal fertiliser.

The leaves that fared the best were fed with no fertiliser at all, though the RHS does not recommend this method.

The unfertilised lettuce was small, tough and bitter, making it less attractive to both people and slugs - negating the point of growing the lettuce for human consumption.

Hayley Jones, who supervised the experiment and leads the RHS's research on slugs and snails, said: "Slugs didn't like it, but people wouldn't like it either, so it's unfortunately not a solution.

"It highlights why slugs are such a problem for gardeners because we want to grow things that they love. We want big, juicy leaves, and so do they."

A garden snail - Tonci MALETIC/Naturimages
A garden snail - Tonci MALETIC/Naturimages

She said it was hard to say exactly why the method didn't work.

"The way the products work is that the calcium enrichment is absorbed into the leaves and that fortifies the plants with calcium. It could be that it makes them tougher - calcium is an element that is used in plant natural defences, so that's the theory.

"Why it didn't work could be for many reasons - it could be that actually it's not getting absorbed, it could be that there's not enough of it. It could be that it was being absorbed, but that the slugs don't care."

Plants such as spinach, which are naturally high in calcium, are meant to be unpalatable to slugs but are often eaten by them anyway, she said.

Dr Jones plans further tests using different plants and applying it more frequently to see if that makes it more effective.

Ineffective methods

The RHS has previously studied other humane methods, such as surrounding plants with sharp gravel, copper tapes and bark, which were also found to be ineffective.

Earlier this year the pesticide metaldehyde, which had been used to kill slugs in gardens and farms, was banned in the UK because of the threat it poses to mammals and birds.

Slug pellets containing ferric phosphate, which harm slugs without affecting birds, are still available, but they can also affect earthworms and pets if eaten in large quantities.

More recently gardeners have been encouraged not to kill slugs and snails because many species don't actually harm crops and they provide food for birds.