Slugs are among the most costly garden pests in the UK, causing approximately £8 million in plant damage each year, according to Greenhouse Stores. A natural deterrent made from a 22p garlic bulb is an effective and budget-friendly solution.
These gastropods are notorious for devouring plants and can decimate seedlings almost overnight. Signs of slug activity include irregular holes in flowers, leaves, stems, bulbs, potatoes and tubers, along with a silvery slime trail. Slugs remain active all year, favour cool, dark and damp conditions, and are most active after dark.
Garlic's allicin is the key
Peer-reviewed research published in the Crop Protection journal by scientists at the University of Newcastle upon Tyne found that garlic ranked among the most potent natural molluscicides, with the kitchen staple even capable of destroying slug eggs within the soil. A four-pack of garlic bulbs is available for just 87p at major UK supermarkets such as Tesco and Sainsbury's, working out at approximately 22p per bulb.
Garlic's effectiveness is down to allicin, a compound formed only when a clove is crushed, chopped or chewed. Slugs find garlic deeply repellent, and sufficiently strong concentrations can prove fatal. Garlic poses no threat to humans, pets, birds, or other garden wildlife.
Method 1: Boiled garlic spray
To make a simple garlic spray or drench, you need two whole garlic bulbs and two litres of water. According to Sprouts of Bristol, the most effective method is regular application of a garlic spray on your plants. The specialists explain: "A weekly application, plus an extra round after heavy rain, is key. It's easy to make and safe for your plants."
Add the garlic bulbs and water to a saucepan and boil for approximately 15 minutes until the garlic becomes soft. Mash the garlic to release the allicin, then strain out the skins, leaving a cloudy liquid concentrate. Allow it to cool before transferring to a bottle. Dilute two tablespoons of concentrate in five litres of water and apply weekly. Keep the concentrate refrigerated or in a cool, dry place.
Sprouts of Bristol suggest thoroughly coating both the foliage and the soil around plants, applying once weekly from February through to October. Reapply after substantial rainfall.
Method 2: Cold water extraction
Specialists at Chimney Sheep highlight a crucial difference: "Allicin is destroyed by heat. It starts degrading above about 60°C and by the time you've had a good boil on, it's pretty much gone. If you want a really potent brew, the secret is to make a cold water extraction instead. The cold water method keeps the allicin intact, which means the slugs sit up and pay attention to it. Simple, but important."
To prepare a cold water garlic spray, take two complete bulbs of garlic, remove the skins from the cloves and place them in a blender or food processor with 200ml of cold water. Alternatively, crush each clove separately with a garlic press and mix into the water. Filter the mixture through a fine sieve or muslin cloth into a jar, ensuring all solid particles are removed to avoid blocking a spray bottle or watering can rose.
For severe infestations, Chimney Sheep advises applying the solution undiluted to problem areas. Alternatively, dilute about two tablespoons with a full five-litre watering can to cover a broader area. Keep the cold garlic concentrate refrigerated or in a cool, dry location, and prepare a fresh batch each growing season.
