Jade McgeeBefore reaching for a commercial pesticide, it is worth knowing that one of the most effective and environmentally responsible pest control options in the garden sits in your kitchen: garlic. Garlic spray is a well-researched biopesticide that works against a surprising range of common garden pests and certain fungal diseases, costs almost nothing to make, and leaves no toxic residue on edible plants.
Why garlic spray works
The effectiveness of garlic as a pest deterrent is not anecdotal. Multiple published studies have examined garlic-based sprays against common garden pests, including aphids on cotton crops, cabbage flies and housefly populations, and various soft-bodied insects, with results that are generally comparable to synthetic alternatives. The mechanism is well-understood: garlic cloves contain sulfur compounds that, when combined with allicin, another naturally occurring compound in garlic, interfere with the sensory receptors of insects, effectively confusing and deterring them. Sulfur is also a recognised standalone fungicide, which explains why garlic spray shows some effectiveness against fungal problems including powdery mildew and downy mildew.
Garlic spray is considered effective against thrips, aphids, slugs, ants, armyworms, flies and various soft-bodied insects. It will also deter beneficial insects if applied indiscriminately, so targeted application to problem areas is more sensible than blanket spraying across the entire garden.
How to make garlic spray
The basic recipe is straightforward. Separate two to three full garlic bulbs into individual cloves but do not peel them. Place the cloves in a blender or food processor with one cup of water and blend thoroughly until the mixture is completely liquid. Any chunks remaining will clog the spray nozzle, so blend until fully liquefied. Add the remaining water to bring the total to approximately one litre, then strain the mixture twice through cheesecloth or a fine mesh strainer to remove any solids. Pour the strained liquid into a glass jar for storage. This is your concentrated stock.
When you are ready to use it, dilute one part of this concentrate with ten parts of water and add to a spray bottle with a fine mist nozzle.
Useful additions
The basic recipe works well, but several additions can enhance its effectiveness. A small amount of insecticidal soap or castile soap, not dishwashing detergent, helps the mixture adhere to leaf surfaces rather than running off immediately. A few drops of neem oil adds another layer of pest-deterrent properties. Finely chopped hot chillies or a dash of cayenne pepper, blended with the garlic, extends the deterrent effect. Some gardeners also add a tablespoon of vegetable oil to help the spray cling to foliage. Strain carefully if adding chilli, as the fine particles will block a spray nozzle just as readily as garlic solids.
How and when to apply
Apply garlic spray in the early evening rather than during the heat of the day. Plants treated in full midday sun can suffer leaf burn from the spray, particularly if temperatures are high. Evening application also means the spray is in place when many pest insects become active. Coat the vegetation as evenly as possible, paying particular attention to the undersides of leaves where aphids, thrips and whitefly tend to congregate. A pressurised pump sprayer provides more even coverage than a standard hand sprayer and makes managing a larger garden considerably easier.
Reapply at least once a week and after any rain that washes the spray off the foliage. If you are applying to vegetables close to harvest, rinse them thoroughly before eating. The smell dissipates once the spray dries, but the deterrent compounds remain active for a day or two before degrading.
A preventative rather than a cure
Garlic spray is most effective as a preventative measure rather than a treatment for established infestations. If aphids have already colonised a plant heavily, or if powdery mildew has spread across most of the foliage, garlic spray alone is unlikely to reverse the problem. Use it on plants you know are susceptible, particularly during the warm, humid conditions that favour fungal disease and pest populations, before problems appear. A weekly preventative spray on roses, cucurbits and brassicas in particular will significantly reduce the interventions needed through the season.
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