Every September, the same ritual plays out in homes across Britain: horse chestnuts are collected from parks, placed on windowsills and in corners, and expected to keep spiders at bay. It is one of those pieces of folk wisdom that everyone has heard and many people swear by. But is there any truth to it?
The Folklore
The conker-spider myth has been part of British culture for generations. The basic claim is that horse chestnuts (conkers) contain a chemical that spiders find repulsive, and that placing them around your home will keep spiders out.
Some versions of the myth are more specific: the conkers need to be fresh, they need to be split open to release the scent, they should be placed on windowsills where spiders enter. Some people even claim that conkers need to be replaced every few weeks as they dry out and lose their potency.
What Does the Science Say?
In 2010, the Royal Society of Chemistry ran a competition inviting schoolchildren to design experiments testing whether conkers repel spiders. The results? Inconclusive at best.
Some student experiments appeared to show that spiders avoided areas with conkers, while others showed no effect at all. The experiments were not rigorous enough to draw firm conclusions (they were school projects, after all), but the RSC acknowledged that the results did not provide convincing evidence that conkers repel spiders.
The chemical most often cited as the active ingredient is saponin, a naturally occurring compound found in horse chestnuts. Saponins are mildly toxic and do have insecticidal properties in concentrated form. However:
- The concentration of saponin in an intact conker on a windowsill is extremely low
- Saponin is not volatile — it does not readily evaporate into the air, so spiders would need to physically contact the conker to be affected
- There is no peer-reviewed study demonstrating that saponin at naturally occurring levels repels spiders
- Spiders do not have a strong sense of smell in the way mammals do — they detect chemicals through sensory hairs on their legs, not from a distance
Why People Think It Works
The same factors that make people believe in Irish Spring soap for mice apply to conkers for spiders:
- Timing: Conkers fall in September and October — exactly when house spiders are most active (males wander indoors searching for mates). By November, spider activity naturally drops as mating season ends. People place conkers in September, notice fewer spiders by November, and credit the conkers.
- Confirmation bias: If you place conkers and do not see a spider in that corner, you assume it worked. You do not count the spiders you would have seen without conkers, because you have no way of knowing.
- It is harmless and pleasant: Conker-collecting is a nice autumn activity. Placing them around the house is free, easy, and gives a sense of control. Even if it does not work, it does not cost anything — so people keep doing it.
What Actually Keeps Spiders Out
If you genuinely want fewer spiders in your home, here is what works:
- Seal entry points: Gaps around windows, doors, pipes, and vents are how spiders get in. Draught excluders, sealant, and mesh over vents make a genuine difference.
- Reduce outdoor habitats: Clear vegetation, woodpiles, and debris from around your external walls. Spiders congregate near exterior walls and come inside when it gets cold. An open gap between plants and your house reduces the bridge.
- Keep things tidy: Spiders love clutter, dark corners, and undisturbed areas. Regular vacuuming, moving stored boxes, and keeping rooms well-lit discourages them from settling in.
- Use proven repellents: Peppermint oil sprayed around entry points has slightly more evidence behind it than conkers — though it is still not a reliable standalone solution. Our natural spider repellent guide covers the options that have some genuine promise.
- Commercial spider repellents: If natural methods are not cutting it, there are effective spider repellent products available that use proven active ingredients.
Should You Worry About Spiders at All?
Worth remembering: UK spiders are almost entirely harmless. The large house spiders that appear in September (usually Eratigena atrica) are not dangerous. They do not bite, they do not carry diseases, and they actively reduce populations of flies, mosquitoes, and other genuinely problematic insects.
For most people, the issue is not danger but discomfort. If you have a genuine phobia or simply want fewer spiders indoors, the practical steps above will make a bigger difference than conkers ever will.
For seasonal pest-proofing tips that cover spiders and much more, see our autumn pest-proofing guide.
Got a serious spider problem? Find a local pest controller on PestPro Index who can survey and treat your property.