Do coffee grounds repel slugs and pests?
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Not at kitchen strength. Caffeine is genuinely molluscicidal: in a 2002 Nature study, 1 to 2% caffeine solutions killed nearly all slugs and snails within two days. Brewed coffee holds roughly 0.05% caffeine and spent grounds less again, nowhere near that dose. For slugs, iron phosphate baits have actual evidence; for grounds, compost them at a modest share of volume.
What do people believe about coffee grounds?
Spent coffee grounds sprinkled around plants deter slugs and snails, acidify soil for acid-loving plants, and act as a free fertilizer.
Where did the coffee grounds idea come from?
Unusually for a garden myth, from a real paper in Nature. In 2002 researchers found caffeine solutions were potently molluscicidal - 1–2% killed nearly all slugs and snails within two days, and as little as 0.01% deterred feeding. That finding was widely reported and quickly compressed into "use coffee grounds".
What does the evidence show?
The caffeine mechanism is genuine. The dose is not. Brewed coffee contains roughly 0.05% caffeine, and spent grounds less again - nowhere near the 1–2% that produced the kill in the study. The tested treatment and the kitchen remedy are not the same thing.
On soil, Washington State University Extension measured the pH of decomposing grounds across a range of 4.6 to 8.4, so they are not reliably acidifying. Grounds are useful in compost at up to about 20% of volume, with more than 30% becoming detrimental. Used as a mulch they compact into a barrier that impedes water and air movement, and the WSU review explicitly recommends against mulching with them. Growth of alfalfa, clovers, Brassica juncea, ryegrass, geranium and asparagus fern was inhibited.
University of Minnesota Extension adds that grounds have not been shown to consistently lower soil pH - elemental sulfur is the tool for that job.
What works instead?
Compost the grounds at a modest proportion of total volume and use the compost. If you want acidity, use elemental sulfur and a soil test. For slugs, use the methods with actual evidence: iron phosphate baits, or targeted natural actives - and see the beer traps and eggshells entries, because most of the popular barrier remedies do not survive testing either.
Key takeaways
- Caffeine at 1 to 2% killed nearly all slugs and snails within two days in a 2002 Nature study.
- Brewed coffee contains roughly 0.05% caffeine, and spent grounds less.
- Decomposing grounds measured pH 4.6 to 8.4, so they do not reliably acidify soil.
- Grounds work in compost at up to about 20% of volume; more than 30% is detrimental.
- Used as mulch, grounds compact into a barrier that impedes water and air movement.
References
Frequently asked questions
Are coffee grounds good for plants?
In compost, at a modest share. Grounds are useful at up to about 20% of compost volume, with more than 30% becoming detrimental. Growth of alfalfa, clovers, Brassica juncea, ryegrass, geranium and asparagus fern was inhibited by grounds.
Do coffee grounds make soil acidic?
Not reliably. Washington State University Extension measured decomposing grounds across a pH range of 4.6 to 8.4, and University of Minnesota Extension notes they have not been shown to consistently lower soil pH. Elemental sulfur and a soil test are the tools for that job.
Can coffee grounds be used as mulch?
Washington State University Extension recommends against it. Used as a mulch, grounds compact into a barrier that impedes water and air movement.
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