Coffee grounds as pest control and soil fix

The claim

Spent coffee grounds sprinkled around plants deter slugs and snails, acidify soil for acid-loving plants, and act as a free fertiliser.

Where it came 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 the evidence actually shows

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 to do 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.