What plants should I sprinkle coffee grounds on?

None, straight from the filter. Compost the grounds first, then use the compost on anything. Spent grounds still carry about 0.45 percent caffeine, and the one controlled trial of direct application found it greatly reduced plant growth. If you must use them raw, a layer thinner than a centimetre, scratched into the soil around established shrubs, is the most Oregon State's extension service allows.

What plants should not get coffee grounds?

Seedlings and anything you are germinating, because the caffeine left in the grounds can slow or stop germination; cacti and succulents, which want lean, fast-draining soil; and container plants, where a crust of grounds can seal the surface against water and air.

Are coffee grounds acidic?

Not after brewing. Oregon State's soil scientist puts spent grounds close to pH 6, near neutral, because most of the acids left with the coffee. They will not acidify soil for blueberries or hydrangeas in any lasting way; any change is small and temporary.

Do coffee grounds keep slugs away?

Grounds, probably not. The research behind the claim used caffeine in solution: a 2 percent solution killed 95 percent of snails in one trial, and brewed coffee is the form the extension service describes using. Spent grounds are about 0.45 percent caffeine by dry weight, so a sprinkle cannot reach the concentrations the study used.

When should I put coffee grounds on my plants?

When they have been through the compost pile, which takes a few months. Add them as you make them, keep them under a fifth of the pile by volume, and balance them with dry leaves or straw. Spread the finished compost in spring or autumn like any other.

Are coffee grounds good for tomatoes?

As compost, fine. Raw on the soil, no: the direct-application trial covered vegetables and found reduced growth, and tomatoes are among the plants the BBC Gardeners' World guide on the same search page tells you to keep raw grounds away from. Feed tomatoes finished compost instead.

Brewing Science

Coffee Grounds for Plants: What Is in Them, Where They Go

By Coffee Studies Editorial·Published November 13, 2026·4 min read

Quick answer

Used coffee grounds help plants when you compost them and can hurt them when you sprinkle them on. Spent grounds are about 2.3 percent nitrogen, close to pH 6 rather than acidic, and still about 0.45 percent caffeine, which is why direct application slowed growth in a controlled trial and why Oregon State's extension service caps them at 20 percent of a compost pile. Keep them off seedlings, skip cacti and succulents, and use brewed coffee rather than grounds on slugs: the slug studies used a caffeine solution far stronger than a sprinkle of grounds can give.

Figure

What a spent espresso puck still contains

Values in grams per 100 g of dry grounds

Oils (lipids)

12.5

Nitrogen

2.3

Chlorogenic acids

0.48

Caffeine

0.45about 80 mg in an 18 g double-shot puck

Averages across 50 spent espresso samples from 14 brands collected in coffee shops (Cruz and colleagues, 2012). The authors note wide variation between samples in the water-soluble parts, caffeine included, because espresso extraction is incomplete and uneven. The puck figure is 0.45 percent of 18 g.^[1]

The grounds you tip out after a brew are not spent. By a laboratory count of 50 espresso pucks from 14 brands, they are still 2.3 percent nitrogen, 12.5 percent oils, and 0.45 percent caffeine, which means a single double shot leaves about 80 mg of caffeine in the puck, nearly a cup of coffee's worth. ^[1]

That caffeine is the whole story of coffee grounds in your garden. It is why they feed a compost pile and starve a seedling, why the slug trick works with brewed coffee and not with grounds, and why the extension services tell you to compost first. Here is what is in them, where they help, and where they do damage.

2.3%

nitrogen in spent espresso grounds, measured across 50 samples from 14 brands^[1]

pH 6

where spent grounds sit after brewing, close to neutral, by Oregon State's soil scientist^[2]

20%

the most of a compost pile spent grounds should make up by volume, per Oregon State Extension^[2]

What is actually in your used grounds?

More than you would think, because an espresso or a drip brew pulls out only part of what the bean holds. The chart is from the one study that measured spent grounds from real cafes rather than a factory.

^[1]

Two of those numbers matter for your garden. The nitrogen is why grounds count as a green in compost, the same role as grass clippings. The caffeine is why they are not a fertiliser you can sprinkle: it is a plant's own chemical weapon against competitors, and it survives brewing.

Are they acidic?

No, not once brewed. Oregon State's soil scientist puts spent grounds close to pH 6, near neutral, because the acids leave with the coffee you drink. Any shift they cause in your soil is small and temporary.

^[2]

So the old advice to pile grounds around blueberries, azaleas and hydrangeas to acidify the soil does not do what you want. If you need lower pH, use sulphur or an ericaceous feed; use the grounds for compost.

Should you put them straight on the soil?

No. A 2016 Melbourne trial applied spent grounds directly to urban vegetable beds, and its title is its finding: direct application greatly reduced plant growth. ^[3] Oregon State's guidance says the same in gentler words: grounds applied before composting can tie up nitrogen while soil microbes break them down and can slow germination and growth because of the caffeine.

^[2]

There is a physical problem too. Fine grounds dry into a crust that sheds water and blocks air, which is the opposite of mulch. If you want them on the surface at all, keep the layer thinner than a centimetre, scratch it into the top of the soil, and keep it away from stems and seedlings.

How do you compost them?

Treat grounds as a green and keep them to a fifth of the pile. That is Oregon State's cap: no more than 20 percent of the pile's volume, because higher amounts can be toxic to plants. ^[2]

What you have	What to do	Why
Daily grounds from a drip or French press	Add to the compost pile with an equal or greater volume of browns (dry leaves, straw, shredded card)	nitrogen-rich green; the browns balance it
A cafe's sack of pucks	Spread through the pile over weeks, never dumped in one layer	keeps grounds under 20 percent of the volume
Paper filters	Compost them with the grounds	carbon, breaks down in weeks
Grounds you want to use now	A thin scratched-in layer around established shrubs, nowhere near seedlings	the only direct use the extension service allows
Grounds for worms	Small amounts in a worm bin, mixed in	worms accept them; a thick layer heats and sours

Finished compost that included grounds is ordinary compost. Use it on tomatoes, roses, houseplants and anything else as you normally would.

Do coffee grounds stop slugs?

Brewed coffee can; grounds, probably not. The claim comes from a 2002 Nature study that found caffeine repels and kills slugs and snails, and Oregon State's summary of it reports that a 2 percent caffeine solution killed 95 percent of snails in orchid growing media, better than a metaldehyde bait. ^[4]

^[2]

Now do the arithmetic. A 2 percent solution is 20 g of caffeine per litre. Spent grounds are 0.45 percent caffeine by dry weight, and most of that stays locked in the grounds rather than leaching into the soil surface a slug crosses. A ring of grounds is a texture barrier at best. If you want the effect the study measured, the extension service describes brewed coffee, applied to the growing medium, not the grounds.

^[1]

What to do

Every brew: tip the grounds and the paper filter into the compost, not the bed.
Keep the ratio: grounds under a fifth of the pile, browns on top of them.
Seedlings, cacti, succulents, pots: no raw grounds, ever.
Blueberries and hydrangeas: grounds will not acidify the soil; use a proper acidifier and give them compost like everything else.
Slugs: brewed coffee on the growing medium, if anything; a ring of grounds is decoration.

The short version

Your used grounds hold about 2.3 percent nitrogen and 0.45 percent caffeine and sit near pH 6. Composted at under 20 percent of the pile, they are a good green. Sprinkled raw, they slowed growth in the one trial that tested it and can crust the soil. Keep them off seedlings and succulents, forget the acid myth, and use brewed coffee rather than grounds if you are chasing slugs.

Watch

See it, not just read it

One video picked for this page. They play right here, full screen on a phone, and you come straight back to the article.

BBC Gardeners' World Magazine59k viewsArit reveals the do's, don'ts, and myths about coffee grounds in the garden, including composting, sprinkling, and slug deterrence.

Hosted on YouTube. Credit belongs to the channels named.

Frequently asked questions

What plants should I sprinkle coffee grounds on?: None, straight from the filter. Compost the grounds first, then use the compost on anything. Spent grounds still carry about 0.45 percent caffeine, and the one controlled trial of direct application found it greatly reduced plant growth. If you must use them raw, a layer thinner than a centimetre, scratched into the soil around established shrubs, is the most Oregon State's extension service allows.
What plants should not get coffee grounds?: Seedlings and anything you are germinating, because the caffeine left in the grounds can slow or stop germination; cacti and succulents, which want lean, fast-draining soil; and container plants, where a crust of grounds can seal the surface against water and air.
Are coffee grounds acidic?: Not after brewing. Oregon State's soil scientist puts spent grounds close to pH 6, near neutral, because most of the acids left with the coffee. They will not acidify soil for blueberries or hydrangeas in any lasting way; any change is small and temporary.
Do coffee grounds keep slugs away?: Grounds, probably not. The research behind the claim used caffeine in solution: a 2 percent solution killed 95 percent of snails in one trial, and brewed coffee is the form the extension service describes using. Spent grounds are about 0.45 percent caffeine by dry weight, so a sprinkle cannot reach the concentrations the study used.
When should I put coffee grounds on my plants?: When they have been through the compost pile, which takes a few months. Add them as you make them, keep them under a fifth of the pile by volume, and balance them with dry leaves or straw. Spread the finished compost in spring or autumn like any other.
Are coffee grounds good for tomatoes?: As compost, fine. Raw on the soil, no: the direct-application trial covered vegetables and found reduced growth, and tomatoes are among the plants the BBC Gardeners' World guide on the same search page tells you to keep raw grounds away from. Feed tomatoes finished compost instead.

References

Every factual claim in this article is drawn from the sources below. See the source library for how we grade evidence.

[1]Espresso Coffee Residues: A Valuable Source of Unextracted CompoundsJournal of Agricultural and Food Chemistry · 2012 · Observational study · Tier 2 · Moderate
[2]Coffee grounds boost soil health and help control slugs (Linda Brewer, soil scientist)Oregon State University Extension Service · Agency guidance · Tier 2 · Moderate
[3]Applying spent coffee grounds directly to urban agriculture soils greatly reduces plant growthUrban Forestry and Urban Greening · 2016 · Observational study · Tier 2 · Moderate
[4]Caffeine as a repellent for slugs and snailsNature · 2002 · Observational study · Tier 2 · Moderate