Coffee Grounds for Plants: Benefits, Risks and Myths

Coffee grounds are often recommended as a free fertiliser: they supposedly feed plants, acidify the soil and keep pests away. Some of this advice holds up, but there are common misconceptions too. Here is how to use coffee grounds so they help rather than harm.

Plants to enjoy: potted plants.

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What is in coffee grounds

Used grounds contain organic matter, a little nitrogen, potassium and phosphorus, plus traces of caffeine and other compounds. Their nitrogen is released slowly as they break down, so they do not give plants a quick feed.

Popular myths

• “Grounds strongly acidify soil.” Fresh ground coffee is acidic, but brewed grounds are close to neutral, so they are of little use for acidifying soil for hydrangeas or blueberries.
• “Grounds are a ready-made fertiliser.” Fresh grounds applied directly around plants can actually slow growth: caffeine and other compounds inhibit seed germination and young roots.
• “Grounds repel slugs and snails.” The evidence is mixed; do not rely on them for protection.

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How to use them properly

• Compost them. This is the best option: grounds are a nitrogen-rich “green” ingredient. Mix them with dry leaves, straw and paper; paper filters can go in too.
• Spread thinly. If you sprinkle grounds on a bed, keep the layer thin and mix it into the soil or mulch. A thick layer cakes into a crust, goes mouldy and stops water reaching the roots.
• Not for seedlings. Do not add fresh grounds to seed compost.
• Houseplants. Grounds go mouldy easily in pots; compost them and feed houseplants with fertiliser instead.

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Other natural feeds and what they really do are covered in “Natural fertilisers for houseplants”. And if you love coffee beyond the cup, try growing a coffee plant on your windowsill.

Other uses

• Odour control: dry grounds in an open jar in the fridge.
• A gentle hand scrub after gardening.
• A natural dye for paper and fabric crafts.

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FAQ

Can I water plants with leftover coffee?

Occasional diluted black coffee will not hurt, but it does little good; never use coffee with sugar or milk.

Which plants like coffee grounds?

Via compost, almost all: it improves soil structure and feeds soil organisms.

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Author: Maria Svitlovir

Graduated from Taras Shevchenko National University of Kyiv, Institute of Biology and Medicine (2001). BSc in Biology, MSc in Horticulture. Writes about plants, works at a garden centre selecting and repotting plants, and contributes to plant reference guides.