Do coffee grounds, eggshells and banana skins actually do anything for the garden?

There is a whole category of gardening advice built on the idea that kitchen waste has hidden powers. Some of it comes from a real fact stated at the wrong scale — eggshells do contain calcium, banana skins do contain potassium — and some of it is simply wrong. The honest summary is that all of these materials are worth composting and almost none of them is worth applying directly.

Step by step

1. Coffee grounds: not acidic, mildly useful.Fresh grounds are acidic; used grounds, after brewing, are close to neutral at around pH 6.5 to 6.8. They do not acidify soil and will not help a blueberry. What they do contain is a little nitrogen — around 2 per cent — and useful organic matter, and they are a good 'green' in a compost heap. Applied thickly and directly they mat and repel water, so keep them thin or compost them.
2. Coffee grounds as a slug barrier: no.The caffeine-kills-slugs studies used caffeine solutions at concentrations far higher than anything in used grounds. In the garden a ring of grounds does very little and washes away in the first rain.
3. Eggshells: calcium, eventually.Eggshell is calcium carbonate and it does add calcium and marginally raise pH — but only once dissolved, and whole or coarsely crushed shells take several years in soil to break down. Ground to a powder they act faster. In a compost heap they are fine and add a little calcium to the finished product.
4. Eggshells for blossom end rot: no.Blossom end rot on tomatoes and peppers is a calcium delivery problem caused by irregular watering, not a calcium shortage in the compost. Almost every growing medium already contains ample calcium. Consistent watering fixes it; eggshells do not.
5. Eggshells as a slug barrier: barely.Sharply crushed shell is mildly deterrent while it stays sharp and dry, which is not long outdoors. Slugs cross it readily when it is damp. It is not a control method.
6. Banana skins: potassium, after rotting.Banana skin is genuinely potassium-rich, at several per cent by dry weight. But it releases nothing until it has decomposed, so burying a skin under a rose does very little for a long time and attracts flies and rodents in the meantime. Compost them, or steep them in water for a fortnight and use the liquid as a weak feed.
7. Tea bags: check for plastic.The leaves are a fine compost ingredient. Many tea bags contain polypropylene in the heat-seal, which does not break down and leaves a mesh of plastic skeletons in your compost. Split the bags and compost the leaves, or buy a brand that is genuinely plastic-free.
8. Epsom salts, wood ash and vinegar: separate cases.Epsom salts genuinely correct magnesium deficiency and do nothing otherwise. Wood ash genuinely supplies potassium and lime. Vinegar kills top growth of small weeds for a day and damages soil life. These are covered properly in their own guides rather than as kitchen folklore.
9. Compost the lot and stop thinking about it.Coffee grounds, eggshells, banana skins, tea leaves and vegetable peelings are all good compost feedstock and the compost is genuinely valuable. That is the whole answer, and it is more useful than any of the direct applications.

Tips

• Coffee shops will often give away used grounds by the sack. As a compost input at that volume they are genuinely worth collecting — high nitrogen, free, and they help a heap heat up.
• If you want to acidify a pot, use an ericaceous compost or sulphur. Nothing from a kitchen will do it.
• The one direct use of eggshells that works is ground to a fine powder and used to raise pH slightly in a container mix. It is a lot of effort for the effect.
• Steeping banana skins or comfrey in rainwater produces a genuinely usable liquid feed. The active ingredient is the same in both cases — potassium released as the material decomposes.
• Nothing here should go into a wormery in large quantities: coffee grounds acidify a worm bin and citrus and onion upset it.

Common mistakes

• Adding coffee grounds to a blueberry pot to acidify it — Used grounds are close to neutral. The plant gets a mat of water-repellent material on the surface and no pH change at all.
• Crushing eggshells into tomato compost to prevent blossom end rot — It treats a symptom of irregular watering as a calcium shortage. The calcium is already there and cannot reach the fruit. Consistent watering is the fix.
• Burying food waste directly in beds — It attracts rats and foxes, goes anaerobic underground, and releases nothing useful for months. A compost bin or a bokashi bucket handles the same material properly.

Questions people ask