How To Use Coffee Grounds In the Garden
Coffee shops often give coffee grounds away free to gardeners, as they’re a waste product they would
normally have to pay to dispose of. For coffee-loving gardeners like me, this freely available resource
sounds like a real boon. But some gardeners suggest that using coffee grounds could be ineffective or,
worse, harmful to plants.
Mulching is incredibly beneficial but it’s notoriously difficult to come by compost, straw or other organic
matter in large enough quantities at a low enough price. Using free coffee grounds seems like the perfect
solution, but some gardeners have found that using coffee grounds directly on the soil has had a
disastrous effect on plants. However this seems to be linked to using thick blankets of it to mulch around
plants and over seeds.
The reason for this could be that coffee beans contain caffeine, which is said to suppress the growth of
other plants to reduce competition for space, nutrients, water and sunlight. How much caffeine actually
remains in used coffee grounds is debatable, and some plants will be more sensitive to caffeine than
others. It would be sensible to avoid spreading coffee grounds around seeds or seedlings as they may
inhibit germination and growth.
There is a more obvious reason why using coffee grounds alone for mulching could be detrimental. Like
clay soil, coffee grounds consist of very fine particles that are prone to locking together. This turns them
into a barrier that will resist water penetration and eventually result in plants dying of thirst.
The solution is to mix coffee grounds with other organic matter such as compost or leafmold before
using it as a mulch. Alternatively, rake your coffee grounds into the top layer of soil so that they can’t
clump together. Variable particle sizes is key to good soil structure.
Coffee grounds are often said to be acidic but this can vary a lot, from very acidic to slightly alkaline, so
don’t expect them to acidify higher pH soils.
Many of us will have dumped the cold remains of a forgotten coffee in a plant pot at some point, and then
perhaps wondered if it was the wrong thing to do! But it turns out that coffee grounds contain a good
amount of the essential nutrient nitrogen as well as some potassium and phosphorus, plus other
micronutrients. The quantity and proportions of these nutrients varies, but coffee grounds can be used
as a slow-release fertilizer.
To use coffee grounds as a fertilizer sprinkle them thinly onto your soil, or add them to your compost
heap. Despite their color, for the purposes of composting they’re a ‘green’, or nitrogen-rich organic
material. Make sure to balance them with enough ‘browns’ – carbon-rich materials such as dried leaves,
woody prunings or newspaper. Your compost heap’s tiny munchers and gnawers will process and mix
them effectively, so using coffee grounds in this way is widely accepted to be safe and beneficial.
Many vermicomposters say that their worms love coffee grounds, so small quantities could also
regularly be added to a worm bin if you have one. Paper coffee filters can go in too.
An oft-repeated nugget of advice is to spread used coffee grounds around plants that are vulnerable to
slug damage. There are two theories why: either the texture of the grounds is abrasive, and soft-bodied
slugs prefer not to cross them, or the caffeine is harmful to slugs so they tend to avoid it.
However in an experiment slugs took just seconds to decide to cross a barrier of coffee grounds! The
same researcher also sought to find out if coffee grounds would repel ants, with similar results – ants
may not particularly like coffee grounds, but they won’t scarper out of your garden to get away from
Composting with coffee is a great way to make use of something that would otherwise end up taking up
space in a landfill. Composting coffee grounds helps to add nitrogen to your compost pile.
Composting coffee grounds is as easy as throwing the used coffee grounds onto your compost pile.
Used coffee filters can be composted as well.
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