Coffee Grounds in the Garden...what you need to know!

Coffee grounds can be a useful addition to any garden. We usually keep a small container outside the shop in the spring / summer months full of used coffee grounds. As long as you know what you’re doing this addition can really help fertilise your plants. The best way to reuse these coffee grounds is to add them to your compost. 

coffee on the compost heap

Fertilising

Composting with coffee is a great way to make use of something that would otherwise get thrown away. Coffee is nitrogen rich and is a great way of adding this important compound to your soil. Nitrogen helps to form proteins in plants, an essential building block without this, plants will wither and die. It also helps to produce chlorophyll which makes leaves greener. What better way to grow and build life than recycling the exhausted part of another plant!

pH

As we know coffee can be quite high in acidity, this is however, before it has been mixed with water. As acid in the coffee is soluble it will be taken away with the water and the remaining coffee grounds will have a pH closer to neutral. Using fresh, unwashed coffee grounds therefore can lower the pH of the soil (higher acidity). This is great for plants that love acidic soil such as peppers, potatoes, rhubarb, strawberries etc. However, if you don’t want to alter the pH of your soil too much the best thing to do is to rinse fresh coffee grounds and reduce the acidity before adding to your compost or only use spent coffee grounds. As washed grounds have a pH nearer to neutral pH or 6.5 they will not affect the acid levels of the soil and can be used on all plants.

One thing to note is that tomatoes especially do not respond well to coffee grounds and can cause growth stunts.

Pesticide

Luckily caffeine is a natural pesticide. The coffee plant ‘Coffea’ has evolved to use caffeine as a deterrent to bugs and pests when growing. When ingested by a pest the caffeine paralyses or kills it and so pests have evolved to be able to detect caffeine and avoid ingesting it. Used grounds are therefore in theory a natural deterrent for pests found in the UK such as slugs and snails.

coffee in the garden and soil

Limitations

There are a few reasons for adding grounds to your compost rather than directly onto your top soil.

  • Household pets - If you have dogs or a cat that like mine likes to investigate EVERYTHING, caffeine can actually be harmful to them if ingested in large quantities so best to avoid adding grounds directly to the soil.
  • Absorption - Adding to directly to the soil won’t immediately provide your plants with nitrogen. The nitrogen in un-composted coffee grounds has low bio-availability, therefore direct application of coffee grounds won’t help to fertilise your plants and provide them with the nitrogen they need.

In summary, coffee can be a great addition to a compost heap as long as it is used in conjunction with other organic matter. Use in moderation and do your research into what plants will thrive and what plants to avoid.

Written by Emma Wickstead

Crosby Coffee Customer Accounts Manager

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