How To Use Coffee Grounds In The Garden?

Coffee grounds are a common kitchen item that you that is often thrown out, however, because this is an organic material can you effectively use it in the garden to enhance the growth of your plants? What are coffee grounds used for?

Coffee grounds can be used in a wide number of applications in your garden which includes as an addition to compost, a fertilizer, a mulch, an acidifying agent, as a pest repellent and it can even be used to grow mushrooms. The wide range of uses occurs primarily because of the composition of the coffee grounds which is provided below, in combination with the pH which is around 6.0,

ComponentPercentage Range
Carbon (C)40 – 45%
Nitrogen (N)1.5 – 2.5%
Potassium (K)0.06 – 0.13%
Phosphorus (P)0.01 – 0.03%
Calcium (Ca)0.01 – 0.02%
Magnesium (Mg)0.15 – 0.25%

As you can see from the table coffee grounds primarily consist of carbon but also contains nitrogen and several minerals including phosphorus, magnesium, and calcium.  This composition means that coffee grounds are suitable fertilizer for plants that produce leafy growth such as lettuce, chard, and spinach.

But fertilization of fruiting plants such as tomatoes, courgettes, or pumpkins will be less effective because there is a relatively low level of phosphorus present which helps to produce flowers and therefore fruit. However, there are still some benefits in adding coffee grounds to these plants because of the presence of calcium ions which will aid in reducing the chances of bottom-end rot which can happen on tomatoes growing especially in the greenhouse and courgettes in particular.

How To Use Coffee Grounds as A Pest Repellent?

Coffee grounds are an effective pest repellent and can be used in several different ways depending upon the nature of the person that you have and specific climatic conditions within your garden.

The most common way to use coffee grounds is as a pest repellent against slugs and snails who do not like crawling across gritty materials to get to plants. Therefore, being able to apply the coffee grounds in a ring around your most sensitive plants will be an effective way to discourage slugs from attacking the plants.

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The other effective material that comes out of the kitchen, which can also be used for exactly the same purpose is eggshells. They can be crushed up to create a gritty barrier along with the coffee grounds if you are a little bit short on coffee grounds.

The other common use for coffee grounds is as a repellent for airborne pests such as mosquitoes who do not like the smell of coffee in general. The presence of mosquitoes is generally more of a problem for those people that have water in their garden.

The most effective way to apply coffee as an insect repellent is either to burn it as incense or put it into very hot water and sprayed in the areas where the mosquitoes are likely to reside. The less effective method is to simply apply the coffee grounds around the garden for other purposes which will produce some odour to repel mosquitoes but will not be overwhelming and therefore it will be less effective.

How To Use Coffee Grounds as An Acidifying Agent

As mentioned earlier in the article coffee has a pH of around 6 which means it can be used to some degree to lower the pH of the soil and therefore acidify it. This is most useful for plants such as azaleas and blueberries which like to have a pH is as low as 5. 

However, because the pH of coffee grounds is not all that low it is not the most effective acidifying agent that you could use in the garden, but it will contribute to the reduction of the pH particularly in chalky soils.

Other materials that you can use that are more effective are pine needles, if you have trees nearby or alternatively the best thing to use is sulfur. Sulfur, over time, will acidify soil due to the production of sulphuric acid. However, is important to note the addition of Sulphur will not acidify the soil instantly, it does take time. 

Using Coffee Grounds as A Mulch

Coffee grounds can be used as a mulch in the garden to exclude light and therefore suppress weeds. However, it would not be my first choice primarily because very few home gardeners would have enough coffee grounds to be effective.

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However, the mulch that I would highly recommend is compost which coffee grounds can be added to contribute turn the mixture. The main reason that mulch is recommended over things like straw is because it is easier to maintain because the weeds can be more easily removed from a compost mulch.

This is because you can use a Dutch hoe to quickly run across the surface of your garden bed and remove any tiny weeds that are developing. This is a much faster method than picking through straw by hand to remove weeds as they develop. Additionally, the level of nutrients within compost is significantly higher so the addition of compost of acts as both a mulch and a soil conditioner.

Using Coffee Grounds in Compost

Coffee grounds can also be added safely to compost with a range of other materials. As you can see from the table above this should be brown material rather than green material because it is high in carbon content. 

To optimise the rate of decay in the compost it is best to place materials in thin layers of green and brown in the compost bay. For most gardeners composting will take the best part of 9 months as it is a cold compost. A cold compost just means that the size of the compost heap is not large enough to create and sustain high temperatures. As a result of this, the composting process does take a lot longer. To generate temperatures above 50 or 60 degrees you need a compost heap that is approximately a metre by a metre in size.

Using Coffee Grounds to Grow Mushrooms

Another unusual application for coffee grounds is as a substrate to grow mushrooms on.  Many growers use around about 20% coffee grounds combined with 80% pasteurised straw.  The pasteurisation of the growing medium is a critical step in growing mushrooms safely because it kills any other spores from wild mushrooms that can be potentially toxic.

To pasteurise the growing medium, you can place the material into a large pot with water and boil it which will kill any bacteria and spores. Once the mixture has been pasteurised it can then be drained and squeezed the material until it is damp but not soaking.

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The material has cooled then mushroom spawn can be mixed through the medium and filled into plastic bags. These bags can be hung up in a relatively dark location such as a shed or garage. To allow the mushrooms to grow out of the medium you need to poke holes through the plastic bag. 

After several weeks the mixture will start to go white and then progressively mushrooms will start to appear on the side of the bag and they can be harvested as needed for several months.

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