If you are either a garden lover or a coffee lover, you will be aware of the debate around coffee grounds for plants. For some, used coffee grounds are a soil enhancer par excellence, while for others they are about as beneficial as pouring weed killer straight on your favorite flowers or vegetables.
So, how do we determine fact from fiction and truth from anecdote? In this post we will dig deep into the question of are coffee grounds good for plants? We will think about using coffee grounds as fertilizer, coffee grounds in compost, and adding coffee grounds to soil in general.
Why the Debate around Coffee Grounds for Plants Matters
There are at least two reasons why the use of coffee grounds in the garden is significant and worthy of consideration.
There is growing awareness that as humans we have a responsibility towards the planet we live upon. The way we live will impact the environment and future generations. Especially as a Christian, I feel strongly that we have a responsibility to care for this world.
Each year an estimated 6 million tons of coffee grounds waste is produced globally. Read that sentence again. 6 million tons! And that is without even considering takeaway coffee cups, coffee pods, coffee filters, or other forms of coffee related waste.
I love coffee, I drink multiple cups every day without fail. But our love of coffee comes at a price. So if we can find a way to dispose of used coffee grounds responsibly, and that also helps the environment, then we have struck gold. The added advantage is that if coffee grounds are indeed good for the garden, then that is something realistic that many of us can implement.
So the debate matters because our planet matters, and the consequences of our consumer choices matter.
The second is not necessarily so weighty morally, but will matter to a certain niche of people. And that is that the debate matters because soil quality is absolutely essential for good growth in the garden. As all the gardeners out there will know, your soil quality, or lack thereof, has a direct impact upon the healthy growth and development of plants.
So, as a budding gardener myself, I want to know whether adding my coffee grounds to the compost or using coffee grounds as fertilizer is going to help or harm my garden. So the debate matters for the gardeners out there and the soil that is receiving the spent coffee grounds.
Are Coffee Grounds Good for Plants?
The Popular Answer
So you have probably come across people or businesses advocating to mix coffee grounds in the garden for soil structure, nutrients, soil quality, and to attract earthworms. In fact, it is common to see the local coffee house put out bags of used coffee grounds for aspiring gardeners to use.
Behind this advocacy of adding coffee grounds to soil there is often anecdotal evidence by veteran gardeners, as well as confirmation by trusted sources.
And certainly it makes sense; coffee grounds contain nitrogen which is essential for healthy soil, fresh coffee grounds are acidic and so one might assume they could affect the PH level of the soil, and the composition of spent coffee does seem to fit it for use as mulch. Similarly, we know that some plants such as hydrangeas, rhododendrons, azaleas, and blueberries like acidic soil.
And certainly, there is some truth to these claims regarding coffee grounds and plants. Studies have shown that coffee grounds can moderate soil temperature, aid soil structure, and hinder the spread of pathogenic fungi. So coffee grounds have the potential to be used positively for soil enhancement. But will coffee in garden actually help your plants to grow?
The Scientific Answer
Over the last few years a number of universities have conducted studies to determine the exact relationship between the use of coffee grounds in the garden on plant development, earthworm populations, and soil quality.
And in general, the studies have come back fairly conclusive in the negative. The very name of one 2016 study makes this point explicitly. The paper is titled: ‘Applying spent coffee grounds directly to urban agriculture soils greatly reduces plant growth.’
Basically, the scholarly consensus to date is that the detrimental effects of coffee grounds in the garden outweigh the positive aspects. In particular it was found that coffee grounds (whether used as mulch, fertilizer, or added directly to the soil) hinders plant growth regardless of soil type. It also negatively impacts seed germination.
It was also proven in another study that coffee grounds in compost rather than attracting earthworms, can actually drive them away.
In part, these detrimental aspects of coffee in the garden can be attributed to the caffeine left in spent coffee beans. This makes sense in regard to what we know of the propagation of wild coffee plants. Coffee cherries that fell to the ground and decomposed released toxins that inhibited the growth of competing plants, allowing the coffee plants to grow more populously.
So the caffeine in the coffee grounds that you might be thinking about putting into your garden may actually hinder rather than aid plant growth.
As to some of the other claims, such as coffee grounds repel slugs and dogs and cats, science has basically shown that these are fairly loose claims. In one experiment, slugs crossed a line of spent coffee grounds with barely a moment’s hesitation. So while it may not be their favorite material, it certainly isn’t a one stop repellent.
Similarly, the use of coffee grounds for mulch has its own pitfalls. If applied too thickly, coffee grounds can actually clump together (like clay) and hinder much needed water getting through to plant roots.
Much of these same claims have also been proven by homegrown experiments done by passionate gardeners.
Coffee Grounds in the Garden, Summing Up
In summary, science as well as experience sadly does show that coffee grounds are more likely to hinder rather than help plant growth. While the potential is there, the studies show that you should be very cautious about adding coffee grounds, especially in large volumes to your garden.
Coffee grounds deter earthworms and hinder plant growth and seed germination. While they have been touted as just what your garden needs, the studies have shown that it is simply not true and is more likely to harm rather than help your plants.
What you Should do With Coffee Grounds in the Garden
There are however a few legitimate options.
The first is in composting. Many of the detrimental effects of used coffee are lessened through the composting process. There are still mixed views about the use of coffee grounds in composting. Many advocate it so long as the amount of coffee does not exceed 20% of the overall pile. However, the coffee grounds have been proven to decrease earthworm activity in composting piles.
In my experience, there are two reasons that people may choose to have compost piles. The first is soil enhancement. These gardeners are very careful about what goes into their compost bin, as well as the ratio of brown (carbon rich materials) to green material (nitrogen rich materials). This compost is regularly and intentionally placed in the flower or vegetable garden to aid soil quality. I would recommend leaning on the safe side and not using coffee grounds in this sort of pile.
However, the other and probably more popular reason for composting is to help the environment and responsibly dispose of degradable materials from the kitchen and house. These compost bins are not usually carefully monitored and are only occasionally if ever added to the actual garden. Their main aim is responsible disposal of waste. Coffee grounds are great for this sort of compost pile (again provided that they don’t exceed 20% of the overall pile). If you just use your compost to get rid of waste responsibly then absolutely add your coffee grounds and paper coffee filters to this pile. It won’t harm the breakdown of compostable materials and will break down itself over time.
The other surprising use of coffee grounds for gardening is in regards to fallow land. If you have a patch of land that you intend to use later but don’t have anything currently in, then coffee grounds can be a great help in slowly aiding soil structure and slowing weed growth. To do this sprinkle coffee grounds over the fallow land and then mix into the soil. Coffee grounds do help soil structure and hinder weed growth in fallow land.
What you Shouldn’t do with Coffee Grounds in Garden
The detrimental effects of ground coffee on soil is impacted by both the volume of coffee used, as well as the developmental stage of the plants in question. As such it is recommended that you never put coffee grounds in soil that you intend to germinate seeds in. This will hinder growth significantly at a stage where plants are already weak and vulnerable. The same is true for seedlings.
It is also recommended that you never add coffee grounds directly into your garden or touching any of your plants. While the effects of adding coffee may not be devastating, especially if you only add small amounts, it simply isn’t helping in the way you think it is. The verdict is out.
Apart from the above mentioned exceptions, coffee grounds are not good for your garden and should not be used in this way if you want a flourishing garden.
So I am truly sorry to have to say coffee grounds for plants is certainly going to be a no go in my garden, and probably should be in yours too.