Close Up of Coffee Beans
Organic Gardening

Are Coffee Grounds Good for Plants?

Looking for a way to use your used coffee grounds? There are countless articles you can find boasting about the benefits of coffee grounds, but are these accounts true?

This article was originally posted on July 13, 2022 and updated on November 10, 2022 to include more information.

Reducing our Footprint and Waste

Truthfully, I’m always looking for ways to reduce our footprint and waste. What can be recycled, what can’t be. What can be composted, what can’t be. Coffee grounds are definitely compostable. It’s another way for us not to have to send something else to the landfill. We originally had a Keurig. Everything seems great with your Keurig until you find out just how much waste there is with it. We were constantly throwing out coffee pods. Yes, we even considered reusable coffee pods, but they just didn’t seem practical for us. Sure, you can recycle plastic….or so you think.

Did you know that in the United States, we generated 40 million tons of plastic in 2021, but only 5 to 6 percent of it was actually recycled? 85% of plastic waste wound up going to the landfill, and 10% of it was incinerated.

Even as a developed country, the United States fails in any effort to recycle. In fact the United States, as well as the rest of the world, should be striving to be more like Sweden. Sweden has been inching its way towards a zero waste lifestyle for years. They virtually have no trash. They even convert food waste into biogas. Although biogas has not yet reduced Sweden’s emissions, it so far increased their consumption of biogas.

“The increase is due to the doubling of imports during the year, of which two-thirds from Denmark. Swedish biogas is mainly upgraded to biomethane (also known as renewable natural gas RNG) and used for natural gas vehicles (NGV).
About one-third of the imports were used as vehicle gas, while the remainder mainly replaced natural gas use in industry and heating. (1)”

With all of these things in mind, we opted to switch out our Keurig for a Breville espresso machine and grind up coffee beans.

Composting & Directly Applying Coffee Grounds in the Garden

There are a lot of questions when it comes to composting. Everyone always wants to know what items can and can’t be composted. I’ve compiled a list of items of what can and can’t be. (It’s not a complete list though!)

When you’re composting there are 3 main ingredients for your mixture: browns (such as leaves and twigs), greens (such as grass clippings), and water. A compost pile always needs equal parts of browns and green. Water plays an important role in breaking down the pile. However, too much water can throw the balance of the pile off. There shouldn’t be a strong, foul odor coming from the compost pile. A pile that is too wet shows little to no breakdown of material and smells.

What CAN be CompostedWhat CAN’T be Composted
FruitsCoal or coal ash
VegetablesBlack walnut leaves and twigs
Coffee grounds and filtersDisease plants
Shredded cardboard**Plants infected with insects
Shredded paper**Meat
Shredded newspaper**Fish
Grass clippings*Dairy products
Shredded leaves**Yard waste treated with chemicals
Wood chipsPet waste
sawdust
teabags

*If grass clippings are put into the pile, the pile needs to be hot enough to kill any weeds that may be present in the clippings. 

**Shredding these items speeds up the decomposing process. By leaving these items whole, it take a lot longer for them to break down.

Coffee grounds are one of the items you should definitely compost, or at least put them directly into the soil in your garden. We definitely take advantage of doing so. We’ve actually done both composting and adding directly to our garden. 

Coffee beans spilling from a bag onto a table
Whole coffee beans

Do Coffee Grounds Have any Benefits for Your Plants? The Pros and Cons

Yes, coffee grounds do play a role in the garden.  Contrary to what many may think, used coffee grounds are not acidic. This is surprising considering coffee itself is acidic. There is also a difference between fresh coffee grounds, or unbrewed coffee grounds, versus used/spent coffee grounds. Fresh coffee grounds are slightly acidic, while used coffee grounds are neutral in pH. They contain nitrogen magnesium, calcium, potassium, and other trace minerals. In spite of minerals, they’re not exactly a medium to grow your plants in.

What the Studies Show at Oregon State University and the University of Melbourne 

According to Oregon State University Extension Service,

“coffee grounds contain a small amount of nitrogen (2%), help with soil structure, are pH neutral (6.5 to 6.8 pH), and help to repel slugs and snails, (2).”

They warn not to use them as a nitrogen fertilizer in spite of containing nitrogen.  Results from the study  showed poor germination rates and stunted growth. 

Another study, “Applying spent coffee grounds directly to urban agriculture soils greatly reduces plant growth”, conducted by the School of Ecosystem and Forest Sciences at the University of Melbourne. found stunted growth with different vegetables, and even noted the suppression of weeds when spent coffee grounds (SCG) were used.During the study, the SCG was either applied directly to the soil or composted.

“Freshly incorporated SCG inhibited the growth of broccoli, viola, sunflower, radish and leek, even when applied at volumes as low as 2.5%. Subsequent weed growth was also suppressed, which emerged as a positive relationship between increasing SCG volume applied and increased weed suppression, (3).”

Anything to help lessen the weeds in my garden without the use of harsh chemicals and pesticides is a bonus.

The Benefits

As both the Oregon State University (OSU) Extension Service and the University of Melbourne stated, coffee grounds can help the overall structure of the soil and repel pests. OSU even states that they promote the growth of microorganisms. Some plants enjoy acidic soil like blueberries. Some can even be watered with coffee because of their love for acidity.

The Potential Negative Impacts

However a third study, “Evaluation of three composting systems for the management of spent coffee grounds”, published by K. Liu and G.W. Price from the Department of Engineering, Nova Scotia Agricultural College, evaluated three different composting systems. In their study they used spent restaurant and ready-to-serve coffee, and examined the impact on in-vessel composting, vermicomposting bins, and aerated static pile bin composting. Liu and Price found, 

“Earthworm growth and survival (15–80%) was reduced in all the treatments but mortality rates were lower in coffee treatments with cardboard additions. A decline in earthworm mortality with cardboard additions was the result of reduced exposure to organic compounds and chemicals released through the decomposition of spent coffee grounds, (4)”.

These findings are contrary to other information you can find from other sources that claim earthworms love coffee grounds. Ultimately it’s up to you to decide whether or not to use coffee grounds in your garden.

How to Use Coffee Grounds for Your Plants

So how can we use coffee grounds in the garden?

Based on the referenced studies, it’s clear that coffee grounds should not be directly applied to plants or used as a nitrogen fertilizer to grow seedlings. Applying directly to plants or used to germinate seeds results in poor plant growth. Here are some ways you can use them.

Tips for Using Coffee Grounds

  1. Put coffee grounds in your compost. It’s a way to add green material to your compost pile and they’ll continue to break down during the composting process.
  2. Apply coffee grounds directly to your soil. Consider using the coffee grounds in a section you’re not growing in. Either sprinkle them on top of the soil or mix them in. Only a thin layer should be put down at a time. Putting too much coffee grounds in one section can cause water not to penetrate the soil because of their water resistance.
  3. Use in a compost tea.
  4. Use for pest control. Sprinkle around plants to ward off slugs and snails with their abrasive texture, and even cats. Cats don’t like the smell of coffee.
  5. Fresh (unbrewed) coffee grounds may help acid-loving plants since it is slightly acidic.
    a. Examples of acid loving plants that may benefit from coffee grounds are: blueberries, azaleas, and rhododendrons.

FAQ: Other Considerations and Tips

1. If I use coffee grounds, how much is added to the soil?

As stated above, too much grounds added into the soil can prevent the soil from absorbing water. When they become wet and then dry, it becomes a solid, clumpy, mass. Spread in a thin layer and mix with other brown and green materials.

2. Do coffee grounds attract pests?

No, they do not attract pests. In fact they deter them because of the strong smell they emit. Coffee grounds help control pests such as slugs, snails, ants (including fire ants), wasps and termites

3. Are there any animals that coffee grounds deter?

They deter cats, rabbits, squirrels, and in some instances, deer.

  1. “Biogas “made-in-Sweden” reduced emissions in 2018.” Sherrard. https://bioenergyinternational.com/biogas-made-in-sweden-reduced-emissions-2018/
  2. “Coffee Grounds and Composting”. Oregon State University Extension Service. 30 Jun. 2022
  3. Hardgrove, Sarah J., Livesley, Stephen J. (2016)Applying spent coffee grounds directly to urban agriculture soils greatly reduces plant growth. Urban Forestry & Urban Greening , Vol. 18, 1-8, https://doi.org/10.1016/j.ufug.2016.02.015 .
  4. K. Liu, G.W. Price (2011). Evaluation of three composting systems for the management of spent coffee grounds. Bioresource Technology, 102(17), 7966-7974. https://doi.org/10.1016/j.biortech.2011.05.073 .

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