Coffee Grounds in the Garden – Are They Safe to Use?

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Robert Pavlis

Is it safe to use coffee grounds in the garden? Coffee grounds are routinely recommended for the garden but in the last couple of years I’ve seen several articles about the possible harm coffee grounds do to plants and soil. Just because it is free organic material does not mean it is something you should be using.

In this post I will look at several of the claims made agaisnt using coffee grounds in the garden and determine their validity.

Coffee Grounds in the Garden - Are They Safe to Use?
Coffee Grounds in the Garden – Are They Safe to Use? These are beans and not grounds, but I love the picture!

Harmful Effects of Coffee Grounds

Coffee grounds are the remaining solid material left after making coffee. Here is a list of the claimed problems with coffee grounds.

  • High acidity will change soil pH too much
  • High nitrogen level stunts the growth of fruits and flowers
  • Caffeine kills plants
  • Kills earthworms
  • Antibacterial properties kill microbes in soil

This sounds like a horrible thing to add to the garden, but what is the truth?

Claimed benefits of coffee grounds, see: Coffee Grounds in the Garden

Coffee Grounds Have High Acidity

I have discussed this in detail in a prior post, called Coffee Grounds Acidify Soil.

Coffee Grounds are not highly acidic. They might be slightly acidic depending on source, but not enough to change the pH of soil, unless you garden on very sandy soil.

Plant Science for Gardeners by Robert Pavlis

Coffee Grounds Have High Nitrogen Levels

Coffee grounds contain 1-2% nitrogen, 0.3% phosphorous and 0.3% potassium along with a variety of micronutrients. They have a C/N ratio of 20-to-1.

They do not have a high nitrogen level. A value of 1-2% is about twice the amount in most organic material, but far less than synthetic fertilizer. Since the grounds are the result of pouring water through them, they will contain very few free ions. All of the nitrogen will be tied up in larger molecules, making it safe to be near plants.

The C/N ratio is ideal for a compost pile, providing a near perfect ratio.

Caffeine Kills Plants

Coffee grounds contain about 6 mg/gm caffeine.

I found the argument that caffeine kills plants surprising since coffee and tea come from plants. These plants and many others produce caffeine as a natural insecticide. Some gardeners even spray it on plants to kill bugs.

Can it really harm plants if plants produce it?

Testing caffeine for allelopathic effects found it stunts the growth of bacteria, fungi, seedlings and plants. Most of these studies use high doses of caffeine and perform the tests in pots. These results can’t be extrapolated to a garden setting.

A study which amended soil with coffee grounds, in both pots and in the ground, tested five horticultural plants (broccoli, leek, radish, viola and sunflower). They found that coffee grounds stunted plant growth, including the growth of weeds. Similar results have been found by others.

This effect is short term. After 12 months the grounds actually produce an increased crop when compared to controls.

There seems to be little evidence that soil amended with fresh coffee grounds will kill established plants.

Coffee grounds in amended soil will affect seedlings or newly planted crops, although the effects are short term, and in the long run they improve plant growth. It is not clear that caffeine is the cause.

It is much less clear how a coffee-ground mulch affects plants. Although some gardeners have reported anecdotal evidence that a mulch harms plant growth, I could find no scientific study that looked at this.

Coffee Grounds Kill Earthworms

People doing vermicomposting regularly, recommend the addition of coffee grounds to the worm bin, provided you do not add too much.

A research study showed, “coffee grounds can be decomposed through vermicomposting and that it improves the quality of vermicompost produced”, but another study that looked at worm populations in three composting systems found that coffee grounds increased worm mortality when coffee grounds were the only food source – which is not what most people do.

Provided coffee grounds are provided in moderation and mixed with other food scraps and/or paper, they should be fine in a worm bin.

I have no idea how coffee grounds affect earthworms in the soil but given the above research it seems likely that when used in moderation, they will have limited if any effect.

More about vermicomposting here.

Coffee Grounds Kill Soil Microbes

Caffeine and several other chemicals in coffee have antibacterial properties so there is no doubt there is some effect on soil microbes. The real questions is, are the effects significant?

Consider that the worms in vermicomposting depend to a great degree on the bacterial activity in their guts. Most of the digestion that takes place inside the worm is actually done by bacteria. If coffee grounds had a significant negative impact on bacteria, worms would either not eat it, or they would die.

Given the fact that in or on soil, the coffee grounds are even more diluted than in a vermicompost bin, they will have an even smaller effect. Also keep in mind that all plants produce antimicrobial chemicals and we have no concern about leaving them in or on the soil.

Coffee grounds probably kill some microbes, but the added organic matter will also feed them. I doubt this is a concern for soil health unless huge amounts of grounds are used.

Coffee Grounds in the Garden – Are They Safe to Use?

As you can see most of the concerns about coffee grounds are unfounded.

There is good evidence that amending soil with coffee grounds can negatively affect plants. So don’t do it.

Mulching with coffee grounds probably will not harm plants, but keep it away from seedlings and new small plants. To be on the safe side keep them out of the vegetable garden. Mulching landscapes should be OK, but don’t use more than an inch at a time.

The best option is to compost coffee grounds first. When mixed with other organic material they decompose in a couple of months and become harmless to plants. It can then be used anywhere in the garden.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

57 thoughts on “Coffee Grounds in the Garden – Are They Safe to Use?”

  1. Frist, thank you for a great blog. It is very refreshing to have a source validated information in the jungle of gardening advice. I just wanted to add about coffee grounds that you can use them to create slug barriers. Caffeine is toxic to slugs. We have a large problem with an invasive slug species where Inlive, and I use coffee ground barriers with success in my garden – and yes, I have made controls. Spraying a caffeine solution on the ground is supposed to be more effective, but I prefer using the coffee grounds and drink the coffee. I am concerned about possible effects on beneficial insects, though, so I only use them where I really need to. Here’s a link to the research:
    https://www.nature.com/articles/news020624-8

    Reply

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