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 against using coffee grounds in the garden and determine their validity.
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 the 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?
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 the source, but not enough to change the pH of the soil, unless you garden on very sandy soil.
Coffee Grounds Have High Nitrogen Levels
Coffee grounds contain 1-2% nitrogen, 0.3% phosphorus, 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 that 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 that there is some effect on soil microbes. The real question 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 done by bacteria. If coffee grounds had a significant negative impact on bacteria, worms would either not eat them, 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 ground 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.
I have been using coffee grounds for almost six years on all my trees and veggies everyone of them doing great and have tons of warms. The year that I didn’t use coffee grounds, there was no warms and my lemons were half size. I went back again and used coffee grounds and my lemons were huge. Mixing coffee grounds with horse manure works great for chards, parsley, Long green beans, radish, green pepper and roses, also fig tree loves coffee grounds. I don’t believe the study that says coffee grounds hurts plans because of my experience with it, I have 22 trees and i use over 200 pounds of coffee grounds every month from Starbucks after dump it in 55 gallons of water and let it compost for a week and then watering my whole garden with it.
Stopping to use coffee grounds for one year will not be responsible for the loss of all the worms and smaller fruit.
You can’t compare one year with another without running controls. And not accepting a scientific study because you have anecdotal information is not sound logic.
I made my first batch of compost in a wheel barrow. ground leaves and grass clippings. The mass heated up then cooled down after about a week. I added maybe 10 lbs of grounds from Starbucks (they save it for gardeners). It heated right back up again. My worms haven’t complained except for those wanting cream and sugar. They don’t get “straight” grounds. Its mixed with other organic debris from gardening or cooking. Best fertilizer yet.
I dumped coffee grounds in my fern bed, all winter. The ferns grew out of control this summer. Lol
I used coffee grounds to control aphids on Swamp Milkweed. Can’t remember where I read that this would work. It did, last year. This year, I think I used too much – the Milkweed was aphid free, but it did not flower well and hardly set seed at all. So I will be more judicious next year.
There is no evidence coffee grounds control insects. It is very common to have years with high numbers of aphids and years with low numbers.
Acttualy in my office I have several Kalanchoes that had been full of aphids for a couple of years. I tried spraying them with chemicals – the aphids would drop off then come back again. In a last try to save them before I’d throw them in the trash, I added to half of them leftovers from my tea. Green, black, oolong, whatever I had I’d just dump it in there instead of plain water. It saved them, the half that received tea shed all the aphids. I am now trying to save the other half too. It’s by no means a scientific study and cannot be linked exclusively to caffeine content of tea, but I personally believe caffeine is the one.
We garner buckets of coffee grounds from a local coffee shop. It is then spread throughout the garden and raked into the horrible sandy soil. So far, no plants have died from this venture! In fact, mixing the grounds has bulked the soil raising some beds that had shrunk. Can’t all be bad! Thank you for allaying our fears about using coffee grounds in gardening.
Read Japanese beetles like coffee grounds since there have been some increase with the beetles I stopped with coffee grounds.
But that does not mean one caused the other.
For a few years, we have added used coffee ground “tea” to our whiskey barrel tomatoes.
Just keep adding our used grounds to a 5gallon bucket of standing brew.
Maybe our imagination but our love apples seem more acidic (a good thing) and the plants are larger.
Coffee grounds contain nutrients – fertilizer makes things grow bigger.
I use coffee grounds by the 5 gallon buckets full. 100s of 5 gallon buckets over the last four years. They are excellent additions to the garden!
I prepare my potatoe hills the Fall before planting. They are well broken down before Spring.
I compost them with hot, cold, and vermi-composting.
I use them to grow mushrooms. Then I make potting soil from the pasteurized spent “leftovers”.
Coffee grounds spread too thickly or up against seedlings is a very bad practice. Germination inhibition, to stem rot. They must age first.
CG are an ideal addition to my clay soil. I routinely spread a thin layer under cardboard, the underlayment for my wood chip paths. It gives you a beautiful layer of spongey humus. Every 2-3 years, my paths become my planting rows. I plant right in the broken down humus/compost. And start my paths, anew.
They do adversely affect my strawberries, if used fresh. I tried using them for slug control. The strawberries nearest the CG were not productive. I am sticking to slug traps and DE. However, the strawberries growing in soil amended the year before are very prolific berry producers.
Moderation. Broadcasting. Allowing time for them to breakdown via any composting method. Vermicomposting directly in potato hills + waiting 5/6 months… very nice spuds indeed. Worm by products, the castings, are probably every bit to credit for the prolific spuds.
I have used coffee grounds combined with other fruit and veggie scraps in compostable kitchen bags in the flower beds of two different homes. Over time the beds became very good soil where originally it was dead old clay. I have had a lot of people tell me it was bad, but my flowers disagreed and were better every season. Thanks for a great article I will be sharing with several people I know. I add coffee grounds directly to roses as a mulch, do you think it should be composted first?
Thanks again, Jo.
Does not need to be composted for established plants.
I have used coffee grounds along with my compost this year on my potatoes and have been happy with my crop ( Marris Pipers) . Next year I’m planting Yukon Gold and will compost with added coffee grounds.