Last Updated: April 7, 2023
Coffee grounds have been used by home gardeners to help “solve” a wide variety of issues. There are so many claims about what coffee can do for your tomatoes, from adding nutrients, to using it as a mulch, and so many more.
In this article, I want to clarify what coffee can (and cannot) do for your garden plants. Specifically, I will answer the question: are coffee grounds good for tomatoes?
Coffee Grounds For Tomatoes
Why does everyone want to use coffee grounds in the garden? Well, the source of the many myths about the benefits of using coffee stems from its popularity.
Coffee is one of the most commonly consumed beverages, with 62% of all Americans drinking it daily. That amounts to hundred of millions of cups, every day! As a result, a lot of coffee grounds are produced.
Coffee grounds are simply the by-product of brewing coffee. They are natural and organic, and are otherwise disposed of as trash. So why not re-purpose them and add them to the garden, right?
It is hard to let something like your daily coffee grounds go to waste when they can benefit your garden. So let’s dive into the most common myths that are out there and the evidence for or against each.
Myth #1: Coffee Grounds Add Nutrients
Coffee contains about 2% nitrogen, and trace amounts of phosphorus and potassium. Since coffee grounds are an organic material, these nutrients have to be broken down in order to be accessible to plants.
As a result, coffee will add nutrients for tomato plants, but not quickly. The bacteria in your soil has to slowly decompose the grounds first, providing a slow-release fertilizer.
One study showed that adding spent coffee grounds directly to the soil actually reduced plant growth. There are other factors that could have been in play here, but it is not reassuring to see negative results.
Tomatoes are also fast-growing, nutrient hungry plants, so it is much better to use a more concentrated formula such as Miracle-Gro Organics. If you simply must use the coffee grounds, add them to your compost pile first, allowing the nutrients to be released before adding to the garden.
If you plan to use coffee grounds as a fertilizer, be sure they are finely ground. Also, amend them into the soil in early spring before transplanting your tomatoes. This will ensure there is enough time for the coffee to break down and release usable nutrients.
Myth or fact? This myth holds some water, but there are certainly better fertilizers for tomatoes. I like to amend my soil with slow release, purpose-made organic nutrients at the beginning of each season. It is best to compost coffee rather than adding directly to the soil.
Myth #2: Coffee Grounds Acidify Soil
Coffee is commonly called ‘acidic,’ but the grounds are typically measured to be only slightly acidic (between 6.5 and 6.8pH). For all intents and purposes, this is neutral.
For tomatoes, the ideal pH is between 6-6.5, so adding coffee grounds will not acidify alkaline soil. The amount required would be far too much, so coffee is not useful for lowering soil pH.
There may be exceptions with specific batches of coffee grounds, but you must test for pH first. Compare this to a more common acidifier, sulfur. Sulfur can acidify your soil, though it may not be permanent.
If you are struggling with alkaline soil, the best option is to use a purpose made acidifier. Or, grow your tomatoes in containers using bagged potting soil.
Myth or fact? This is a myth. Most coffee grounds alone will not reduce soil pH. Use another method and understand that different soil requires different treatment.
Myth #3: Coffee Grounds Are A Good Mulch
Mulching your tomato plants can be highly beneficial to your plants. It can help reduce the likelihood of soil-borne pathogens getting on your foliage by preventing soil splashing.
It can also help improve water retention in the soil, reducing evaporation from the surface. Mulch can also insulate the soil, keeping the roots from getting to cold or too hot.
But using the wrong material for mulch can be a bad thing. Let’s look at the essential qualities of a good mulch:
- Suppress weeds
- Allow water through to reach soil
- Insulate the ground and prevent erosion
With a thick enough layer of coffee grounds, they will certainly suppress weeds. The finely ground coffee will compress and form a layer over your soil, preventing seedlings from accessing sunlight.
However, the compaction of coffee grounds can lead to a hydrophobic surface, preventing water from getting through. As a result, using coffee grounds as a mulch around tomatoes could prevent them from receiving water.
In short, coffee alone does not make for good plant mulch. Instead, you could use add some coffee grounds to straw or woodchips to help regulate moisture content.
Myth or fact? Myth – coffee grounds can eventually repel water. There are much better mulches for tomatoes such as grass clippings, woodchips, or straw.
Myth #4: Coffee Grounds Deter Pests
While there have been studies indicating that concetrated caffeine kills slugs and snails, there is less evidence showing that coffee grounds are strong enough to do the same.
Caffeine is almost synonymous with coffee, so it is obvious why this myth was born. However, the lower concentration of caffeine in spent coffee beans likely reduces the potency of the chemical.
Slugs can certainly be a pesky pest for tomato growers, so I imagine a small sprinkling around the base of a tomato plant couldn’t hurt. But don’t think this will be a quick and easy solution if you are dealing with an infestation!
There is also very little evidence that coffee can help deter anything else, from flea beetles to aphids to ants. If you are looking to deter pests, you will likely have to be specific about the pest in question and use alternate means.
Myth or fact? Myth – I have not seen evidence that coffee grounds will deter pests on tomatoes (or other garden plants).
Myth #5: Coffee Grounds Cure Disease
Like any tomato grower, you want to avoid plant diseases every year. Serious diseases can destroy your crop completely, so there are many recommended practices to avoid them in the first place.
One recommendation you may find on the internet is coffee grounds. They are sometimes said to prevent and even cure fungal diseases on tomato plants.
While there are studies that show an increase in microbial activity due to organic diversity, it does not directly correlate coffee grounds to disease prevention.
Myth or fact? Given the lack of conclusive research, I can’t be confident in this claim. Coffee grounds may help fend off disease in certain situations, but it is difficult to be sure.
Myth #6: Worms Eat Coffee Grounds
You have likely heard that worms are a good thing to see in the garden. They aerate the soil and break down organic material into simple nutrients for plants to use.
This is true! And if you have your own vermicompost, you can give the worms some food by adding coffee grounds.
Worms will consume the coffee ground (possibly getting a coffee buzz), and break them down. Just make sure they are moist by adding some water.
Myth or fact? This one is fact – worms are nourished by used coffee grounds. However, be sure not to add too much coffee to your worm farm.
Myth #7: Coffee Grounds Are Good For Compost
Finally, I want to discuss the benefits of adding coffee to your compost pile. Thankfully, this is not a myth! This is probably the best way to use your spent coffee grounds for growing tomatoes.
Like any other organic material, coffee grounds will easily compost. This is the process of adding the right proportions of nitrogen rich and carbon rich materials in a pile.
Over time, the organic materials will begin to break down with the help of bacteria, releasing essential plant nutrients. After a few months, compost is ready to be amended into garden soil where tomatoes and other plants will benefit greatly.
Fact: Coffee grounds are considered a ‘green’ material for compost, meaning they are nitrogen-rich (as opposed to carbon-rich).
It is understandable to want to use your spend coffee grounds for your tomato plants. The grounds would otherwise go to waste, so why not sprinkle them into the garden?
However, with all of the myths out there, I recommend using caution. The best way to use your coffee grounds is to compost them with a variety of other organic materials.
Good uses for coffee for tomatoes:
- Compost them. Have I said this enough now? The best use for them is by composting. Add them to dried leaves, grass clippings, cardboard and other compostable materials to make a nutrient-rich material for the garden.
- Add small amounts to soil. I am all about experimenting, so if you feel inclined, try comparing results while using coffee grounds as fertilizer.
- Feed worms. If you vermicompost, the worms will consume coffee grounds and break them down.
If you have run any tests using coffee grounds for tomatoes, I’d love to hear about them! I have nothing against coffee grounds – they are organic and widely available, so why not figure out how they can be used for good in the garden?