Are Used Coffee Grounds Good For Tomato Plants

Growing tomatoes is a non-negotiable must for my garden each year. I can never seem to have enough tomatoes – beefsteak, saucing, cherry, heirlooms – I want them all! As a suburban homesteader, I am also a huge fan of composting and using everything I can in my garden rather than throwing things in the trash.

I remember hearing about the benefits of using coffee grounds directly in the garden many years ago. But is it really beneficial? Should you be using them just when composting instead? While this can be a controversial topic, I’ve arrived at my own answers based on my first hand experience.

So, let’s take a deeper dive into the world of adding coffee grounds while growing tomatoes in your garden this season. We will look at the short and longer answers to this question, as well as busting a few myths!

The Long Answer + Myth Busting

So let’s take a look at the longer answer to this question, as well as busting a few myths that are out there today. If you do a search online today, you’ll find countless articles on all the benefits coffee grounds can add to your garden. From fertilizing, to mulching, to disease prevention, it seems that just a dash of them is the answer to all your gardening ills!

But does the science actually back up these widespread claims? In my experience, they do not. In fact there are studies that have been done that show that adding coffee grounds directly to your garden is harmful, and not beneficial. Let’s take a look at some of the top myths out there, and how they stand up.

Myth 1: They Add Nutrients to Your Tomatoes

Hand Holding Soil
Some gardeners claim that spent or unspent grounds add nutrients to the plants.

This is definitely the most common claim made about using coffee grounds in your garden. Coffee grounds contain about 2% nitrogen and trace amounts of phosphorus and potassium. These macronutrients are really important to the growth of your tomatoes. It’s definitely understandable why many believe coffee to be an excellent slow-release fertilizer for tomatoes.

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In theory, this should work. As the grounds are broken down in the soil, they will naturally release nutrients. They won’t provide immediate nutrients, but over time, they will increase levels in the soil.

However, if you’re looking to replace your fertilizer with coffee grounds, you may want to rethink it. Even the most average slow-release fertilizer designed for tomatoes has much higher concentrations of nitrogen, phosphorus, and potassium.

The miniscule 2% concentration in coffee grounds won’t do your plants any favors in the early growth phase when tomatoes need a lot of nitrogen to stimulate foliage growth.

Tomatoes are fast growing, nutrient hungry plants. Do your plants a favor and invest in organic tomato-specific fertilizers that are designed to help your plants grow efficiently at each stage of their growth cycle.

Myth 2: They Acidify Soil

Adding grounds to tomato plant
Coffee grounds are actually pH neutral. Because the acid that is in the composition of coffee beans is washed out during preparation.

This is probably the second most common myth around adding coffee grounds to your tomato plants. Tomatoes thrive in a slightly acidic environment. They are happiest at a pH level of 6-6.5.

You can easily test your soil at home with a pH meter or soil test strips to see where your soil naturally falls. If your soil is too alkaline, you will need to amend it with a soil acidifier (this is much easier to do when planting tomatoes in containers).

Coffee is thought of as acidic so it’s no wonder that it was thought of as a ‘hack’ for soil acidification. Unfortunately, they are only slightly acidic and are often considered to have a more neutral pH level (most often around 6.5-6.8).

Myth 3: They Make Good Mulch

Organic mulching tomatoes
Some gardeners feel they can help soil from drying out.

Mulch is incredibly important in your garden. It’s especially important in and around tomato plants. Mulch provides protection against excessive heat, helps soil retain moisture, prevents water splash back (which can spread fungal disease), and discourages weeds.

You can use a lot of organic materials as mulch. Straw, grass clippings, wood chips, and crushed up leaves are all things you can repurpose from your yard (if it hasn’t been treated with herbicide/pesticide).

On paper, it looks like coffee would be a great addition to this list. It’s finely textured like coconut coir, it’s an organic material, and it slowly releases macronutrients over time. Coffee should be a winner! In actuality, adding 1-2 inches of coffee grounds to your garden as mulch can cause more problems than it solves.

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The number one issue is water. As they sit, coffee grounds compact and form a hard layer over the soil. This can cause a hydrophobic surface. Instead of helping the soil retain water, those hardened grounds prevent water from getting to the soil … and the roots of your tomatoes. If the layer is thick enough, it can also cause aeration issues in the soil, which can suffocate the roots of your plant.

Myth 4: They Are a Good Weed Barrier

Grounds added to Plant
Coffee grounds have the ability to prevent the germination of weeds.

Keeping your garden clear from weeds can seem like a never ending task. The good news is that you can companion plant certain flowers and herbs with your tomatoes to take some of the work off your plate. Trailing nasturtiums, marigolds, chives, and basil all make excellent weed barriers around your tomatoes and have the added benefit of pest control as well!

But the question is whether coffee grounds would make a good weed barrier. In the previous section, we discovered that the hard, compact layer created by coffee ground ‘mulch’ prevented the soil from receiving adequate water and air.

On paper, this would indicate that it would make an excellent weed barrier! Weeds won’t grow where there is no water or air. While this is true, we also now know that this condition is detrimental to your tomatoes. If the action you take kills your tomato plants, it doesn’t really matter if weeds grow or not.

Myth 5: They Deter Pests

Hornworm on a Tomato Plant
Some gardeners believe that coffee grounds can deter pests from coming after your tomato plants.

In 2002, the University of Nebraska conducted a study to look at the effect of using caffeine as a repellent on snails and slugs. They found that at certain levels, concentrated caffeine becomes a lethal neurotoxin to the pests. This research has, of course, been extrapolated to mean that coffee grounds have the same effect … because coffee has caffeine.

The problem with said extrapolation is that used, it’s been shown that they have minimal levels of caffeine. If you sprinkle grounds around your plant, snails and slugs will just roll right over them and still eat all your juicy tomatoes.

There is also zero evidence that they deter other tomato pests like aphids, flea beetles, or white flies. If you’re looking to deter pests or eradicate an infestation, you’ll be far better off focusing on remedies that are focused on that particular pest. You can also try companion planting your tomatoes, which is natural, and works as a natural deterrent.

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Myth 6: They Prevent Fungal Disease

Fungal dangerous diseases of tomatoes
The verdict is still out on if coffee grounds actually prevent fungal diseases.

Fungal diseases can be devastating to your tomato plants. Often, by the time you notice a problem through leaf curling or yellowing, the disease is fairly established. Many diseases are fatal to a plant unless caught in the earliest stages.

In 2016, a publication called Plant Signaling and Behavior published a study looking at the efficacy of concentrated levels of caffeine on fungal activity in soil. We know that fungal disease is often transferred to tomatoes by soil-contaminated water splashing back on the underside of leaves. If we could lessen the fungal activity, that would be great!

The problem is the same, however, with the slug issue mentioned before. Concentrated levels of caffeine are just not possible in coffee grounds. Amending soil or adding them to compost will likely all help the general health of your soil, but that won’t achieve the same results as the study.

Other studies like this one, have looked at how different soil amendments (including coffee grounds) increase the microbial activity in your soil, thereby suppressing fungal diseases. The study, unfortunately, only looked at one type of disease – Fusarium Wilt – and only in spinach. So while the results were encouraging, the potential impact is unknown for other plant species or other fungal diseases.

Myth 7: Let’s Compost!

coffee ground using as compost in the garden
If used properly, they can make an excellent composting material.

Let’s talk about composting. This is an area where I 100% wholly advocate for using your spent coffee grounds!

A healthy compost pile needs a particular ratio of carbon to nitrogen materials to break down efficiently and provide your plants with the nutrients they need. This really just means that your compost needs a healthy mix of ‘green’ and ‘brown’ materials.

They are considered a ‘green material’ (those high in nitrogen) just like food scraps, grass clippings, and eggshells. Brown materials – shredded paper, straw, or fall eaves – are carbon rich and provide the other side of the composting equation.

A general rule of thumb is that up to 20% of your compost pile can be made up of coffee grounds. It would be kind of shocking if you actually achieved that level, so don’t worry about measuring this. Simply aim for a healthy mix of greens and browns; your compost will do the heavy lifting!