Are coffee grounds good for tomato plants? Yes and no.

Ever wondered if tossing those leftover coffee grounds onto your tomato plants is actually a good idea? Well, you are not alone. I was teaching a beginner gardening class at our local library a few weeks ago, and this very question came up and sparked a lively discussion.

I know there are a lot of myths out there when it comes to gardening, and sometimes it can get confusing. I'm going to clear things up if you've ever wondered, "are coffee grounds good for tomato plants?"

Jump to:

• Are Coffee Grounds Good for Tomato Plants?
• What Research Says About Spent Coffee Grounds
• What Happened When I Tested Coffee Grounds on Tomatoes During a Kansas Heatwave
• Not all coffee is created equal
• Benefits of Coffee Grounds for Tomato Plants
• Disadvantages of Coffee Grounds for Tomato Plants
• Ideal Soil PH for tomatoes
• Signs tomatoes are benefiting from added coffee grounds
• How to add coffee grounds to the garden
• Other kitchen scraps that can be used in the garden?
• Making Compost for Tomatoes
• Frequently Asked Questions
• MORE GARDENING POSTS YOU'LL LOVE
• Comments

Are Coffee Grounds Good for Tomato Plants?

It's a complex answer, but yes, coffee grounds can be beneficial for tomatoes. Let's talk about the benefits first.

Coffee grounds are packed with nutrients that tomato plants love, like nitrogen, potassium, and phosphorus. They help improve soil structure and promote microbial activity, which can lead to healthier, more robust tomato plants. Plus, they may even help deter certain pests. It's like giving your juicy tomatoes a natural pick-me-up!

Moderation is key! Don't get in the habit of emptying your coffee maker basket over the same spot every day, okay? Once is fine and probably beneficial, but doing it repeatedly is going to have very negative consequences.

One of the biggest gardening myths online right now is the idea that coffee grounds are some kind of miracle tomato fertilizer. They can absolutely be useful in the garden, but more is not better.

In my experience, small amounts occasionally added to compost or soil can be beneficial. Thick daily applications directly around tomato plants? That’s where gardeners often run into problems.

It's also a good idea to conduct a soil test to determine the pH level of your soil before adding coffee grounds. This will help you adjust the application accordingly to ensure optimal growing conditions for tomatoes.

What Research Says About Spent Coffee Grounds

One reason there is so much conflicting gardening advice online is that many people confuse brewed coffee with spent coffee grounds. The brewing process removes many of the acidic components found in coffee beans, leaving behind spent grounds that are much closer to neutral than many gardeners realize.

Scientific studies suggest that coffee grounds are most beneficial when used as a compost material rather than applied heavily around plants. While coffee grounds contain nitrogen-rich proteins and trace amounts of essential plant nutrients, those nutrients are not immediately available to tomato plants. Instead, soil microbes must first break down the organic matter through the natural composting process.

This is why many gardeners see better results when coffee grounds become part of a healthy compost pile instead of being used as a thick top dressing around plants. Once composted, coffee grounds can help improve soil structure, increase organic matter, and contribute to healthy soil that supports strong root growth.

In my experience, the biggest difference is fresh grounds versus composted grounds. A small amount of composted coffee grounds mixed into garden beds is very different from repeatedly dumping fresh coffee grounds around the base of tomato plants. While composted grounds can be a valuable soil amendment, excessive layers of fresh grounds may create a dense surface that can interfere with soil moisture, airflow, and overall soil quality.

For most home gardeners, the best way to incorporate coffee grounds is to add them to a compost pile along with other brown materials and organic matter. Over time, the finished compost becomes a healthy mix that can be worked into the tomato garden to improve soil health naturally.

What Happened When I Tested Coffee Grounds on Tomatoes During a Kansas Heatwave

This year, my tomato plants are struggling, big time. Between a late hard freeze that delayed planting by several weeks, extreme heat, relentless wind, and ongoing drought conditions here in Kansas, even normally reliable plants are looking stressed. Curled leaves, slowed growth, blossom drop… it’s been rough.

And because coffee grounds are constantly recommended online as a “natural tomato fertilizer,” I decided to actually test them in my own garden instead of just repeating gardening advice from the internet.

I’m currently comparing:

• tomato plants with no coffee grounds
• tomato plants with composted coffee grounds
• tomato plants with fresh used coffee grounds

I wanted to see if coffee grounds would actually improve plant health, moisture retention, and growth during stressful growing conditions… or if too much could actually make things worse.

One thing I’m already noticing? Moderation matters. A light application mixed into compost behaves very differently than thick layers of fresh grounds dumped directly around plants.

I’ll continue updating this post with photos and observations throughout the season as the experiment progresses.

Not all coffee is created equal

And one more thing, remember that not all coffees are created equal when it comes to gardening. Generally, any type of coffee grounds can be used, but some coffee beans may be a better food source for tomato plant growth than others.

For example, organic, unflavored coffee grounds are typically the best choice, as they contain fewer additives and chemicals.

Avoid using flavored or artificially processed coffee grounds, as these may not provide the same benefits and could even harm your plants. 

Disadvantages of Coffee Grounds for Tomato Plants

Fresh coffee grounds, while generally beneficial for plants like tomatoes, can inadvertently increase the risk of fungal diseases in the soil if not applied correctly.

One way this can happen is through their moisture-retaining properties. Coffee grounds have a tendency to hold onto moisture, creating a damp environment that promotes the growth of fungi, including harmful ones that cause diseases like mold or fungal rot. 

A Note on Drought Conditions

During periods of extreme heat and drought, I’ve also noticed that thick layers of fresh coffee grounds can sometimes dry into a crust-like layer on the soil surface. Instead of helping moisture penetrate the soil, excessive coffee grounds may actually make watering less effective if they are overapplied.

This is one reason I personally prefer using small amounts mixed into compost or lightly scratched into mulch instead of dumping large amounts directly around tomato plants.

Coffee also contains caffeine, even if you think you've squeezed every last drop out of the grounds. A study in the Journal of Agricultural and Food Chemistry stated there can be up to 8 milligrams of caffeine per gram of used coffee grounds, which is about the equivalent of a cup of strong black tea. Another study found that excessive amounts of caffeine killed the earthworms in the soil.

Furthermore, the slow decomposition rate of coffee grounds can contribute to the problem. As they break down, they can create pockets of organic matter where fungal spores can thrive, exacerbating the risk of fungal diseases in the soil.

To mitigate these potential issues, it's important to use coffee grounds in moderation, ensure they are properly mixed into the soil, and monitor soil acidity and moisture levels to prevent the development of fungal diseases.

Ideal Soil PH for tomatoes

Tomatoes kinda like their soil a bit on the sour side, you know? Not too acidic, but just a tad. Think somewhere between 6.0 and 7.0 pH level, which is slightly acidic to neutral. 

Many gardeners assume coffee grounds dramatically acidify soil, but spent coffee grounds are often much closer to neutral than expected. While repeated heavy applications can still affect soil conditions, coffee grounds are generally not considered a reliable way to lower soil pH.

Excessive acidity can also create conditions favorable for certain fungal diseases. If the soil becomes too acidic due to an overabundance of coffee grounds, it may disrupt the balance of beneficial microorganisms, further increasing the risk of fungal issues. 

They can handle a bit of acidity or a touch of alkaline soil, but they really thrive when the soil is just right. Adding too much coffee grounds can throw off the pH and create issues for the tomatoes, companion plants and soil microbiome.

Signs tomatoes are benefiting from added coffee grounds

If your tomato plants are loving the coffee grounds, you'll likely see some positive signs, like healthier-looking leaves, vibrant green color, and robust growth.

On the other hand, if they're not so keen on the use of coffee grounds, you might notice signs of nutrient deficiencies, such as yellowing leaves or stunted growth.

Keep an eye on your plants and adjust your coffee grounds usage accordingly to keep them happy and thriving!

How to add coffee grounds to the garden

1.  Collect Coffee Grounds: Start by collecting used coffee grounds from your coffee maker or local coffee shops. Make sure to use organic, unflavored coffee grounds for best results.
2. Allow Coffee Grounds to Cool: If you're collecting coffee grounds from your coffee maker, allow them to cool down to room temperature before using them in your garden.
3. Prepare the Tomato Plant: Choose a mature tomato plant that is already established in your garden. Avoid applying coffee grounds to newly planted or young seedlings, as they may be more sensitive to changes in soil composition.
4. Apply Coffee Grounds: There are a few different methods for applying coffee grounds to tomato plants:
   • Sprinkle Method: Simply sprinkle the coffee grounds around the base of the tomato plant, making sure to spread them evenly in a thin layer.
   • Mix into Soil: Alternatively, you can mix the coffee grounds directly into the soil around the tomato plant. Use a garden trowel or rake to gently incorporate the coffee grounds into the top few inches of soil.
5. Avoid Excessive Application: Remember to use coffee grounds in moderation. While they can be beneficial for tomato plants, applying too much at once can lead to issues with soil pH and nutrient imbalances. A thin layer of coffee grounds spread around the base of the plant is typically sufficient.
6. Monitor Plant Health: Keep an eye on your tomato plants after applying coffee grounds. Look for signs of nutrient deficiencies or excess acidity, such as yellowing leaves or stunted growth. If you notice any problems, adjust your application of coffee grounds accordingly.

Other kitchen scraps that can be used in the garden?

While coffee grounds can be useful, they're just one of many compostable kitchen scraps. Banana peels, eggshells, vegetable scraps, and other compost materials can all contribute to healthy soil when added to a well-managed compost pile.

Making Compost for Tomatoes

Building compost for tomatoes is a fantastic way to enrich the soil and provide essential nutrients for healthy growth. One approach is to gather organic material from your kitchen and yard, such as vegetable scraps, coffee filters, grass clippings, and even paper coffee filters. Read my post Cheap Ways to Add Nitrogen to the Soil if you want to learn more!

Layer these materials in a compost bin or pile, making sure to add a small amount of each component to maintain a balanced mix. Consider adding used tea bags, which are rich in organic matter and can break down quickly to create nutrient-rich compost.

Over time, the compost will decompose into a nutrient-rich mixture that can be used to feed your tomato plants, ensuring they thrive throughout the growing season. Remember to turn the compost pile regularly to aerate it and speed up the decomposition process.

The picture below is from last fall. We had a hard freeze in the forecast, so I picked every tomato from the raised bed and let them ripen slowly indoors.

Frequently Asked Questions

MORE GARDENING POSTS YOU'LL LOVE

Once you've found yourself bitten by the garden bug, you'll find yourself wanting to know everything! Check out my post How to Grow Tomatoes, Guide for Growing Potatoes or How to Save Sunflower Seeds. And don't forget about Composting Egg cartons!

Happy gardening!

Meg