Just like Peter Pan, some tales just never get old. They’re recycled from generation to generation with little to no scrutiny, so it’s hard to tell whether these are actual gardening ‘hacks’ or not.
If you’re trying to figure out whether adding coffee grounds to your tomato plants is a good idea, stick around as the newest MythBusters are in town!
Are coffee grounds good for tomato plants?
Coffee grounds are quite good for tomato plants because they contain 2% nitrogen and other trace minerals. However, they don’t have enough nutritional value to bring any immediate or visible improvements.
Hence, your coffee grounds are better off added to your compost bin or vermiculture.
True or False? Debunking Ways of Using Coffee Grounds for Tomatoes
Rather than throwing it away, coffee grounds have been known to bring a ton of benefits to the garden, from adding nutrients to deterring pests. But how true are these claims?
Read on as we debunk the various ways coffee grounds can be repurposed in the garden.
1. Coffee grounds can be used as fertilizer: FALSE
Many believe that coffee grounds are a great nitrogen-rich fertilizer because they contain a ton of nutrients that’ll be beneficial for your tomato plant’s development.
Er, we think this is a bit of a stretch. According to research, coffee grounds only have about 2% nitrogen and even smaller amounts of phosphorus, potassium, and other trace minerals.
Nevertheless, in theory, adding coffee grounds to your soil should provide your tomato plant with additional nutrients, right?
While these are important nutrients for tomato plant growth, coffee grounds simply don’t have enough nutritional value to bring any immediate or visible improvements in the quality of your soil or tomato plant.
As a slow-release fertilizer, coffee grounds are, well, slow. They’ll be able to boost the nutrient content in your soil over time as they break down.
However, we don’t think relying solely on coffee grounds is a good idea. This is because most slow-release fertilizers have higher concentrations of nutrients and minerals.
We think many gardeners will agree that using slow-release fertilizers made for tomato plants will benefit your tomato plants more than the measly 2% that coffee grounds have.
This is especially true if your tomato plant is already in its early stages of growth and needs a lot more than 2% of nitrogen to grow healthy foliage and produce a lot of fruit sets.
Believe it or not, another study revealed that adding coffee grounds straight onto your soil can lessen plant growth. While there are several other factors to take into consideration, this still isn’t a good sign.
Luckily, all hope isn’t lost. You can still use coffee grounds as a “fertilizer” by ensuring that it’s finely ground before adding it to your soil a couple of weeks before you transplant your tomatoes.
Don’t forget to mix it every now and then so that it doesn’t compact. Doing this before you add in your tomato seedlings will give the coffee grounds enough time to break down and slowly release their nutrients into the soil.
2. Coffee grounds can be used as mulch: FALSE
The biggest issue with using coffee grounds in your mulch is that you’ll need buckets full to be able to make a big enough impact. This is because coffee grounds only contain small amounts of nutrients.
To get all the benefits of mulch, you’ll need a thick layer of about 2 to 3 inches. We’ll leave you to do the math of how much coffee grounds that’ll require.
Even then, using coffee grounds in large quantities is tricky because it has the tendency to compact and harden the soil, which is definitely not what we’re going for.
One of the biggest advantages of mulch is that it’s great at retaining moisture. But how will your mulch do that if it’s compacted?
If anything, it’s going to stop moisture from reaching your soil.
Here’s how you can get around these side effects: Gently mix small amounts of coffee grounds onto your soil, no more than a half inch, before adding a thicker layer of mulch.
Alternatively, you can add a little bit directly into your mulch. This way, your coffee grounds can act as a slow-release fertilizer without compromising the quality of your mulch or soil.
3. Coffee grounds can be used as compost: TRUE
Adding coffee grounds to your compost is probably one of the most popular “hacks” in the gardening community. Thankfully, this one’s true and may be one of the best ways to use it!
All composters know that a healthy mix consists of a ratio of brown materials such as dried leaves and straw which are high in carbon and green materials like grass clippings and food scraps which are high in nitrogen.
Between these two, coffee grounds fall under green materials because they contain nitrogen. Coffee grounds are also known to boost microbial activity, which is great for breaking down compost materials.
It’s generally recommended to add no more than 15 to 20% of coffee grounds into your compost mix. Otherwise, you could risk causing the mixture to get too acidic or hinder decomposition.
In retrospect, 20% is a ton of coffee so we don’t think going overboard is anything to worry about.
Don’t forget to let your compost sit for a hot minute and do its thing before incorporating it into your soil. By this time, your coffee grounds have released all their nutrients.
4. Coffee grounds can be used as soil amendment: TRUE
Research performed by Sunset Magazine found that coffee grounds do a pretty good job of improving your soil’s structure.
In both the short and long run, tilling your soil with up to 35% coffee grounds will help enhance the physical properties of your soil.
This is especially good news if you’ve got alkaline soil and acidity-loving plants.
With that said, keep in mind that tomato plants prefer only slightly acidic soil with a pH level between 6.5 and 6.8. Even then, this is considered fairly neutral.
So while coffee grounds can improve the structure of your soil, you’ll need to use them sparingly so as not to cause pH imbalances.
This is especially true if the soil in your area is known to be acidic. If you’ve got more alkaline soil, you could get away with adding a bit more coffee grounds.
5. Coffee grounds can prevent fungal diseases: TRUE
Even the smallest indication of fungal disease can have gardeners scrambling to find a cure. More often than not, it’s already too late to save them, even at the earliest stages.
Hence, it’s no surprise that gardeners will try anything under the sun to reduce harmful fungal and bacterial activity, ultimately preventing diseases from spreading.
Luckily, ground coffee can help with that. As they break down, they prevent diseases from developing in the soil. These include Sclerotinia, Fusarium, and Pythium species.
Don’t be alarmed if you notice a blue-green colored fungus-like coating developing on its surface. This is a mold called Trichoderma that’s known to suppress the growth of microorganisms that cause fungal rots and wilts.
6. Coffee grounds can suppress weed growth: FALSE
Using coffee grounds to suppress weed growth? That sounds too good to be true. Unfortunately, it is.
In order to prevent weeds from growing around your tomato plant, you’ll need a thick layer of coffee grounds to prevent the germination of seeds, sun exposure, water, and air, which are all needed for weeds to grow.
However, a thick layer of coffee grounds tends to compact, which can hinder water and airflow. This could cause problems for your soil and tomato plants.
So while coffee grounds naturally contain chemicals that suppress weed growth, the amount you’ll need isn’t ideal for a healthy environment for your tomato plants.
7. Coffee grounds deter pests: TRUE
Don’t you just hate it when you don’t get to enjoy the juicy tomatoes that you’ve been growing all season?
Once slugs, snails, and all sorts of pets get into your garden, it’s hard to eradicate them. While coffee grounds have been known to be a great repellent against these pesky critters, their concentration levels are too low to cause a significant change.
On the bright side, studies have revealed that high concentrations of caffeine can kill slugs. Imagine what a cup of black coffee can do!
Though if you’ve got a rather large infestation on your hands, you may want to bring out the big guns and save your coffee grounds for your compost bin instead.
8. Coffee grounds can acidify soil: FALSE
Another popular way to use coffee grounds in your garden is to increase the acidity levels of soil, which is great for tomato plants as they love acidic soil.
Coffee grounds are usually lightly sprinkled onto the soil to slightly raise the pH level.
However, most coffee grounds have a pH level of around 6.5, which is just shy of neutral. Hence, they aren’t acidic enough to greatly impact the pH level of your soil.
It also isn’t recommended to add ingredients that can interfere with your soil’s pH level willy nilly without doing a soil test beforehand. Otherwise, this could bring more harm than good to your tomato plant.
9. Coffee grounds can be used to vermicompost: TRUE
Humans aren’t the only ones who can’t get enough of coffee!
Did you know that worms absolutely love coffee grounds? In fact, coffee grounds can take up about 25 to 50% of a worm’s diet.
Hence, adding coffee grounds to your vermicompost pile can give your worms additional nourishment and function as an organic pesticide.
While you can add both used and unused coffee grounds to your vermicompost pile, the former is a better option as it’s less acidic. If you’ve only got unused coffee grounds, add them moderately.
We suggest adding crushed egg shells to your vermicompost pile to help neutralize the coffee grounds, keeping the pH level of your soil in check.
Having mentioned that, the amount of coffee grounds you add should be no less than 1/8th of your bedding material or compost. Spread a thin layer and mix thoroughly to avoid clumping.
To prevent this, be sure to promote air circulation by turning the pile regularly, but be careful not to accidentally harm your wriggling friends while you’re at it.
If you’re just starting to add coffee grounds, we highly recommend adding small amounts so as not to shock your worms. This will also allow you to see how well they take to the coffee beans.
FAQs
You can add organic matter to the soil of your tomato plants biannually. This also goes for compost, manure, and similar materials.
You can add coffee grounds directly onto the soil of your tomato plants, but it may take a while for it to break down. It’s better to gently mix coffee grounds into the soil for optimal absorption.
Adding liquid coffee is another great source of nitrogen for tomato plants. However, this may be too concentrated, so you’ll need to dilute it.
Depending on how strong your coffee is, mix around ¼ coffee and ¾ water or a 1:1 ratio.
Too much coffee grounds can cause ventilation, drainage, and disease issues that could kill your tomato plant. When adding coffee grounds, ensure that you break them apart to prevent clumping.
