Are Coffee Grounds Good for Plants? – The Ultimate Guide

Are coffee grounds good for plants? As a gardening lover, I thought I knew the answer. Boy was I wrong. I spent 11 hours researching both sides of the argument for and against using coffee grounds on plants.

Keep reading to learn if using coffee grounds in your garden is a good idea.

The Cons of Using Coffee Grounds in Your Garden

There are a lot of articles on the Internet about whether you should add coffee grounds to plants. Some are for it, but quite a few are against it.

The truth is more nuanced than a blanket yes or no. 

Why? 

Two reasons: Caffeine and acidity.

Let’s look at the no’s for a moment because they can have negative effects on plants when not considered:

1. Raw coffee grounds are acidic.

Here’s the thing with coffee’s acidity: the grounds are water-soluble. That means most of the acidity is in the steaming cup of coffee you used those grounds to make. Not the spent grounds.

While brewed black coffee has a pH of 5 (acidic), used grounds only have a pH between 6.5 to 6.8 pH. vanilla. Or “neutral”, to be exact.

Yet, if you use fresh grounds, the brewing process has yet to dilute the acidity. 

You’re putting high pH levels straight into your soil, which can turn leaves brown or kill your plants.

I’m willing to bet not even blueberry bushes like it that spicy.

2. Caffeine can prevent nearby plants from growing.

Coffee’s caffeine is “allelopathic”, meaning it controls and inhibits nearby plant growth.

In nature where survival of the fittest is the law, this is great news for coffee. 

But in your garden, you’ll need a bit of extra knowledge to use grounds without harming other plants.

Then Why Use Coffee Grounds for Plants? 

OK, so at this point, you might be asking: 

‘Lauren, why would I use coffee grounds to fertilize plants if there’s a chance they could turn my roses into jitterbugs or kill off my prized hydrangeas?!’

I hear you. And that is the argument a lot of people make, but we can’t stop there.

After all, there’s a reason everyone’s grandparents told us their green thumbs were thanks to coffee grounds:

1. Coffee grounds contain nutrients in them that are super potent
2. Recycling them is better for the environment than throwing them away
3. And they’re free if you’re a coffee drinker 🙋‍♀️

Wait: How kickass are nutrients found in coffee?

According to the University of Wyoming, coffee grounds contain the following by volume:

• 2% nitrogen
• 0.06% phosphorus
• 0.60% potassium 
• Micronutrients like calcium, copper, magnesium, and zinc

These nutrients make spent coffee grounds in gardens and landscapes an attractive, inexpensive fertilizer.

Can all plants benefit from coffee grounds?

Not all plants will benefit from coffee grounds. Acid-loving plants like azaleas and blueberries tend to respond well. But coffee grounds are neutral and do not significantly alter soil pH, so they are generally safe for most plants.

What Plants Don’t Like Coffee Grounds?

Not all plants appreciate the use of coffee grounds. But you should especially keep coffee grounds away from these:

• Asparagus
• Chinese mustard
• Geranium
• Lavender
• Rosemary

What Plants Like Coffee Grounds?

Acid-loving plants like:

• Aloe vera
• Azaleas
• Blueberries 
• Cabbage
• Carrots
• Grass
• Hydrangeas
• Parsely
• Potatoes
• Roses
• Rhododendrons
• Strawberries
• Tomato plants

How to Use Coffee Grounds as Fertilizer on Plants

Bacteria and fungi need to break down fresh coffee grounds in the soil before your plants can actually absorb their nutrients. 

That’s why most experts agree it is best to add them to a compost pile—not straight to your topsoil.

If you do want to add coffee grounds directly on top of your soil, use a nitrogen fertilizer like manure or bloodmeal along with the coffee grounds. 

That’ll give plants an immediate boost of nitrogen while the grounds slow-release into the soil as they decompose.

When sprinkling old coffee grounds on top of the soil, do so lightly. Less is more, particularly if you’re adding another fertilizer along with it. 

To help decomposing coffee grounds along, use a garden trowel to mix coffee grounds into the soil so they aren’t lying on top. 

Pro tip: People on social media swear that crushed egg shells, diced banana peel, and spent coffee grounds make the best fertilizer.

Do Plants Like Coffee Grounds as Mulch?

Straight up: It’s not a good idea.

Remember the concern about coffee grounds being allelopathic? That comes into play when we start thinking about using grounds on soil as mulch. 

Soil needs to be biologically active for plants to grow in it. Coffee can energize the soil. 

But coffee grounds also contain caffeine. And too much caffeine kills off soil’s important microorganisms—essential for plant growth.

Grounds also clump together when they’ve dried out (kind of like peat moss or clumpy clay soil). These clumped mats block oxygen and can kill your plants.

If you’re keen to add grounds on top of mulch, simply sprinkle the grounds directly onto another type of mulch (like bark).

Composting Coffee Grounds is Best for Plants

Coffee grounds also work well with vermicompost (adding worms to your compost pile).

From all the research I’ve done, adding coffee grounds to your compost pile seems like the best way to cycle coffee into your garden for one big reason: 

When composted, coffee grounds have time to decompose their remaining caffeine. The decomposition process also makes nutrients like nitrogen more absorbable to plants.

Putting on my science hat again: One study looked at four different concentrations of grounds in compost. 

The research found that compost made with 40% coffee grounds was best—and emitted the least amount of greenhouse gasses.

Remember: In the world of composting, coffee grounds count as a green material. Your compost ratio should be 1/3rd green and 2/3rd brown.

Pro tip: Paper coffee filters can be composted right along with the coffee grounds used.

Are Coffee Grounds Good for Indoor Plants?

Possibly, but they are not the best choice. Your houseplants love warmer temps and higher humidity: But so do pests!

Coffee grounds can attract mealybugs, fungus root gnats, and thrips due to the added moisture.

Your coffee grounds can also clump after they dry out, which prevents oxygenation. 

Pro tip: One thing you can do to avoid moss-like clumping is to only use coffee that’s ground on the coarsest setting available. Fine grinds tend to form bigger mats.

On the plus side, coffee smells better than other types of fertilizers out there (or compost that’s a bit smelly).

What About Coffee Grounds on Plants in Pots?

For the safest way to use coffee grounds in a potted plant that is outdoors, coffee grounds must be mixed into your potting soil before planting.

Keep in mind that coffee grounds retain water, so you might try a bit of gardener’s sand along with the grounds if you’re concerned about too much water.

Combine coffee grounds and soil with a bit of sand or perlite to control moisture retention, improve aeration, and support drainage. And, depending on where your plant is, water potted plants with coffee grounds less often.

 Here are some bonus tips for using used coffee grounds safely with potted plants:

1. Consider your plant’s needs.

Different plants have different requirements for soil moisture. If your potted plant prefers drier conditions, you may want to avoid using coffee grounds altogether. However, if your plant enjoys moisture, it might love coffee grounds as long as you control the water frequency.

2. Monitor moisture levels.

Since coffee grounds can increase water retention, it’s important to adjust your watering schedule accordingly.

Rather than watering your potted plants with coffee grounds as frequently as you would with plain water, check the soil moisture level and water only when needed. This will prevent overwatering and potential root rot.

3. Apply composted coffee grounds.

If possible, it’s best to compost your coffee grounds before using them in potted plants. Composting helps to break down the grounds and reduces the risk of acidity buildup, which can be harmful to some plants.

Remember that every plant is unique, and some may respond differently to coffee grounds. Always monitor your plant’s health and adjust your care practices accordingly. 

Other Creative Uses for Used Coffee Grounds

Coffee Grinds As Pest Control 

Caffeine remaining in your grounds can be an effective pest deterrent in your garden—even against slugs and snails. 

Coffee also contains diterpenes, which are toxic to insects like mosquitoes (another reason coffee is magical).  To use coffee grinds as a pest control method, sprinkle them around the areas where you notice slugs and snails. The caffeine acts as a natural repellent and deterrent, making them less likely to venture into your garden.

For mosquito control, you can create a natural insecticide spray using coffee.

1. Start by boiling a pot of water and adding a few tablespoons of coffee grounds.
2. Let the mixture steep for a while, then strain it into a spray bottle.
3. Spray this solution around outdoor areas where mosquitoes tend to gather, such as around stagnant water or near plants.

Using coffee grinds as a pest control method is an eco-friendly and affordable option that not only helps keep pests away but also provides added benefits to the soil.

The grinds act as an organic fertilizer, releasing nitrogen and other nutrients into the soil as they break down.

So, next time you brew your morning cup of coffee, don’t throw away the used grounds. Instead, put them to good use in your garden as a natural pest control method. 

Pro tip: You can also burn used, dry coffee grounds in a fire-safe bowl (kind of like incense) and it acts as a natural outdoor bug repellent!

Can Old Grounds Act As a Weed Killer?

Yes! Coffee’s allelopathy can work for you as an effective natural weed killer.

Yep, that’s right: Generously apply old grounds on young weeds or seeds for the best results. 

But be careful because you can kill nearby plants if you’re too heavy-handed applying grinds to weeds. Try using coffee grinds on weeds in isolated areas like your driveway—not in the middle of your garden.

To use coffee grounds as a weed killer, here’s what you can do:

1. Collect used coffee grounds: Instead of throwing away your coffee grounds, save them in a container until you have enough to use as a weed killer.
2. Identify the target weeds: Select the weeds that you want to eliminate. It’s best to focus on young weeds or areas with weed seeds.
3. Apply the coffee grounds: Generously sprinkle the used coffee grounds over the weeds or weed-prone areas. Make sure to cover the weeds thoroughly.
4. Be cautious about nearby plants: Keep in mind that coffee grounds can also inhibit the growth of desired plants. Avoid applying coffee grounds near plants you want to keep or in the middle of your garden. Use it primarily on isolated areas like driveways or walkways where you want to control weed growth.
5. Follow up if necessary: Coffee grounds may not provide instant results and may require repeated applications for stubborn weeds.
6. Monitor the effectiveness, and if needed, reapply the coffee grounds.

It’s important to note that coffee grounds alone may not completely eradicate all types of weeds. They are more effective in preventing weed growth and acting as a preventive measure.

For tougher or persistent weeds, you may need to combine coffee grounds with other natural weed control methods. Always use caution and follow proper disposal methods for coffee grounds, as excessive amounts can alter soil pH or nutrient levels.

Final Thoughts

Whether you make coffee at home every day or don’t drink it yourself, coffee grounds are a beloved presence in long-time gardeners’ toolkits.

Yet, it’s important to know about using coffee grounds because adding too many or in the wrong way can do more harm than good.

The best way to stimulate plant growth with coffee is by composting used grounds.

Mix used grounds into your compost pile. Then fertilize your garden with compost instead of putting coffee on top of your soil. 

This reduces the risk of caffeine in coffee inhibiting other plant growth, which remains a risk if you’re scattering coffee grounds on the soil or using leftover diluted coffee around the plants.

Happy brewing and composting!

Coffee Grounds in Gardens – FAQs