Coffee Grounds in the Garden – Uses, & When NOT to Use Them

Can you reuse coffee grounds? In this guide we will go through the potential uses for used coffee grounds and dispel a few myths along the way.

Can Coffee Grounds Be Used To Boost Plant Growth?

There is a plethora of articles out there acclaiming used coffee grounds as a miracle substance for plant growth. Many advocate using spent used coffee grounds around plants to help them grow healthy and strong. Coffee is often touted as the secret to growing prize fruits and vegetables, or for making flowers bloom abundantly all summer long.

Unfortunately, science does not back up this garden myth. Used coffee grounds do contain core nutrients nitrogen, phosphorus and potassium, along with a range of micronutrients needed for plant growth. These nutrients are tied up in larger molecules and so, like other organic matter, could potentially be a good slow-release fertiliser. Adding used coffee grounds will not boost plant growth short term, however, but rather add long-term to the health of your soil.

But while N content 1-2%, the P and K amounts are much more variable, and generally rather low. This mix can actually encourage leafy growth to the expense of flower and fruit formation. What is more, used coffee grounds also contain caffeine. Caffeine is the main reason why using coffee to fertilise and boost your plants is not a good idea.

Coffee_Grounds by How Can I Recycle This / CC BY 2.0 Used coffee grounds are a common waste product, so what should we do with them? 

Caffeine originally arose as a mutation in plants. This mutation gave certain plants an edge because the caffeine in their leaves falling around them had an effect on the surrounding soil which made it more difficult for other plants to grow nearby. Caffeine has been shown in numerous studies to suppress plant growth. As one study states: applying spent used coffee grounds directly to urban agriculture soils greatly reduces plant growth.

A study also showed that used coffee grounds had a negative, rather than positive effect on seed germination of certain plants. Caffeine reduces germination rates in a number of different plants by tying up the nitrogen in the soil.

Clearly then, it is generally not beneficial to spread used coffee grounds around some of your prized produce and plants!

Coffee Grounds for Roses?

Should you sprinkle coffee grounds on your flower bed for roses? While coffee grounds can lower the pH in the soil and attract worms, which is in turn good for roses, too much can create a nitrogen burn which is very bad for your roses. Dilute your coffee grounds with water (1 cup of coffee grounds per 1 gallon), and sprinkle in moderation.

Can We Use Them To Reduce a Weed Problem?

Got a weed problem? Coffee grounds are unlikely to be the solution.

Since the caffeine in those spent coffee grounds suppresses germination and plant growth, should we be using it on our weeds? You might imagine that coffee grounds could be used to limit the growth of those plants we do not want in our gardens.

Unfortunately, it is not quite that straightforward. There are a range of complex interactions between different plants, and elements of the soil ecosystem. There are no guarantees that adding coffee will have any notable effect on the weeds you want to reduce or eliminate. Some may grow worse, but some may grow better. It is really rather difficult to say.

The best way to reduce a weed problem is to weed little and often, and mulch well with a good-quality organic mulch. Mulching is one of the most important practices in a ‘no dig’ garden.

Are Coffee Grounds a Good Mulch To Use Around Plants?

So, we’ve established that the caffeine in used coffee grounds can suppress plant growth. But this effect is not universal. Not all plants will be affected negatively. So should we consider using used coffee grounds as a mulch around certain plants? This is an idea often bandied about on gardening sites and garden forums.

Bad news again – used coffee grounds are not an ideal mulch material. As a slow release fertiliser, they could potentially be added to a mulch containing other organic materials with beneficial effect. But used coffee grounds should not be used as a mulch on their own. They are fine, and therefore compact too easily. This means that they can form a hard layer that does not allow the soil beneath to breathe. A compacted coffee grounds mulch can also make it more difficult for water/rainwater to reach the topsoil where it is needed.

Add used coffee grounds to other materials like dried leaves – but don’t use them as a mulch on their own.

Will Coffee Grounds Help Acidify Alkaline Soil?

Some people are convinced that adding used coffee grounds will make soil more acidic. If they have acidic soil already, they may see this as a reason not to use them in the garden. If they have alkaline soil, they may be hopeful that adding them will help redress the pH imbalance.

Oh dear! This too is a myth. Used coffee grounds may differ a little in their pH level, but generally speaking they are NOT highly acidic, but only mildly so. They are usually only just on the acidic side of neutral, with a pH that generally falls between 6.5 and 6.8. The acid in the coffee is water soluble, so it is in what you drink, not what you throw away.

Should We Add Coffee Grounds to Our Compost Heaps?

Compost Heap at Capacity by Alan Levine / CC0 1.0 You can add coffee grounds to your compost – but only in small quantities, and balanced with plenty of carbon rich material. 

One of the most common ways to dispose of coffee grounds is simply to add them to your compost heap. Coffee grounds are an excellent nitrogen source for composting. They have a C/N ratio of 20-to-1. Sustained temperatures of 140 to 160 degrees Fahrenheit for up to two weeks were recorded when coffee grounds were 25% of the material in the compost pile by volume, in informal trials with OSU/Lane County Extension Service.

But you should not throw too many coffee grounds on your compost heap all at the same time. It is important to keep your compost in balance. When you add nitrogen rich substances like coffee grounds, you should also add carbon rich materials. You might, for example, layer coffee grounds and other kitchen waste with carbon rich compostables like dry leaves or shredded card or paper.

It is important not to add too much at one time to your compost heap. Even when you are diligent in adding carbon to the compost as well, coffee grounds release organic compounds and chemicals which can increase the death-rate of earth worms. Adding too much at one time could also kill off beneficial microbes.

So, yes, used coffee grounds in moderation can aid in creating good quality compost. But you should only ever add it in small quantities, and should always be sure to balance the heap by adding a wide range of both nitrogen rich and carbon rich materials.

Can We Use Coffee Grounds To Deter/ Control Pests in the Garden?

Can coffee keep slugs away? Perhaps not.

There are also plenty of articles and comments out there that tell you that you can use coffee grounds to deter or control a wide range of pests.

Anecdotal evidence might suggest that used coffee grounds repel slugs and snails. But the fact of the matter is that there is no scientific evidence to back this up. Coffee grounds will not kill slugs. Nor will they deter them if the small scale studies that have been undertaken are anything to go by.

Placing coffee grounds around ant hills is also said to encourage them to move elsewhere. A researcher did some experiments to determine whether coffee grounds would repel the ants in his garden. His conclusion was that though ants did not like the coffee grounds, they simply went around them, moved them out of the way, or ignored them. So they were not effective – certainly with the type of ant he had in his garden.

Use Coffee Grounds to Keep Animals Away

Many animals are repelled by the smell of coffee, including cats. Scatter coffee grounds around your garden to prevent any cats from using it like their own personal litter box. Large amounts of coffee grounds may be harmful to dogs, but there is conflicting research to say how much they have to ingest before having a reaction. So, it’s best to avoid doing this if you have a dog that likes to eat or chew on anything laying around. 

How Else Might We Use Them in Our Gardens?

So coffee grounds may not be quite as useful in your garden as the many articles on the subject would suggest. Yes, they can be beneficial when used in moderation as part of a mixed organic mulch, or when added in small quantities to a composting system. But many of the other claims made about them are spurious. Or the scientific evidence is lacking.

But the good news is that you definitely do not have to throw away used coffee grounds. You can also use used coffee grounds as a scrub to scour a crusty barbecue or fire pit grill. You can also use it as a gentle abrasive to get rid of stuck food on pots and pans.

Another idea to consider is using coffee grounds to stop you from slipping on paths or patios. Its gritty texture can help you to keep a sure footing.

You can also try burning coffee grounds to keep mosquitoes at bay. Place a layer of coffee grounds in a fire-proof container, and set them alight. Once they are smoking, smother them with a damp paper towel. The smouldering coffee grounds should help you sit and enjoy your garden without being bothered by these flying pests.

There are also a lot of crafty things that you can do with spent coffee grounds. One excellent example that could help you keep your garden looking great is to use coffee grounds, vinegar and wire wool to make a natural wood stain and give wood in your garden a natural, weathered look.

If none of the ideas above quite fits the bill, you will also find plenty of great ways to use coffee grounds inside your home – from soap making to arts and crafts. Whatever happens, you should never find yourself throwing the coffee grounds away. Make the most of your cup of coffee!

Plants That Like Coffee Grounds

There are several plants you can have in your home or outside that love coffee grounds because they do better in higher acidity soils. They include but are not limited to:  

• Azaleas
• Blueberries
• Cabbage
• Carrots
• Hollies 
• Hydrangeas
• Lilies
• Radishes
• Rhododendrons

These are all acid-loving species that grow best when you put them in acidic soil. It’s a good idea to avoid putting coffee grounds on plants like alfalfa, clovers, and tomatoes because they don’t do well with a higher acid content. 

Hydrangea by Tony Alter / CC BY 2.0 You can actually change the color of your hydrangea flowers by altering the soil’s makeup, especially the acidity. Higher acidity levels will give you blue-tinged flowers. 

How To Use Coffee Grounds In Hanging Baskets & Containers

Coffee grounds work well as a slow-release fertilizer for hanging baskets and planters as well as in the ground. You can easily turn it into one of  your secret weapons to help get full, lush, thriving hanging baskets and containers all season long. 

Along with adding worm castings, you can add a few tablespoons of coffee grounds to all of your containers and pots every few weeks. All you have to do is simply sprinkle them on the surface of your container’s soil. When you water the containers, the nutrients found in the coffee grounds will slowly release and leach into the soil. As they do, the root system will soak them up for a nutrient boost. 

How To Use Coffee Grounds In Flower Beds

Just like your vegetable gardens, you can use coffee grounds when you have annuals in your flower beds. Adding a tablespoon or two of coffee grounds in each hole will help to nourish the plants. It also continues to help you build your soil up each year so you won’t have to amend it as much. The grounds provide a host of trace nutrients, and they also introduce structure to the soil. The structure is essential to getting good drainage, and it helps create channels for water, air, and nutrients to funnel into the plant’s roots. 

You can also add coffee grounds to the base of each plant as a slow-release fertilizer like you would in your garden or container. Each time you water the plants or if it rains, it’ll make the nutrients flow down to the soil. It also helps protect against slugs. . 

How To Use Coffee Grounds Grounds On Perennials, Shrubs and Trees

Coffee grounds work well on perennials, bushes, and shrubs too. You can add a few tablespoons to the surrounding soil when  you plant your perennials to help boost the nutrient content and soil structure to encourage long-term growth. For larger shrubs or bushes, you can add a few coffee filters with grounds all at once around the hole you dig to plant them. The filters will break down by themselves. As this setup slowly breaks down, they’ll release nutrients to the roots.

Using Fresh Coffee Grounds Successfully

So far, we’ve talked about using spent coffee grounds, but what about fresh ones? Take note that this isn’t always recommended, but it should be okay in certain situations, including: 

• It’s possible to sprinkle fresh coffee grounds on any plants that love acid like hydrangeas, azaleas, lilies, and blueberries. A lot of vegetables also like slightly acidic soil, but tomatoes usually won’t respond well to this addition. Root crops like carrots or radishes typically respond well, especially when you mix the fresh grounds into the soil in the spring. 
• Using fresh coffee grounds is thought to be a natural weed suppressant too because it has a few allelopathic properties. This is why tomatoes don’t respond well to them. It could also suppress fungal pathogens. 
• Sprinkling fresh, dry grounds around your plants and on the top soil will help deter a few different pests. It won’t fully eliminate any of them, but it can help with keeping slugs, rabbits, and cats away. This is thought to be because of the caffeine content in the grounds. 
• Instead of using unbrewed, fresh coffee grounds that could negatively impact your plants, you might want to go with used decaffeinated coffee. 

Precautions About Using Coffee Grounds in the Garden 

There are a few precautions that come with using coffee grounds in your garden, and it wouldn’t be fair if we left them out because we want to show you both the benefits and downsides of using them. Tgey include but are not limited to: 

Dogs and Coffee Grounds

A word of warning; coffee grounds might not affect any pests in your garden as much, but they can be harmful if your pet chooses to ingest them in high enough amounts. It’s very difficult to say how much they’d have to have before it caused caffeine poisoning because the amount of caffeine left will vary. But, if you have a dog that loves to dig and eat things that they shouldn’t, you want to avoid sprinkling coffee grounds directly onto the soil. Instead you can bury them in a compost heap. 

High Acid Content 

We totally get that it feels good to do something to your coffee grounds besides tossing them into the garbage. People who sing the praises of coffee grounds aren’t wrong when they claim that these grounds are packed full of nutrients like nitrogen. Generally speaking, adding organic material to your soil is good for the garden because this material gives bacteria something to feed on, break it down, and inject even more nutrients into the soil. 

However, there are a few warnings you want to heed. Coffee grounds have a very high acid content, so you only want to use them on plants that like higher acid in the soil. If the soil already has a higher nitrogen content to it, the extra addition by adding coffee grounds could actually stunt the flower or fruit growth. So, it’s a good idea to test your soil before you add anything to it to see where your levels are. 

General Warning Information for Coffee Grounds

In order for you to understand why caffeine can be bad for your garden, you have to first understand why certain plants produce caffeine. You most likely already know that both chocolate and coffee have caffeine, even if it does come from completely different plants. The plants aren’t related, but they used convergent evolution to produce caffeine independently. This simply means that two species evolved the same trait by themselves, and it’s a sign that the trait could be useful. For caffeine, the main purpose is competition because it can kill off any plant in the surrounding area. 

While you may assume that you got every last drop of  it out of the grounds because you use a French press, you’d be incorrect. The Journal of Agricultural and Food Chemistry put out a study that found that every gram of used coffee grounds can have up to eight milligrams of caffeine left in them, depending on how long you let the ground steep. So, after you brew an espresso shot, the grounds will still have roughly half as much caffeine as you’d get in a cup of tea. 

This is why you want to apply the coffee grounds with extreme caution. In fact, Urban Forestry & Urban Green Gardening put out a study that said that applying coffee grounds directly to the soil could stunt the growth of the plants. Also, a second study found that spiking your compost with coffee grounds could actually kill beneficial earthworms. Finally, coffee grounds can contain antibacterial properties too. 

Frequently Asked Questions 

Coffee Grounds by Steve Snodgrass / CC BY 2.0 Since there is conflicting information involving adding coffee grounds to your garden, it’s common to have a few questions about this process. 

1. What plants love used coffee grounds?

The plants that like coffee grounds include blueberries, roses, carrots, azaleas, rhododendrons, radiehs, cabbage, hydrangeas, hollies, and liles. They all like to have a higher acid content in the soil, so they can do well with an additional dose. 

2. Can you add too many coffee grounds to the garden?

Yes, it’s possible to add too many coffee grounds to the garden. Most people recommend a tablespoon or two a few times a year. If you’re constantly throwing coffee grounds out in the garden, you stand a high chance of stunting your plants growth or killing them because of the high acid content. 

3. Can you add coffee grounds to potted plants?

Yes. You can add coffee grounds to your indoor and outdoor potted plants by sprinkling them on the surface of the soil. Once you water the plants, it’ll encourage the coffee grounds to release their nutrients. 

Bottom Line

Gardening and coffee grounds do go together naturally. No matter if you’re using them around the yard or doing coffee ground composting, you may find that coffee grounds can give your garden as much of a pick me up as a cup of coffee does for you.