There is a lot of misinformation out there about using coffee grounds in the garden. How can we separate the fact from the fiction? Is it a good idea to use coffee grounds in our outside spaces? If so, how and where should we use them? This article should help us get to the bottom or the issue (or should that be the cup). We’ll take a look at some of the common uses for coffee grounds that are sometimes recommended, and talk about whether or not we should consider them.
Can Coffee Grounds Be Used To Boost Plant Growth?
Coffee grounds are a common waste product, so what should we do with them?
There is a plethora of articles out there acclaiming coffee grounds as a miracle substance for plant growth. Many advocate using spent 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. 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 coffee grounds will not boost plant growth short term, however, but rather add long-term to the health of your soil.
But while nitrogen 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, coffee grounds also contain caffeine. Caffeine is the mean reason why using coffee to fertilise and boost your plants is not a good idea.
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 grown. As one study states: applying spent coffee grounds directly to urban agriculture soils greatly reduces plant growth.
A study also showed that 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 coffee grounds around some of your prized produce and plants!
So, 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 coffee grounds can suppress plant growth. But this effect is not universal. Not all plants will be affected negatively. So should we consider using coffee grounds as a mulch around certain plants? This is an idea often bandied about on gardening sites and garden forums.
Bad news again – 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 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 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 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. 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?
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, 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 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 go around them, move them out of the way, or ignore them. So they were not effective – certainly with the type of ant he had in his garden.
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 coffee grounds. You can also use 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 on 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.