There are a great many articles out there proclaiming that coffee grounds are 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. But will they really help plants in your polytunnel to grow?
Are Coffee Grounds Good For Plant Growth?
Unfortunately, science does not back up this garden myth. Coffee grounds do contain core nutrients nitrogen, phosphorus and potassium. They also contain a range of micronutrients needed for plant growth. Like other organic matter, however, coffee grounds will break down slowly over time. This means that they won’t give plants a quick boost. Rather, they will break down to provide a slow-release fertiliser to improve the soil over time.
The nitrogen content in coffee grounds is 1-2%. But the phosphorus and potassium amounts are much more variable, and generally rather low. This means that adding coffee ground around your plants can actually encourage leafy growth to the expense of flower and fruit formation – not great for those prize tomatoes – for example.
Unfortunately, coffee ground also contain something that is not good for plant growth: caffeine. 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.
Do Coffee Grounds Make Good Mulch?
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 in a polytunnel garden?
As a slow release fertiliser, they could potentially be added to a mulch containing other organic materials with beneficial effect. But it is not a good idea to use coffee grounds as a mulch on their own. They compact too easily, reducing air flow to the soil and preventing water from reaching plant roots where it is needed.
If you do decide to mulch certain plants in your polytunnel with coffee grounds, to boost the fertility of the soil for nitrogen-hungry plants, make sure you mix the coffee grounds well with other organic materials. Better yet, compost them first and then use the compost as mulch material.
Adding Coffee Grounds to the Compost Heap
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 carbon to nitrogen ratio of 20-to-1. Informal trials have shown that sustained temperatures of 60-70 degrees Celsius for up to two weeks were recorded when coffee grounds were 25% of the material in the compost pile by volume. So coffee grounds could be used to great effect in hot composting systems.
In a typical cold compost heap or bin, however, coffee grounds should only ever be added in moderation. Yes, you can add some coffee grounds to your compost heap. But be sure to keep the compost balanced and add only a little at a time. Layer coffee grounds and other kitchen waste with carbon rich compostables.
Really, this bears repeating – do not add too many coffee grounds. They 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.
Other Ways To Use Coffee Grounds
Coffee grounds are not quite as useful in a garden as some people would have you believe. But as an organic material, they can have limited use in a mixed mulch, and can be added in small amounts to your compost heap. But what do you do with the rest of your coffee grounds if you have too much too much to use up in the ways mentioned above? Here are a few suggestions:
Use coffee grounds to scrub or scour a barbecue or grill, tools, or pots
Spread some coffee grounds on a path or patio to stop you slipping.
Use coffee grounds to stain wood in your polytunnel or elsewhere in your garden and give it an attractive weathered look.
Get crafty with coffee grounds – there are a wide range of ways to use them in crafts.
You can also use them to make a number of natural beauty and cleaning products.
What do you do with your spent coffee grounds? Let us know in the comments below.
Elizabeth Waddington is a writer and green living consultant living in Scotland. Permaculture and sustainability are at the heart of everything she does, from designing gardens and farms around the world, to inspiring and facilitating positive change for small companies and individuals.
She also works on her own property, where she grows fruit and vegetables, keeps chickens and is working on the eco-renovation of an old stone barn.