If you're an avid coffee drinker, if you brew your coffee from bean to cup, then you're likely accumulating a lot of ground coffee. Day after day, week after week, you chuck it into the bin. But, as you brew your morning coffee and look through your window onto your garden, have you ever wondered: are coffee grounds good for plants? Should I be tipping them into the garden instead of the bin?
Search online for the answer, and you'll find a bunch of conflicting views.
Some praise coffee grounds as the saviour of gardening. Others warn against using coffee grounds: they'll kill your plants!
What's the truth?
Here at Presto, we've done the research.
Which Plants Do Not Like Coffee Grounds?
Aside from the delicious flavour, what's the main reason we drink coffee? Caffeine! That little buzz in the morning or afternoon pulse. But why do plants even contain caffeine? What's the purpose?
Well, caffeine is found in many plants, from tea to coffee and through to cocoa. When it first evolved, it gave these plants an edge over the competition by… killing the nearby plants. As the leaves fall, the caffeine 'poisons' the surrounding soil, suppressing plant growth by tying up the nitrogen.
I know what you're thinking. How much caffeine is left in used coffee grounds?
Surprisingly, quite a lot.
In a 2012 study at the University of Navarra, spent coffee grounds contained up to 8.09 mg of caffeine per gram. Add that up over the 5 grams required for an espresso, and you're looking at 41 mg of caffeine. That's equivalent to a cup of tea.
So, rather than spreading it over your plants, you'd be best to use coffee grounds as a weed killer. (Try boiling them once again for a potent weed-killing spray.)
Which Plants Like Used Coffee Grounds?
There are a few exceptions to the rule, however. Plants are as complex and diverse as animals. No two are alike, and they're all fussy growers – requiring wildly divergent conditions.
Plants such as Lily of the Valley (Convallaria majalis) and Azaleas (Rhododendron sp.) will thrive in acidic soils, helped by coffee grounds. Meanwhile, fruits and vegetables like wild strawberries and blueberries, sweet potatoes, parsley, and peppers all thrive in coffee ground fed soils.
If you're fancy using up your coffee grounds, mix them with compost or mulch – or dig them straight into the soil. You'll need around a half-inch of grounds per four inches of mulch or fertilisers. Oh, and let them cool before you use them.
Do Coffee Grounds Damage Plants?
As we've mentioned, coffee grounds inhibit the growth of plants, particularly seedlings. But the effects of caffeine are more extensive. If plants are matured, the outcome is likely to be less pronounced. However, for younger plants, caffeine suppresses root growth, preventing plants from taking up adequate water and nutrients. Fewer roots equal weaker plants.
Nor do the effects stop there.
Being mildly antibacterial, caffeine mucks around – no pun intended – with the good bacteria in the soil. At first glance, soils don't look like much. But microscopically, it's one of the most complex substances on the planet. Bacteria, fungi, and microscopic critters work together to provide fertility and health to plants. Upset this balance, and you'll cause havoc for the soil's functioning.
That's also true for earthworms: nature's vacuum cleaner. Worms break down compost and mulch, providing nutrients for the plants. Coffee grounds kill the worms.
Can You Put Too Much Coffee Grounds in Your Garden?
Thus, without caffeine, the beneficial effects of coffee grounds are remarkable. As their toxicity declines, their benefits increase. First, as the grounds mineralise, the macronutrients are released, creating a boon for microorganisms. They digest these nutrients, making them available to plants – the cycle of life. Also, as toxicity decreases, earthworms rejuvenate with no negative effects on their health.
So, coffee grounds are miraculous later but deadly at first.
How to rectify the problem?
Try boiling them again. Then, drain off the liquid to remove more of the caffeine. It'll also break down their structure, making them easier to digest. You'll likely lose some minerals. But that's a small price to pay to decrease the toxic effect.
Just don't go overboard. If you dump heaps of coffee grounds, you'll overpower the soil. Remember, you're using a ratio of 1 to 4. For every one spoonful of coffee grounds, mix in four spoonfuls of soil or mulch.
Forests on Caffeine: A Story of Recovery
There is one place you can tip the world's coffee grounds en masse: tropical forests.
A recent study published in Ecological Solutions and Evidence revealed that truckloads of coffee pulp from local factories had a remarkable effect on Costa Rica's degraded land. In two short years, canopy cover shot up to 80 per cent, compared with 20 per cent in untreated sites. Overall, the canopy was four times taller.
The coffee pulp eliminated invasive grass species, enabling the forest to recover. Nutrients such as carbon, nitrogen and phosphorus were also elevated: helping to supercharge the surrounding trees.
As the saying goes, one man's waste is another man's treasure.
If the results can be replicated, it offers a miraculous boost to forest recovery. Now that's a good news story!
The Bottom Line
In short: caffeine is bad for plants; coffee grounds are fantastic. If you're able to squeeze out all the caffeine by boiling the coffee grounds a second time, you'll suffer fewer ill effects. Then, mix with four parts soil to one-part grounds and spread around your plants.So, if you've got left-over grounds from our House Espresso beans, give it a try. Just use sparingly.