Are Coffee Grounds Food for Plants?
Updated: Mar 25, 2021
Generally speaking coffee grounds are not good for plants in the raw post coffee making format. The grounds contain residual caffeine which actually lowers plant growth. Also coffee grounds contain high levels of acidity, and nitrogen. Nitrogen is normally good for plants but in too high concentrations can damage plant growth. Like many things though - the answer depends on the situation.
The Problem with Coffee Grounds for Plants
It feels good to do something with your morning coffee waste aside from throwing it in the garbage. Many gardeners who write about it are correct when they say it's full of soil-friendly nutrients like nitrogen, which is essential for plant growth. Generally, adding organic material to the soil is good for your garden, since bacteria will feed on it and break it down into more nutrients the plants can use.
But even coffee-ground gardening advocates include a few words of warning. Coffee grounds are highly acidic, they note, so they should be reserved for acid-loving plants like azaleas and blueberries. And if your soil is already high in nitrogen, the extra boost from coffee grounds could stunt the growth of fruits and flowers. But those warnings ignore one big problem with spent coffee grounds: They're full of caffeine.
While it’s true that Coffee grounds contain several key minerals for plant growth — nitrogen, calcium, potassium, iron, phosphorus, magnesium and chromium and also might help absorb some heavy metals, like in most situations the answer depends on the circumstances.
One Horticulturist ran an experiment in their personal Garden following the advice of popular gardening blogs. And the results? Well, here’s the deal. The crop yield and growth of pretty much everything in the coffee bed became noticeably worse within about two weeks of application. Plant growth slowed, some developed leaf yellowing, others defoliated and died. Seedling germination in some cases was almost completely inhibited. While some species looked OK, none of the plants in the coffee group proved better than my basic control. But it’s just adding organic matter. What went wrong?
So I had a look at the scientific literature, myself. Coffee grounds are of course a rich source of caffeine – in fact they can be richer than coffee itself, depending on brewing technique. One of the key functions of caffeine in the plants that produce it is allelopathy – the ability to reduce competition from surrounding species by suppressing their growth. Caffeine is packed into coffee seeds for the very function of suppressing the germination of other seeds.
There is a stack of studies to suggest it also stalls root growth in young plants, preventing their uptake of water and nutrients. Yet others have shown it has antibacterial effects (so much for boosting soil bacteria).
One Solution then is to mix coffee grounds with other organic matter such as compost or leaf mold before using it as a mulch. You can also add your coffee filter (if that's how you brew) to the compost at the same time.
In one study compost made with coffee grounds mixed with Acacia dealbata L. shoots and wheat straw. At different time intervals during composting, carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) emissions were measured and selected physicochemical characteristics of the composts were evaluated. During the composting process, all treatments showed a substantial decrease in total phenolics and total tannins, and an important increase in gallic acid. Emissions of greenhouse gases were very low and no significant difference between the treatments was registered. The results indicated that
Spent coffee grounds may be successfully composted in all proportions. However 40% concentration, was the treatment which combined better conditions of composting, lower Greenhouse Gas emissions and better quality of end product.
Fertilizing Successfully with Coffee Grounds
Most people think that brewed coffee grounds are acidic—which is true—but the amount can vary a lot. Actually, fresh grounds have a higher acidity level. This means you shouldn’t rely on spent grounds to greatly alter your soil’s ph. There are acid-loving plants, however, that would appreciate a boost from fresh grounds, plants like hydrangeas, azaleas, rhododendrons, blueberries, carrots, and radishes. Just sprinkle some grounds onto your soil and either rake or lightly scratch it in. Tip: tomatoes are not coffee fans.
The number one mistake people make when using coffee grounds with plants is using too much. The added nitrogen and potassium in the coffee grounds is good in moderation only, she says. “You really want to dilute it and use it sparingly.
So just like for Humans, coffee seems to be okay in moderation. And additionally the same way some people love coffee, and others don’t - Some plants have more affinity
for coffee than others.