QUESTION: Should I put coffee grounds in my garden? I’ve heard conflicting advice. — Vivica B.
ANSWER: Coffee grounds should only be used in your garden if you’re growing acid-loving plants, and even then it should be used carefully. You can also mix coffee into compost if you carefully limit how much coffee you use. These are the only two ways you should use coffee in the garden, however.
Resist the urge to put coffee grounds in containers or mix it into soil for your plants—except for acid-loving plants like blueberries, rhododendrons, or azaleas. Even then, follow our directions carefully.
We know this runs contrary to the advice millions of gardeners have heard about adding coffee grounds directly to soil, mixing coffee grounds into soil blends, or side dressing plants with coffee. As it turns out, there are a few reasons that coffee isn’t good for most of your plants.
Proponents of adding coffee to garden soil often say it contains the nitrogen plants need. It’s true that a bit of nitrogen is good for plants. However, it’s easy to give your plants way too much nitrogen if you’re amending soil with coffee. And while up to a certain amount, nitrogen boosts plant size, too much nitrogen actually inhibits flowers and fruit production.
Coffee’s high nitrogen content isn’t always beneficial. The acidity of coffee along with its nitrogen makes it possible for there to be a mold bloom wherever coffee is used in the garden. This mold bloom comes along with a threat to plant health and the health of the soil. That’s why we recommend you follow our instructions for adding coffee to compost if you want to use it in your garden. Just keep reading﹘you’ll find those instructions at the end of this article.
Another theory is that the caffeine in coffee helps plants grow larger or promotes flowering. Intuitively, this makes sense. If some coffee gives us energy and increases our productivity, why wouldn’t it be the same for flowers and plants?
It’s this line of thinking that causes some gardeners to pour liquid coffee onto plants or add used coffee grounds to their soil. Caffeine is a performance enhancer for people, so why shouldn’t it also enhance the performance of plants?
Unfortunately, when scientists studied plants that produce caffeine on their own, they found that the performance of nearby competing plants was actually inhibited. While the study focused on caffeine-producing plants and not on plants treated with caffeine, we can safely assume the results are similar in both cases.
And used coffee grounds do contain enough caffeine to make a difference. After squeezing as much liquid as you can from used coffee grounds, the caffeine is equivalent to that in a cup of tea.
You may have heard that coffee grounds are antibacterial and believe that this is good for soil. After all, humans use antibacterial products to help us fight disease and keep healthy. Some people believe that adding coffee can help plants stay healthy since it is antibacterial. The truth is that bacteria are hard at work in the soil breaking organic material down into components that plants can process.
Using anything antibacterial, coffee included, will kill these beneficial bacteria along with any bacteria that pose a threat to plant health. One study showed that when coffee grounds were used in compost, earthworms were killed, too. Coffee wreaks havoc on the natural process of decomposition that takes place in your soil because it kills the microorganisms working to break things down into nutrients for your plants.
For this reason, we don’t recommend adding coffee directly to your soil, either on top of the soil where plants are growing or as a side dressing. And when coffee is used in compost, we start from scratch instead of adding coffee straight to the compost heap. The amount of coffee we recommend using is also limited, as too much coffee can be detrimental to plant health and soil health.
How to Use Coffee in Compost for Acid-Loving Plants
The only plants we recommend using coffee with in the garden are acid lovers like blueberries, rhododendrons, and azaleas. Even with these plants, it’s important to follow a recipe that limits the amount of coffee used in the garden. Coffee grounds have been tested as having a pH level higher even than acid-loving plants can tolerate, so it’s important to keep their use to a minimum.
You’ll notice we only give instructions for adding coffee to compost and then giving the compost to your plants. We don’t recommend using coffee directly on your soil, either in its liquid or ground form. We also don’t recommend side dressing or mulching with coffee grounds. Making a compost mixture with the coffee and using that mixture to amend your soil ensures that the coffee is a beneficial amount and that plants don’t get too much nitrogen or acid.
For every 10 pounds of coffee grounds you add to your existing compost, add one cup of agricultural lime or hardwood ashes. Of course, if you’re using less coffee, you’ll need to adjust the amount of lime or ashes to match.
If you don’t have a compost pile already, there’s a simple way to make a coffee compost you can use in the garden. Simply mix shredded leaves with 10 to 25 percent coffee or ashes. Then add agricultural lime or hardwood ashes at the ration of one cup per 10 pounds of coffee grounds.
This mixture can be used on acid-loving plants as you would use any finished compost. Spread it over the surface of soil as a mulch, mix it in as a side dressing, or use it as a soil amendment. But keep its use to the acid lovers in your garden.
Coffee is just too much for most plants, and it’s easy to use too much coffee unless you follow a recipe like the one we’ve provided. And only acid-loving plants will really benefit from coffee. It contains too much acid for other plants.
Using coffee comes along with the risk of mold bloom and can kill beneficial microorganisms in the soil as well as earthworms. On plants that don’t love acid, these risks just aren’t outweighed by any benefit. You may decide not to use coffee on acid loving plants, either, because of these risks.
There are plenty of other ways to increase soil acidity for these plants that don’t come along with the risk of mold bloom and killing beneficial bacteria and earthworms. There’s also the risk of caffeine or too much nitrogen inhibiting the production of foliage, fruiting, and flowering in your plants.
We just can’t recommend using coffee willy nilly on the top of soil, as a liquid, or as a mulch or side dressing unless it’s mixed into a compost recipe like the one we’ve given. We also can’t recommend using coffee unless it’s on acid-loving plants. Ultimately, it’s your decision whether to use coffee in your garden at all. After reading this article, you’re going into the decision armed with knowledge about the risks and benefits that come along with using coffee in the garden, so you can make a wise and informed decision.