With spring upon us in many places, we’re all getting our gardens going and making sure our soil is in the best possible shape. Soil amendments in the home garden are always a topic of conversation in gardening circles, and lately online there’s been a renewal of interest in using coffee grounds in the garden. While there are many websites and blogs listing the advantages and benefits of using coffee grounds in your garden, what’s the truth? Do coffee grounds actually help? What are the real pros and cons of using coffee grounds in your garden? Let’s dig in!
Coffee grounds (better referred to as “spent coffee grounds”) are what’s left after you make coffee, as opposed to ground coffee which is the ground-up roasted coffee beans you use to brew coffee with. We wanted to cover this up front, as it can get confusing. Using ground coffee in your garden is unlikely—why not make coffee with it?—and if you must do it please compost it in a slow manner as it takes much longer to break down than coffee grounds.
With the definitions and such out of the way: do coffee grounds help your garden? While many sources make great big claims about the wonders of coffee grounds for gardening, the reality is a lot more complicated. While there are plenty of uses for coffee grounds, there are some potential drawbacks as well. This all has to do with how coffee grounds work with your soil, and in particular how caffeine works with plant life. Many plants—including coffee and tea plants—evolved caffeine in part as a way to deter competition from other species. This means that caffeine in the soil deters plant growth for many species. So while coffee grounds contain many of the nutrients that plants need to thrive, adding too much directly to your soil can be detrimental to plant growth and root development particularly in smaller plants and seedlings. This is why we recommended that you use fresh coffee grounds or coffee beans as part of your compost heap, and then to let them compost quite slowly so that the caffeine can break down over time. It’s also why we’ll recommend that you be careful in adding spent coffee grounds directly to your soil.
In addition to containing caffeine, coffee grounds are also acidic, and adding too much to your soil can change its pH and thus have an impact on the health of your plants. This is neither good nor bad; depending on the conditions of your soil and what you’re growing you may want to acidify the dirt a bit. However, it is a property that spent coffee grounds bring to the table and it’s something that savvy gardener should be aware of when using coffee grounds around their plants.
Finally, there’s the question of worm health. Worms are a vital part of most gardens—vermiculture or the raising of worms is an entire industry of its own—and worms do a lot to promote soil health. And worms do love to eat spent coffee grounds, but as with so many things in this life too much of a good thing can be harmful. Most sources we found recommend that worms consume no more than 50% coffee grounds as part of their overall diet. This is something to take into account if you raise worms at home or if you have them in your compost heap or garden.
So, what’s the best way to use spent coffee grounds in your garden? While a bit of coffee grounds as a top dressing along with other compost or some spent coffee grounds as a soil amendment, the best place for your coffee grounds is in your compost heap. Unlike ground coffee (see above) spent coffee grounds break down fairly quickly and release useful nutrients and nitrogen into your compost. Their caffeine breaks down as they compost, preventing it from becoming a problem for your plants. While acidification remains an issue, you can deal with that by adding plenty of “green” matter to your heap and turning it regularly. Testing your compost’s pH is a must as you also test your soil.
We hope this blog helps de-mystify the use of spent coffee grounds in your garden. If you use coffee grounds in your garden space or compost, please let us know what your experiences have been. We love to hear from you!