Gardening is a rewarding endeavor, especially for newcomers excited to grow thriving plants. However, the plethora of "quick-fix" solutions can be misleading and unfortunately spread like wildfire through social media groups and blog posts, casting shade on the real info out there. One popular myth that spreads like wildfire through social media involves the use of coffee grounds as a miracle remedy for various garden challenges. This article is about addressing these misconceptions and uncovering the science behind them.
Myth 1: Coffee Grounds Acidify Soil for Acid-Loving Plants
It's commonly believed that adding used coffee grounds directly to the soil will lower its pH, benefiting acid-loving plants like blueberries and azaleas. However, according to Linda Brewer, Senior Faculty Research Assistant II, Department of Horticulture at OSU, "contrary to popular belief, it’s a myth that coffee grounds are acidic and will lower the pH of the soil. After brewing, the grounds are close to pH neutral, between 6.5 and 6.8. Research shows that whatever change coffee grounds bring to the soil is short-lived Therefore, they are not effective in altering soil pH for acid-loving plants."
People may try to claim their their rhododendrons, blueberries and other acid loving plants may thrive due to the coffee grounds they use, but the research that has been conducted suggests otherwise and that the spent coffee grounds is not the standalone reason for their acid loving plants performing well. My rhododendrons have been performing well for well over a decade and have never once been fed with anything. I could have been using coffee grounds around the plants for the last ten years, which I have not been, and then I may have been under the false impression that the reason they did so well was because of the coffee grounds, which would be falsely attributing coffee grounds to their success. Be wary of people claiming that their rhododendrons have been doing well because they have been feeding them for years or decades with spent coffee grounds. Without a control group, they have no way to prove that.
Myth 2: Coffee Grounds as a Rich Fertilizer
While coffee grounds contain nitrogen and other nutrients, their nutrient levels are relatively low, making them insufficient to serve as a standalone fertilizer. According to Linda Brewer, incorporating coffee grounds directly into the soil in large quantities can lead to nitrogen immobilization, where microbes consume available nitrogen to decompose the grounds, potentially depriving plants of this essential nutrient. Combining this with the fact that spent coffee grounds usually only have about 1% of available nitrogen, adding too much can do more hard than good. According to Mendes in Environmental Science and Pollution Research (2024), "Unwashed spent coffee grounds may exhibit inhibitory effects on microbial activity and root development... High salinity levels observed in certain spent coffee grounds formulations can create localized soil conditions harmful to plant growth, particularly during germination and early seedling development... spent coffee grounds, as raw plant waste, is characterized by a strong phytotoxic impact, associated inter alia with caffeine, tannins, and polyphenols content." As a result, a more effective approach is to compost coffee grounds with other organic materials. This allows the grounds to decompose, reducing phytotoxic effects and transforming them into a nutrient-rich amendment that improves soil structure and fertility.
That is not to say that it is bad to add spent coffee grounds directly to soil as it can increase earth worm activity, albeit it is not their favorite food source. The point is that more is not better and there are more effective methods to best utilize coffee grounds. With that in mind, if one were to use it in their own homemade compost, it should not be the bulk of the compost either. According to Dr. Chalker-Scott from the University Of Arizona in "Using Coffee Grounds in the Garden", one should "limit coffee ground content to no more than 20% of the total compost volume - more than 30% has often been detrimental. Additions of diverse materials to compost should ensure a diversity of microorganisms." Coffee grounds can be good, but the better long term approach is to learn how to make compost and use coffee grounds in your compost. We have seen people make comments about going to their local coffee shop on a weekly basis and collecting tens or hundreds of pounds of spent coffee grounds to mix into their soil, and according to Brewer, “You really need to take the recommended dosages to heart. I’ve visited a site where a raised bed was ruined by too much coffee grounds. Like most kitchen waste, it is a fine amendment for the garden, but like anything else, coffee grounds can be overdone.”
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