As a student of gardening for quite a while now, I read a lot, attend classes, and regularly get head-to-toe-dirty while working in my yard. And, even though I often have questions, I’ve developed a bit of confidence in my ability to maintain healthy garden beds and keep our woodland plants thriving. Then, I read an article titled, ”Whatever you do, don’t put coffee grounds in your garden.” (update: the article has been removed from the internet).
Are Coffee Grounds Really That Scary?
Wow, that’s a pretty strong stand on coffee grounds, maybe I should change my ways! Almost every day, I have several scoops of brewed grounds from our French coffee press that need to go somewhere. Occasionally, I’ll spread them around the blueberry bushes, hydrangeas, or azaleas. But, more often, I’ll toss them onto the compost pile. But, I wanted to know what is the best and safest way to use coffee grounds in the garden. According to the article:
- Coffee grounds are highly acidic, so if your soil is high in nitrogen then coffee grounds could retard the growth of plants
- There can be up to 8 milligrams of caffeine per gram of brewed coffee grounds, and a high amount of caffeine in the soil can kill plants
- Even composted coffee grounds applied to soil can reduce plant growth
- Composted coffee grounds kill earthworms
- Coffee grounds have antibacterial properties, therefore, will kill good bacteria- Keep coffee grounds away from your garden
That’s a lot of negative stuff my coffee grounds could be doing in my garden! At this point, I’m confused and curious. The last thing I want to do is put more stuff in the trash bin that will end up in the local landfill. So, I read other resources, including from our local University of Georgia County Extension office as well as other experts in the field. Here’s what I learned.
What Are The Experts Saying?
- According to Josh Fuder the University of Georgia Extension, “Coffee grounds (average pH of 6.1): This material is less acidic than 80 percent of all soil tests I see. This is a decent soil amendment at 21 percent carbon, almost 2 percent nitrogen by weight in slow release form and low in all other nutrients.
- Use in the compost pile or incorporate into soil before planting. Do not use as mulch because the fine particles can bind together and inhibit air and water movement to the soil.”
Also, Linda Chalker-Scott, Extension Urban Horticulturist and Associate Professor, Puyallup Research and Extension Center, Washington State University, concluded that:
“You can safely use coffee grounds up to 20% of total compost volume. More than that may be detrimental. Twenty-percent total volume is sufficient for effective disease suppression and for building soil, and should not acidify your soil (pH levels vary according to type of soil, microorganisms present, rainfall, nature of compost and many other factors).”
“Using coffee grounds alone as mulch is fine, but don’t lay them on too heavily, as they compact easily and may actually serve to block air and water exchange. Its best to use coffee grounds in combination with an organic mulch, or layered underneath a mulch like wood chips to avoid compaction.”
Using Coffee Grounds For Maximum Benefit
Bottom line: I will continue to lightly spread some coffee grounds around my acid-loving plants as I have been doing. However, now I know the best and safest way to use coffee grounds is to compost them. Therefore, in the future, I am going to go the safe route and put the bulk of my coffee grounds (considered green compost material) into the compost pile balanced with brown compost material to break down first. It is working well so far as our compost pile is full of earthworms. I also revisited our Soil Test Report and confirmed that our soil pH is in an acceptable range to utilize the composted coffee grounds.
I’ve listed more resources than I usually do. But, there’s a lot of information available to help us understand how to successfully utilize coffee grounds and avoid sending compostable material to the landfill.
Baking up a better soil makes for a successful garden by Josh Fuder
Will coffee grounds acidify my garden soil? by Todd Heft
Which Items Are “Greens” and Which Are “Browns”? by Colleen Vanderlinden
The Four Things You Need to Know About Soil pH by Lee Reich