Coffee lovers, good news! According to research from Washington State University, those grounds you’re brewing every morning can have a second life in your garden. Before you dump your next batch, here’s what science confirms about using coffee grounds in your garden:
The Benefits Of Coffee Grounds Are Real!
- Coffee grounds moderate soil temperature and increase water retention like other good mulches
- They bind pesticide residues and heavy metals, preventing them from spreading.
- They increase the availability of essential nutrients like nitrogen, phosphorus, iron, and zinc.
- Earthworms love them, which improves soil structure and aeration
- The microorganisms on decomposing grounds may help suppress some plant diseases
Forget These Myths
- Coffee grounds are NOT reliably acidic – their pH changes as they decompose
- Some growers suggest, but NO scientific evidence shows, that coffee grounds repel cats, kill slugs, or prevent weeds
- They WON’T always act as a nitrogen source until properly composted
- They’re NOT suitable for areas where you’re growing from seed (they can inhibit germination)
How to Use Them Correctly:
In Compost:
- Use no more than 20% coffee grounds by volume in your compost pile
- Let the grounds cool before adding to compost (heat can kill beneficial microbes)
- Don’t add them to vermicomposting bins (can harm worms in confined spaces)
As Mulch:
- Apply a thin layer (no more than 1/2 inch) of coffee grounds
- Cover with a thicker layer (4 inches) of coarse organic mulch like wood chips
- Never use thick layers of coffee grounds alone – they compact easily and can block moisture and air
The science shows coffee grounds can be beneficial garden additions when used correctly. Just like brewing the perfect cup requires proper technique, so does using those grounds in your garden!
So next time you finish brewing, remember your plants might enjoy those grounds almost as much as you enjoyed the coffee!
Based on research by Dr. Linda Chalker-Scott, Washington State University Extension
