According to a handy fact sheet from Washington State University, Coffee grounds will buzz your garden. Coffee grounds build humus, boost nitrogen, phosphorus and zinc, bind pesticides and toxins, prevent bacterial and fungal infections and feed earthworms. Authored by Dr. Linda Chalker-Scott, Extension Urban Horticulturist and Associate Professor, this peer-reviewed pamphlet also provides a set of suggestions for using coffee grounds in the garden:
- Coffee grounds should be composted before used as a soil amendment but can be used fresh as a mulch.
- Fresh grounds are phytotoxic, so keep them away from direct contact with roots.
- Coffee grounds will not necessarily make your soil more acidic.
- Don’t use coffee grounds where you are starting seeds.
- Despite rumors, coffee grounds do not repel pests.
- Let coffee grounds cool before adding to compost bin so you don’t kill beneficial microbes. And don’t let coffee grounds amount to more than 20% of your compost pile.
- Don’t add coffee grounds to vermicomposting bins.
If you’re using coffee grounds as a mulch Chalker-Scott has two suggestions:
- Apply a thin layer (no more than half an inch) of coffee grounds. Cover with a thicker layer (four inches) of coarse organic mulch like wood chips (Chalker-Scott 2015). This will protect the coffee grounds from compaction.
- Don’t apply thick layers of coffee grounds as a standalone mulch. Because they are finely textured and easily compacted, coffee grounds can interfere with moisture and air movement in soils.