Coffee grounds aren’t good for plants. Experiments in Melbourne, Australia, documented plant growth of broccoli, leek, radish, viola and helianthus that were grown in sandy, sandy clay loam, and loam soils amended with spent coffee grounds at 2.5%, 5%, 10% and 25% rates. Every one of the plants grew poorly when soils were amended with spent coffee grounds, although the grounds did increase moisture-holding capacity. The authors believe that the grounds may cause a phytotoxic effect. It’s possible that spent coffee grounds can be composted and blended with other soil additives and organic materials, but for now, don’t do it unless you seek to thwart plant growth. It should be noted that leek performed poorly in all tests, even controls.
Applying spent coffee grounds directly to urban agriculture soils greatly reduces plant growth by Sarah Hardgrove and Stephen Livesley in Urban Forestry & Urban Greening.
December 15, 2016 NewTerrain.