Coffee grounds are a source of nitrogen, and the brewing process removes much of the acidity from the used grounds. Japanese maple trees require normal amounts of nitrogen, which coffee grounds may supplement, particularly if the soil is nitrogen depleted (for example, wood chips remove nitrogen from the soil).
Effects of Over-Fertilization
Nitrogen-rich fertilisers may cause excessive growth of leaves with the effect of hindering flower bud growth. If nitrogen is added at the end of the growing season, it will cause, says BackyardGardener, "vegetative growth that will not have a chance to harden off before the onset of cold weather."
Bottom Line
Because too much nitrogen from coffee grounds may be detrimental to a Japanese maple, test the soil to determine if it is depleted. Coffee grounds may help the soil and the tree growth, but they should be used moderately to avoid overfertilization.