Watering plants with coffee can be beneficial, especially if the plant is an acid-loving variety. Coffee grounds and strong coffee contain large amounts of nitrogen, which raises the acidity level of the soil.
Blueberries, azaleas, impatiens and orchids all love acidic soils. Most other plants grow well in soil that has been treated with nitrogen, but moderation is the key to success. If coffee is the fertilizer of choice, watch the plant closely for signs of overdose. Leaves that begin to turn yellow at the tips are indicators of too much acidity in the soil. Simply add more water to the plant's "diet," and the problem should correct itself.