A fun way to use coffee grounds in the flower garden is to experiment with the color of hydrangea flowers. It is common knowledge that sweet (alkaline) soil produces pink hydrangea flowers, while a bit of extra acidity gives you blue flowers.
If your plants happen to have pink flowers, feed them coffee grounds and check whether they show inclination for a color change.
As we mentioned earlier, the acidity of coffee grounds is variable, so you might end up with flowers in a range of colors that fall between pink and blue on the same plant.
Remember that it is not the pH of the soil alone that controls flower color, but the ability of the plant to absorb aluminum salts from the soil which is enhanced by soil acidity. So if your soil is deficient in aluminum, you may want to add some aluminum sulfate for blue flowers.
When you use coffee grounds in your garden, whether it is from your own use or from the coffee shops, you are turning this organic waste into plant food that your garden will love!