Azalea Care & Coffee Grounds

Azaleas, flowering shrubs in the genus Rhododendron that are hardy in U.S. Department of Agriculture hardiness zones 6 to 8, have somewhat finicky preferences when it comes to soil composition and pH. A byproduct of your morning coffee, however, can also function as a soil amendment to help satisfy those preferences.

Azaleas prefer slightly acidic soil with a pH between 5.5 and 6. A study conducted by "Sunset" magazine found that coffee grounds have a pH of about 6.2, a level of acidity that's quite close to that favored by azaleas, so adding coffee grounds to soil with a high pH may help to bring the acidity of the soil closer to a level that is beneficial to the plant. Coffee grounds alone, however, may not be enough to sufficiently lower the pH of basic soil.

Coffee grounds contain substantial levels of several nutrients that azaleas need for growth, including potassium, phosphorus, copper and magnesium. These nutrients are held in the grounds in a form that's readily available to the plants, so the grounds may be a suitable replacement for other sources of these particular minerals in the short term. Coffee grounds also contain a relatively large amount of nitrogen, although most of the nitrogen is in a form that is not immediately available to plants. As the grounds decompose, however, the nitrogen is converted to a form that azaleas can use, making the grounds act like a slow-release fertilizer. Soils with extremely low levels of native nitrogen might need supplemental fertilizer.

The "Sunset" study found that coffee grounds contain about 442 pounds of organic matter per cubic yard, an amount that makes the grounds a significant source of beneficial organic material. In addition, the ratio of organic matter to nitrogen in the grounds is more than sufficient to meet the nitrogen needs of the microorganisms that decompose the organic matter, so as the organic material decomposes, the excess nitrogen becomes available to the plant.

Azaleas like well-drained soil, but they do best when they have moist soil at their root level, so the soil they're planted in should be able to capture and hold water. Coffee grounds have excellent moisture retention capability, and using grounds as a soil amendment may improve the water-holding properties of both sandy and clay soils. Coffee grounds also have a fine texture, so they can also be used to lighten and loosen heavy clay soils.

To improve the composition and nutrient content of the soil in which you intend to plant your azaleas, mix coffee grounds into the soil so that they make up 25 percent to 35 percent of the mixture by volume. Till the grounds into the soil to a depth of 6 to 8 inches prior to planting.

Another option is to use the grounds as an acid mulch, spreading a half-inch layer of the grounds over the surface of the soil; this method introduces acidity and conserves water without altering the soil composition.