Using spent coffee grounds around evergreen plants—which include conifers like pine and spruce, and broadleaf evergreens like rhododendrons and holly—is a frequent topic among home gardeners. This practice is often promoted for its alleged acidifying properties and nutrient content. However, the true benefits of this common kitchen waste lie less in its chemical composition and more in its physical contribution to the overall health of the soil.
Understanding the pH Factor
The most widespread belief is that coffee grounds significantly lower soil pH, making them ideal for evergreens that thrive in acidic conditions. This notion stems from the fact that fresh, unbrewed coffee grounds are indeed acidic, typically registering a pH between 5.0 and 5.5. This acidity is primarily due to organic acids.
Once brewed, most of these acids are extracted into the beverage. The resulting spent coffee grounds, which gardeners typically use, are surprisingly close to neutral, generally falling within a pH range of 6.5 to 6.8. Since 7.0 is neutral, spent grounds have a negligible effect on the long-term acidity of the soil.
The massive volume required to measurably alter the pH around a mature tree would be impractical for the average gardener. Any slight, temporary change in acidity is quickly buffered by the existing soil structure. Therefore, relying on coffee grounds to achieve the low pH required by acid-loving evergreens is ineffective; specific soil amendments like elemental sulfur are a more reliable choice.
What Coffee Grounds Offer Evergreens
The real advantage of applying spent coffee grounds is their value as an organic soil amendment, independent of pH. They are rich in organic matter, which enhances the physical structure of the soil, especially in areas with heavy clay or sand. Incorporating this material helps to improve aeration and drainage, while simultaneously increasing the soil’s capacity to retain moisture.
Spent grounds contain a small but steady supply of plant nutrients, acting as a slow-release fertilizer as they decompose. They typically contain between 1% and 2% nitrogen by volume, along with trace amounts of phosphorus, potassium, and micronutrients like magnesium and boron.
This organic material serves as a food source for beneficial soil organisms, including earthworms and various soil microbes. As these microorganisms break down the coffee grounds, they release the nutrients into a form that the evergreen’s roots can absorb, contributing to overall plant health and vigor.
Proper Use and Application Techniques
The safest and most effective way to utilize spent coffee grounds is by adding them to an active compost pile. Since they are a “green” material, they must be balanced with “brown” materials like dried leaves or wood chips. The grounds should not exceed 20% of the total compost volume to ensure proper decomposition.
When applying grounds directly to the soil surface, use them sparingly to avoid common pitfalls. Coffee grounds are fine particles that compact easily when applied in a thick layer, forming a crust that repels water and prevents air from reaching the roots. This compaction can also encourage the growth of molds or fungi.
To avoid these problems, grounds should be applied using one of the following methods:
- Spread them in a very thin layer, ideally no more than a half-inch thick.
- Mix them into or cover them with a coarser mulch material like shredded bark or pine needles.
- Create a simple liquid feed by steeping two cups of used coffee grounds in five gallons of water overnight. This coffee “tea” offers a gentle, diluted nutrient boost directly to the root zone.