Are coffee grounds effective for hydrangeas?
Written by
Michael Sullivan
Reviewed by
Prof. Martin Thorne, Ph.D.Generally, the use of coffee grounds provides some benefits to hydrangeas, primarily improving soil texture without significantly changing bloom color. Although gardeners widely use the impact of coffee grounds to decrease soil pH, the actual decrease in pH with coffee grounds is minimal, ranging from 0.1 to 0.2 pH units in 0.5 to 1.0 pH units, and should not be relied on for hydrangeas to exhibit noticeable color changes. To increase the acidity of the soil and create color changes, the use of aluminum sulfate is generally recommended. I personally add coffee grounds to compost rather than using them directly.
Soil Benefits
- Improve soil structure increasing drainage and aeration
- Add organic matter supporting beneficial microorganisms
- Provide trace nitrogen phosphorus and potassium nutrients
pH Limitations
- Cause negligible pH reduction insufficient for color change
- Require microbial decomposition to release minimal acidity
- Cannot replace aluminum sulfate for blue blooms
Application Methods
- Mix into compost piles at 20-30% volume ratio
- Spread thin layers under mulch avoiding direct stem contact
- Limit to 1 cup per plant monthly during growing season
Precautions
- Avoid fresh grounds which may temporarily inhibit growth
- Prevent clumping by mixing thoroughly with soil
- Discontinue use if mold develops on soil surface
The benefits of composting can be maximized and risks minimized by keeping them together. Combine grounds with dry browns - dried leaves - in a 1:3 ratio. Degrade for 2-3 months before applying to hydrangeas. This allows the mix to stabilize its pH and avoid nitrogen tie-up in soil. My own compost blend includes vegetable scraps/peels and coffee grounds.
Alternative applications are valuable uses of coffee grounds that do not provide expectations in terms of pH. Sprinkle ground coffee around the plants to deter slugs. Combine ground eggshells with crushed eggshells to create a mulch that includes calcium. Mix into potting mixes for container plants. These alternative uses will utilize coffee grounds and avoid unrealistic expectations and visual color changes.
Soil testing before blending any amendments prevents unintended results from an imbalanced soil amendment. For most situations, collect samples annually from just below the root zone. A soil test will help you ascertain whether the grounds can provide enough acidity for your conditions. Most soils require stronger acidifiers than coffee to produce meaningful changes. I test each spring before adding amendments.
Risks of applying directly are mold growth and soil compaction. Fresh grounds may develop crusts that are resistant to water and impede its penetration. Use aged composted grounds at all times and incorporate completely. Apply cautiously to prevent caffeine from accumulating and negatively impacting plant growth.
Read the full article: How to Care for Hydrangea: Complete Growing Guide
