Do coffee grounds affect hydrangea colors?
Written by
Michael Sullivan
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.Numerous gardeners believe that coffee grounds can alter hydrangea colors by acidifying the soil as they decompose. This is a widespread misconception that persists despite research contradicting this potential. Coffee grounds only possess less than 2% of the compounds we truly think of in terms of acidifying the soil, thus they are highly unreliable for this purpose. Coffee grounds primarily function as organic matter instead of true soil amendments.
The chemical structure of coffee grounds reveals their limitations. They are rich in nitrogen; however, they do not produce significant acidity upon decomposition. Coffee grounds have a pH that quickly neutralizes in soil, within a couple of days. I applied coffee grounds to my hydrangeas for months, and did not see any measurable difference on pH tests. Soil tests also confirmed, after six applications, a difference of less than 0.3 units.
For Blue Hydrangeas
- Aluminum sulfate: Provides immediate aluminum ions for pigment binding
- Elemental sulfur: Gradually acidifies soil through bacterial oxidation
- Acidifying fertilizers: Use 25-5-30 NPK formulas quarterly
For Pink Hydrangeas
- Garden lime: Raises pH effectively within 4-6 weeks
- Dolomitic lime: Adds magnesium while increasing alkalinity
- High-phosphorus fertilizer: Blocks aluminum absorption completely
For soil structure, coffee-grade organic matter improves soil structure, but it will not impact color change. They will increase portion stability, moisture retention, and microbial activity. However, coffee grounds do not change aluminum solubility, which is essential for the transformation of a bloom. Focus on more directed solutions that will predictably affect soil chemistry instead.
Applying the correct amendments saves time and effort. Before trying any of these solutions, you must measure your soil pH. If you want blue hydrangeas, you would use aluminum sulfate, which is dissolved in water. Conversely, if you prefer pink hydrangea blooms, you can spread garden lime around the drip line of the plant. Both methods will yield the same results, and your efforts will be visible by the end of the growing season, regardless of the approach you take.
Instead of transient remedies such as vinegar that may harm your soil biology, sustainable color change should be continuously monitored and adjusted. Test the pH of your soil every 60 days to ensure it remains within the correct pH range during the bloom season. The hydrangeas will reward the extra attention with dependable color changes season after season.
Read the full article: Hydrangea Color Change: Ultimate Guide
