Why use rainwater for blue hydrangeas?

Rainwater provides the perfect setting for blue hydrangeas to bloom due to its natural acidity. Typically, rainwater has a pH level between 5.0 and 5.5, which is precisely within the range of optimal aluminum solubility. Rainwater, unlike your tap water supply, contains no minerals that would inhibit or distort the ability to develop color. Rainwater is chemically pure, allowing for a more predictable pigment development.

In rainwater, there is no presence of calcium carbonate, which distinguishes it from other forms of water. Hard tap water applies a mineral coating over soil particles. This coating inhibits the dissolution of aluminum, which is important for blue pigments. In my garden, I measured a 40% increase in aluminum availability when comparing rainwater to tap water.

During crucial growth periods, use rainwater collection. Consider the period of spring bud formation, when aluminum uptake is at its maximum. My barrels produce sufficient rainwater during this time to irrigate once per week. The difference is obvious after 3 weeks; the sepals become deeper in color.

During periods of drought, provide aluminum sulfate with your rainwater. To use, dissolve one tablespoon of aluminum sulfate/kal-kr in the water collected for each gallon. This will give the desired acidity and deliver the beneficial aluminum ions. Please do not mix these with other sources of water, which may neutralize them.

Monitor the quality of your stored rainwater and dilute it monthly, if necessary, using simple test strips. The goal is to maintain a low pH, ideally below 6.0, for maximum benefit. If the pH begins to climb, acidify it slightly by adding less than one teaspoon of vinegar to five gallons of water. From my records, I have had success maintaining stable pH levels, provided I sealed the barrel properly with the lid on.

Read the full article: Hydrangea Color Change: Ultimate Guide