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Garden Detective: Hydrangeas need right pH for best color

I have four mature hydrangea plants on three separate plots of land around my house. All of them had lots of flowers this year but they were all a dull green color. In the past, I had blue and pink blooms on the plants. I’ve looked for pictures so I could recall exactly which plants were which colors, but alas, I can’t find any pictures of the house during that colorful period of the year.

The only thing that changed is that my husband cut the bushes way back in October of last year. I wondered if they would even give me flowers this year after the butchering he gave them. They did grow and produce flowers. But they are all this dull green or gray color.

How can I get my beautiful color back?

– Ann McCandless, Sacramento

According to UC master gardener Carol Rogala, the color of a hydrangea planted in the garden is affected by the pH in the soil. The blue color is produced in soil strongly acidic (below pH 5.5) and pink is created in a neutral to alkaline soil (pH 7.0 or higher). The lower the pH number, the more acid the soil.

Flower color treatment is not effective unless started way ahead of bloom, typically in late autumn the previous year.

Exact amounts of any amendment to the soil to get the desired flower color will vary depending on your current pH and soil type.

White varieties of hydrangea stay white and cannot be changed.

According to hydrangea experts, it’s not uncommon for hydrangeas that are normally pink or blue to bloom green in some years. No one seems to know exactly what causes this, but often it occurs for only one or two years and then the blooms return to their normal color. The green flowers also may be prompted by heat, particularly a very warm and dry spring.

To help get your bushes back on their appropriate color tracks, check your soil’s acidity and aluminum level.

To obtain a blue hydrangea, aluminum must be present in the soil. To ensure that aluminum is present, aluminum sulfate – available at nurseries – may be added to the soil around the hydrangeas.

Additions of acidic organic material (such as peat and pine bark) can also help lower the pH.

To obtain a pink hydrangea, the plants must not take up aluminum from the soil. If the soil naturally contains aluminum, altering the soil pH is necessary. To raise the pH, add lime or super phosphates to the soil.