Hydrangeas: Getting the (right colour) blues

Forcing hydrangeas for Easter? Our friend Dr. Brian Whipker at North Carolina State University has released a great resource on managing nutrients to ensure your blooms are blue and not bluish-purple.

As you can imagine, more than one factor contributes to bloom colour.  The most important to consider are:

  • The rate of applied aluminum sulfate
  • Substrate pH (in the range of 5.2 to 5.5) and
  • Limiting phosphorus applications


Key take-home messages on controlling flower colour in hydrangeas:

  • Low substrate pH can affect the uptake of calcium (Ca) and magnesium (Mg), so ensure that the micronutrients in your feed are adequate to avoid deficiencies.
  • Phosphorus (P) competes with aluminum for uptake in the plant, so ensure levels are within range. Make sure you are NOT using phosphoric acid to acidify blue hydrangea feed since the level of phosphorus will be too high. Recommended ranges of P are: 2-5 ppm for blue flowers and 10-20 ppm for pink flowers.
  • Potassium (K) levels also impact flower color! Recommended ranges of K are: 150-200 ppm for blue flowers and 100-150 ppm for pink and white flowers.

Brian is working on a project to come up with specific aluminum requirements for hydrangea growers which we hope will benefit our Ontario growers in the near future. Until then, check out the full fact sheet on Nutrient Management Strategies for Ensuring Blue Colouration of Greenhouse Hydrangeas here.

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