Question:
How do I get my hydrangea to produce blue flowers instead of pink?Answer:
Of all the hydrangeas that grow in Iowa, only the bigleaf hydrangea can potentially bloom with blue flowers. The flower color of this species depends on soil pH – blue in acidic soils and pink in alkaline soils. In Iowa, most bigleaf hydrangeas are purplish-pink due to our slightly alkaline soils. Soil pH must be at 5.5 or lower to reliably get blue flowers and in Iowa, soil pH tends to be between 6.5 and 7.5.
Changing soil pH is necessary to get blue blooms in bigleaf hydrangea. Planting shrubs with Canadian sphagnum peat moss can help to lower the soil pH in the root zone. Watering plants with an acidifying fertilizer can also help. Elemental sulfur can be applied to the soil to lower soil pH. Conduct a soil test to determine the current pH and apply these amendments at a rate that will lower the pH to 5.5. Over time, soils will return to their natural state so regular applications of soil amendments will be necessary to maintain the desired pH.
Learn more about changing your soil's pH in this article: How to Change Your Soil's pH.
Learn more about growing hydrangeas in Iowa in this article: Hydrangeas for Iowa.
