|What do I add to the soil to turn the flowers of my hydrangea deep blue?|
|Hydrangea color can depend on the specific variety you are growing, the overall health of the plant, the weather that year, and the soil. Some hydrangeas will change color, to pink or blue or in between, based on the soil pH. If your soil is acidic, the hydrangea flowers will be bluer; if it is alkaline they will be pinker. The soil in your area is naturally somewhat acidic, so your flowers should naturally be blue. A pH of 5.5 should be low enough to bring out the blue.
For the best blue, do not apply lime near your hdyrangea. If you added lime to the soil in the past few years, that would have made the pH more alkaline. Lime will leach through the soil over time and so your flowers should consequently turn bluer. Another cause for poor blue can be planting adjacent to concrete -- which leaches lime.
A slight acidifying effect can be achieved by using a slow release fertilizer for acid loving plants such as Hollytone. Read and follow the label directions for how much to use -- do not be tempted to overapply it. (Regular fertilization can also help the plant show its best color due to being in better health.)
You can also be more precise and check your soil pH by testing it and adjust it according to the test results, if necessary, using granular sulfur. Your local Cornell county extension should be able to help you with the soil testing and interpreting the results.
Enjoy your hydrangea!