Why aren't my Hydrangeas blue?
Before we continue to investigate this question, make sure you have a variety that can be blue (most commonly 'Nikko Blue').
Hydrangeas, along with many other ornamental plants that we use in our landscape, require a more acidic soil than is typically offered in our urban landscape to be able to absorb the nutrients in the soil. To actually turn your hydrangeas flowers from pink to blue, you'll need to fertilize them in some way and make soil amendments to keep them healthy. Peat is a natural product that when added to the soil helps to lower the ph of the soil, making it more acidic. Peat also increases the soil's capacity to hold water and reduces compaction in the soil.
Another natural product that will help your plants get the nutrients they need to grow is compost. When mixed with peat, the two amendments will increase the acidity, moisture holding capacity, and nutrient availability to your plants. These amendments are frequently recommended to our customers before considering any chemical fertilizers, but to get those hydrangeas to turn blue, you will most likely still need another fertilizer. At Earth Care, we recommend using another slow release fertilizer on top of making soil amendments.
We find that Hydrangeas, Rhododendrons, Blueberries and Hollies are the most common plants that need these special considerations if you want to grow them here in the midwest. There are more plants that would benefit from peat and compost alone that may otherwise be growing in the existing soil. With the existing soil conditions, some plants will live, but not really grow well. Adding a 1" layer of mixed compost and peat across a mature plant's root zone is the preferred method of adding nutrients to the soil.