The color of the blooms of the mophead-type hydrangea with the scientific name hydrangea macrophylla is somewhere between blue and violet with a flash of silver. Blue hydrangeas almost defy description. But when you see them, you know there is nothing like those blue flowers.
So how do you make this kind of hydrangea change from pink to blue? The key is to change the acidity or pH level in the soil. The more alkaline your soil is the pinker the flowers. Conversely, a higher acidity level yields bluer blooms.
Make Your Hydrangeas That Beautiful Blue
But before you move on to testing and adjusting soil acidity to make your hydrangeas blue, make sure that you’ve taken these measures to encourage them to thrive:
- Provide protection from the winter cold. Move your hydrangea bush to a sheltered location near the house or behind a windbreak of trees. If you can’t move the bush, place a cage around it and cover it with shredded leaves.
- Prune correctly. Avoid pruning your hydrangea when the buds are forming in the spring. Fall or late winter are better times to prune when its stalks are dry and have finished blooming.
- Fertilize at the right time. Apply an organic fertilizer like Rose-Tone as soon as the leaves of the hydrangea start to come out. And then apply it again when you see buds on the plant. If the bush blooms a second time, it’s a good time to follow with another application of fertilizer.
- Water properly. Too much water produces leaves and no flowers. So, wait until the leaves look a little dry after the sun is off the bush. If the plant doesn’t rebound, it needs water. Try running a hose at the base to avoid encouraging fungus if the leaves get wet.
How to Test the pH of Your Soil
Here is a simple way to check if your soil is acidic enough to grow blue blooms on your hydrangeas:
- Take a handful of the soil from near the plant’s roots and place it in a container.
- Pour distilled white vinegar over the soil.
- If the white vinegar solution fizzes, the pH level is high, and the soil is alkaline. You will definitely need to amend the soil. If it doesn’t fizz, the soil is neutral or acidic, and you may not need to amend it as much.
How to Amend the Soil
Although you could use coffee grounds and ground-up citrus peels to adjust your soil’s acidity, there is a lot of guesswork involved in this method. A more straightforward way to do this is to use aluminum sulfate, which you can purchase at your local garden center. The chemical composition of aluminum is responsible for creating those coveted blue hues.
To use aluminum sulfate for encouraging your H. macrophylla to turn blue, follow these steps:
- Dissolve one tablespoon of aluminum sulfate in a gallon of water.
- Drench the plant’s root zone with the solution in early to mid-spring, but don’t overdo it. You may need to adjust the application depending on when the buds form in your growing zone.
- Mulch your hydrangea with pine needles, pine bark and/or leaf litter.
- Another thing to keep in mind is to avoid applying fertilizers that contain phosphorus. This element binds with aluminum and reduces the effectiveness of the solution you’ve just applied.
If you are willing to take the time and effort to give your hydrangeas the care they deserve, they might just reward you with those big, beautiful blue blooms. All it takes is a little time and know-how on your part.