Avoid This Mulch If You Want Perfectly Blue Hydrangea Blooms

Hydrangeas are gorgeous flowers with seemingly magical properties that turn them different colors. But it isn't garden magic — it's just science. Hydrangea blooms change colors based on the soil pH they're planted in. They range from pink to blue, sometimes turning purple, or even a pink-to-blue gradient. There are several ways you can change the pH of your soil to affect their color, and mulch is one of them. Hardwood mulch can turn hydrangeas pink because of its alkalinity, so steer clear of them if you're in the mood for blue hues.

Mulch is a great way to tip the color scale if your flowers are purple. As the mulch breaks down, the nutrients will enter the soil and change the pH, resulting in a change in color. Breaking down takes some time, so don't expect a change overnight. However, it's a good way to maintain blue flowers, much like how a color-depositing conditioner will make your dyed hair last longer.

Hardwood mulch is made from hardwood trees like hickory, oak, and maple. These mulches can last up to two years, depending on the size of the mulch pieces. It's readily available, making it a popular choice among gardeners and landscapers, but it's not good for hydrangeas if you want them to be blue. Hardwoods, and most other mulches, make the soil alkaline when they start to break down. The alkalinity hardwood adds to the soil isn't good for hydrangeas, and they also contain a lot of manganese, which is harmful to plants in acidic soil.

Hydrangeas change colors based on the aluminum in the soil, which is affected by the pH level. Acidic soil, which hydrangeas need, has aluminum readily available, while alkaline soil doesn't. Aluminum turns hydrangeas blue, which is why alkaline soil will make pink flowers. If you have blue hydrangeas and wish to keep it that way, but you are using hardwood mulch, remove it and replace it with an acidic mulch, otherwise, the colors will slowly change as the mulch breaks down.

If you want to keep your hydrangeas solid blue instead of multicolored flowers, acidic mulch is the way to go — and there are different mulch options to choose from. Mulch made from pine trees, for example, is acidic and readily available. Coffee grounds also make the soil acidic, turning your hydrangeas a beautiful blue.

You can use sulfur and aluminum sulfate to turn your flowers blue faster than mulch alone can. Add aluminum sulfate to water and thoroughly douse your plant with the concoction for the quickest results. Top it off with some acidic mulch, and you'll have blue blooms all year long. 

Make sure your acidic mulch isn't around other plants that need alkaline soil. Choose other acidic soil-loving companion plants like azaleas, daylilies, and gardenias. Though the mulch won't make the other flowers change colors, it will allow them to thrive, and you'll be given the gift of a beautiful flower bed.