Avoid This Mulch If You Want Perfectly Blue Hydrangea Blooms

Avoid This Mulch If You Want Perfectly Blue Hydrangea Blooms

Hydrangeas are beautiful flowers that appear to have magical properties that cause them to change color. However, it’s not magic, it’s science. The color of hydrangea blooms changes based on the pH of the soil in which they’re planted. They can range from pink to blue, with some blooms turning purple, or even a gradient from pink to blue. There are various ways to alter the pH of your soil to affect the color of your hydrangeas, and mulch is one of them. Hardwood mulch can make hydrangeas turn pink due to its alkalinity, so avoid using it if you want blue blooms.

Mulch is an excellent way to change the color of your flowers if they are purple. As the mulch decomposes, the nutrients will mix with the soil and alter the pH, resulting in a change in color. It takes some time for the mulch to break down, so don’t expect to see a change overnight. However, it is an effective way to maintain blue flowers, much like how a color-depositing conditioner can help keep your dyed hair looking great for longer.

Hardwood mulch turns hydrangeas pink

hydrangeas turning pink

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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.

Mulches that will make your hydrangeas blue

blue hydrangeas


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.

✿ Read More About Flowers.

Dr Heidi Parkes

By Dr Heidi Parkes

Senior Information Extension Officer QLD Dept of Agriculture & Fisheries.