The Unexpected Fertilizer To Use For Stunning Purple & Pink French Hydrangea Plants

French or bigleaf hydrangea (Hydrangea macrophylla) is a favorite of many gardeners. It is easy to care for, thrives even in heavy shade, and produces big, beautiful flowers that last almost all summer long. It is tough not to love these plants. But you know what makes French hydrangeas really special? They can change the color of their flowers. If you are a fan of purple and pink shades, you don't need to plan a tour to your local nursery or gardening store just because your hydrangeas are currently blue. All you need to do is raise the soil pH, and soon your bigleaf hydrangea plants that once bloomed in blue will start producing purple and pink flowers. The best part? You can do it with an unexpected fertilizer that many homes already have — wood ash.

Now you might be wondering how this is even possible. The answer lies in how pH affects aluminum in soil. When the pH is below 5.5, it increases the solubility of aluminum ions and their absorption by plants. This turns French hydrangeas blue. However, if you raise the soil pH, it decreases the availability of aluminum to plants, and hydrangeas begin to turn purple and pink. Purple flowers can develop in soil with a pH of 5.5 to 6.5, and pink flowers show up in soil pH 6.5 and above.

So, what is the role of wood ash fertilizer in all of this? Well, wood ash acts as a liming material. It contains a significant amount of calcium, about 25% to 50%, which can increase the pH when added to the soil. Just keep in mind that this only works on French and mountain hydrangea plants. It won't work on other white hydrangea varieties that you might have in your garden.

The very first thing you need to do before adding any wood ash to your garden soil is to conduct a soil test. For pH, you can use the commonly available home-based soil testing kits. However, if you want to get all the details about the condition of your soil, including its pH, lab tests are the better, more reliable choice. Knowing the soil pH is also important before trying this technique because you don't want to raise the pH too much. A pH over 7.0 can lead to iron deficiency and chlorosis in hydrangeas.

Another thing you want to consider before using this method to change the colors of your bigleaf hydrangeas is to make sure that you only use material that comes from untreated wood. Although this is another one of the ways to repurpose fireplace ash around your home and garden, if the ash you have on hand came from wood that was painted or treated with any other products, like preservatives and pesticides, there might be chemical residues present, which could harm your plants. Also, use safety precautions when handling, and wear gloves, a mask, and eye protection. 

Make sure you remove any big pieces of wood before adding it to the soil. To apply, before the growing season, work a thin layer of wood ash into the top few inches of the soil and add water. Only apply once a year, and stay conservative with quantity. Do not put more than 10 pounds of wood ash per 100 square feet per year. You can retest the soil after six months. Also, only apply wood ash to established French hydrangea plants. Applying it to seedlings or young plants can damage the new, tender roots.