How to Make Hydrangeas Purple

While some of the joy of growing hydrangeas either in the ground or in containers is watching the display of large, full blooms year after year, another fascination with the plants is that they are one of the few species where you can alter the flower color by altering the soil. The color of a hydrangea works side by side with the pH scale. Blue flowers grow when the soil is acidic, and pinks flourish in basic, or alkaline, soil. To make hydrangeas purple, which rests closer to neutral, you'll need to find the perfect balance and hit the pH scale just right.

Things You'll Need

• Soil pH testing kit
• Lime
• Aluminum sulfate

Instructions

1. Determine the pH of your soil by consulting with your local garden center on how to test for pH. If your plants are established by at least 2 to 3 years, and it blooms the same color each year consistently, then testing pH may not be necessary.

2. Alter your pH, if needed, by treating the plants in spring before they bloom. Lavender to purple flowers are produced with a 5.5 pH. If your flowers are usually blue, or your soil has a low pH, then you will need to raise the pH. For pink flowers, or a soil with a high pH, you will want to lower the pH.

3. Raise the pH, when necessary, by introducing lime into the soil. Applying 4 oz. of lime to the base of the plant can raise the pH by one point. Use either 2 or 4 oz. of lime in each application to slowly raise the pH until you reach the 5.5 pH required to see purple flowers.

4. Lower the pH, if needed, by adding aluminum sulfate. Follow the package for instructions on using aluminum sulfate carefully. Usually you can use 1 tbsp. per gallon of water around the base of the plant for each application. Because aluminum sulfate can burn the roots if used incorrectly, you may opt to change the pH by adding organic matter, such as coffee grounds, homemade compost or grass clippings.

5. Allow at least 2 to 3 weeks between applications of lime or aluminum sulfate to really give the plant time to adjust to the new soil and change colors. Pay attention over time to how your plant is doing and whether or not your small changes are taking effect. No matter what you do, you will not see instant results, but it is possible to overdo it in the waiting process.

6. Take regular care of your plant with pruning, treating for disease, and fertilizing one to two times a year. A lack of intensity in color can be based on your hydrangea's health, not solely on the soil. If your blooms aren't vibrant, then it may be time to schedule a regular maintenance routine to make a happy plant with bright purple blooms.

Tips & Warnings

• Keep a running calendar or journal to document when you make changes to the soil and how much of a product you used. This will keep you from applying to much of a good thing and harming the plant unintentionally. Remember to be patient and enjoy the process and unpredictability of adjusting your hydrangea's color. Because of the slowness of the process, it can take months to achieve the color you are looking for. Depending on how low your pH is, some gardeners must wait up to two growing seasons to see the results they were striving for.

• While most gardening is simple and natural, not all products used in the garden are. Wear the appropriate gloves to keep harmful substances from coming into contact with your skin

References

• Manipulating hydrangea color
• Changing the color of hydrangeas

Resources

• Caring for hydrangeas and their many varieties