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Hydrangeas are one of the only types of flowers that can change color. The change in hue happens in response to the availability of aluminum. These gorgeous landscaping plants are like pH alarms for the garden. Planting them near pH-sensitive plants allows gardeners to keep tabs on soil pH levels. Blue or bluish-purple blooms typically signify acidic soil (pH under 6.0), while pink or reddish flowers signify pH that’s alkaline (above 7.0). A pH between 6 and 7 will net purple or pinkish-blue blooms.
Hydrangea flowers purchased at a nursery might well change color once they are placed directly in the garden. You can control the color palette by fiddling with soil pH. It’s a fun science experiment and a great way to teach kids about soil chemistry.
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BEFORE YOU BEGIN
Be careful when playing around with soil pH. Many plants are particularly sensitive to pH changes. For some plants, the wrong pH can inhibit nutrient uptake and cause plant stress. This is especially true with edibles. Most vegetable gardens do best with a neutral pH.
Also, not all hydrangea cultivars will change color according to soil pH. Lacecap and Mophead varieties are more likely to respond to soil chemistry. Plants sold with white flowers won’t change color. But if you want to feature a color to complement a nearby ground cover or tree, try experimenting with a hydrangea cultivar’s flower color.
Avoid playing around with soil pH when dealing with newly transplanted hydrangeas. Plants are already stressed from their move and might not respond well to additional shock. Once the plant has grown for a season, take the steps below to try to change the flower color.
STEP 1: Wait until the spring or fall.
Ideally, the best time to apply pH-adjusting products is in the spring or fall. That gives the plant plenty of time to absorb the aluminum ions that produce color changes.
It will be difficult to notice any color-changing effects unless plants are in full bloom. Plus, color changes aren’t automatic. In some cases, changes in hue can take several weeks or months to show up.
STEP 2: Test your soil’s pH during the growing season.
In general, it’s a good idea to perform regular soil tests to check for nutrient imbalances or issues with soil pH. That’s because it’s virtually impossible to tell these values just by looking at the soil.
It’s true that hydrangeas can give you an idea of the pH of the soil. However, not all cultivars respond to pH changes. So if you’re not sure of the variety, it’s a good idea to check and see if pH and bloom color correspond to avoid unnecessarily adding products to the soil.
- pH lower than 6.0 produces blue or light purple-blue blooms
- pH between 6.0 and 7.0 produces purple or blue-pink blooms
- pH above 7.0 produces reddish, pink booms
The lower the pH of the soil, the bluer the flowers, and the higher the pH, the redder they’ll be. While store-bought products can change soil pH, other factors also can affect pH levels. Watering with hard water, for instance, can raise pH.
STEP 3: Adjust the soil’s pH to get the desired color.
To lower the pH of the soil and achieve bluer-toned flowers, use sulfur or aluminum sulfate—both are readily available at garden centers and nurseries. Natural options for increasing acidity and lowering pH include adding coffee grounds or citrus peels to the soil.
To raise the pH of the soil and get pink or reddish flowers, apply ground lime—also readily available commercially. Careful. Too much lime can cause leaf yellowing. Make sure to water well after applying lime.
It might be necessary to reapply products several times and keep in mind that changes in hue may take months to appear. Also, keep in mind that raising pH is generally easier than lowering it. Additionally, some varieties might not change color as readily as others.
STEP 4: Treat soil every 2 to 4 weeks, starting in early spring or late autumn.
Making a drastic color change requires a bit of upkeep. Even after you apply products like aluminum sulfate or lime, the soil eventually will return to its original pH levels. Regular treatments with pH-adjusting products are required to maintain a specific pH level.
STEP 5: Feed your hydrangea with a 25-10-10 fertilizer to change its color to pink.
Applying a 25-10-10 fertilizer in the spring or fall helps prevent plants from taking up aluminum, which affects bloom color. This is because fertilizers higher in phosphorus keep aluminum out of the plant’s system.
Another way to prevent aluminum uptake is to put crushed eggshells around the base of hydrangea plants. The shells slow the absorption of the element and prevent plants from turning blue.
STEP 6: Feed your hydrangea with a 25-5-30 fertilizer to change its color to blue.
A 25-5-30 fertilizer is recommended for those wanting blue hydrangea blooms. A lower phosphorus content allows the plant to access aluminum in the soil. It can be challenging to maintain blue coloration in hydrangeas that are planted in naturally alkaline soil.
If you’re dead set on having blue flowers, consider growing hydrangeas in containers. This growing method makes it easier to control soil pH.
Increased acidity can also be achieved by adding organic matter like:
- Coffee grounds
- Fruit and vegetable scraps
- Grass clippings
- Leaves
Playing around with color can be a delight, and can contribute both to garden color and striking dried flower arrangements. Just remember that drastic color changes might require a lot of effort. Growing in pots can reduce the amount of work needed to maintain specific flower hues. Water can also affect soil pH. Collect and use rainwater to avoid potential contamination from tap water.