How to Turn Your Hydrangeas Blue—the One Trick Gardening Experts Swear By

There's an old gardening myth that you can turn hydrangeas blue by burying pennies or nails in the soil. Unfortunately, this particular wisdom isn't true, but there are other ways you can change these beautiful flowers to a brilliant, cerulean hue. It will take a bit more effort than just dropping some loose change into the dirt, though.

Here, our experts walk us through which hydrangea varieties produce the best shades, along with the secret to perfectly blue hydrangeas year after year.

Which Varieties Are Best?

Generally speaking, if a variety name has “blue” in the name, then it should turn that color with the right soil conditions. However, the best hydrangeas for predictably blue blooms are bigleaf and mountain varieties, according to Ondrea Kidd, the lead floral designer and founder of Sowing Joy Farm. These are a few that she recommends in particular:

• Nikko Blue: This popular Japanese variety is a reliable producer and grows up to 6 feet tall and wide.
• Endless Summer: This variety is known to produce blue blooms with the correct soil acidity on both old and new wood.
• Blue Enchantress: This is a great choice for those needing something hardy down to zone 4, and it can turn lighter blue as the blooms age.
• Blue Jangles or Pop Star: These are both popular dwarf varieties and will stay smaller. 
• Big Daddy: This is a larger variety—a beautiful mophead hydrangea that can reach over 6 feet tall and wide but requires a hardiness zone of 6 or higher.

That being said, keep in mind that not all bigleaf hydrangeas can turn blue, says Stacey Hirvela, the marketing manager at Proven Winners Color Choice Flowering Shrubs. Even the right varieties may not be blue when you first purchase them.

“Some growers will treat them so they are blue at the point of sale, but there’s no guarantee,” Hirvela adds. “That said, once you plant them in [the right] conditions, they will take on the color.”

Soil pH and Aluminum

​​In order for hydrangeas to have that classic blue color, aluminum must be available in the plant’s soil. “Aluminum is naturally occurring in most but not all soils,” Hirvela says. “It tends to be most abundant in clay soils over sandy soils."

However, your soil’s pH level also plays a major role, adds Juliet Howe, a horticulturist and the founder of Twigs Design. Here's what the different pH levels will mean for your blue hydrangeas:

• pH 6 or below: In acidic soil, aluminum is free and available for uptake in its roots. 
• pH 7 or above: When the soil is alkaline, the aluminum in the soil will form tight chemical bonds with the soil particles and is not readily available to be absorbed by plant root systems.
• ph 6 to 7: Neutral soil will generally lead to purple hydrangeas or a pink and purple mix.

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How to Turn Your Hydrangeas Blue

Remember, before you start, your hydrangea has to be the right variety. “White hydrangeas don’t change color at all,” warns Hirvela. “Those that tend more toward red or very deep pink will, at best, develop some purple—but will not go blue at all.”

Once you’ve chosen a hydrangea whose color will change based on soil chemistry, simply follow these steps to optimize its blueish hue. 

Step One: Test Your Soil

Test the soil to figure out where you stand in terms of pH. “My preferred method is using a soil meter,” says Howe. “These handy gadgets generally cost less than $20 and will immediately give you the pH of your soil.”

Step Two: Amend Your Soil

The soil pH must be acidic, Hirvela says, as the aluminum is only free for the plant to take up and metabolize in acidic conditions. “Otherwise, it’s bound tightly to the soil particles and cannot be absorbed by the plant,” she warns.

Therefore, your soil pH needs to be 6 or lower to create a blue hydrangea. “I recommend using aluminum sulfate to increase your soil’s acidity in a predictable way,” says Kidd.

However, Howe says aluminum sulfate has been likened to eating a candy bar for energy. While it works quickly, it will also break down fast and need to be reapplied often. It also comes with some risks. “If applied in excess or too quickly, aluminum sulfate can be toxic to plants, causing root burn and disrupting soil health,” she warns. 

Instead, you can also try elemental sulfur. “It’s a slightly less expensive choice, but it requires time to break down, and so it works more slowly," says Howe.

Pine needles, pine bark, peat moss, or coffee grounds are things you can likely find at home that will also lower the soil’s pH. “These are very slow acting,” Howe says. “It will take a very long time for them to break down."

Step Three: Check Your Soil

Check the pH every three months with your soil meter. “The color of the hydrangea reflects the nutrients that were in the soil at the time when buds were developing,” Howe says. “If the soil has not yet reached the desired pH, reapply according to the directions on your product. If the soil had an especially high pH to start with, then you may start out with purple flowers as the pH lowers.”

Apply your solution monthly from early spring through bloom time. “For continued blue flowers, apply again in late summer or early fall,” Hirvela says. “This is when the plant will start making its flower buds for the following season.”

For a continuous supply of blue hydrangeas, this process will need to be repeated yearly.

Best Care Practices

Here's how to care for your blue hydrangeas once they've bloomed.

• Provide precise sunlight: Hydrangeas need four to six hours of morning to early afternoon sun. Provide an area where they get a good amount of sunlight, but keep them shaded during the hottest part of the day.
• Avoid harsh water: Because water from your hose can be rather alkaline, Howe suggests collecting rain water—which is more acidic—to water your blue hydrangeas.
• Sidestep the leaves: Avoid watering the leaves to prevent fungal disease, Howe says. Instead, water directly at the base near the soil line, which will help promote healthy roots. 
• Prune after bloom: Prune the spent flowers shortly after blooming to maintain the shape of your plant, says Howe.