Will Coffee Grounds Turn Your Hydrangeas Blue? Here's What a Soil Scientist Says

It’s a popular piece of garden folklore: Coffee grounds will change the color of hydrangea blooms, making them the dramatic blue many gardeners covet. The truth is more complicated, according to a soil scientist. Find out if it's worth putting used coffee grounds around your hydrangea plants or not, plus better ways of changing their flower color.

Changing Hydrangea Bloom Colors

You can change the flower colors of some bigleaf hydrangeas and mountain hydrangeas by altering the pH of the soil in which they are growing. Soil pH acts as a gatekeeper for aluminum, a common element found in most soils. Acidic soil, with a pH of 6.0 or lower, makes aluminum more readily available to hydrangeas, causing the blooms to turn blue. Alkaline soil, with a pH of 7.0 or higher, prevents aluminum absorption, resulting in blooms that are pink or red.

Neutral soil, with a pH between 6.0 and 7.0, yields purple hydrangea blooms or a mix of pink and blue flowers, as aluminum is partially available to the plant. White hydrangeas retain their white color regardless of the soil pH.

Do Coffee Grounds Affect Soil pH?

“Spent coffee grounds are only slightly acidic, with a pH of 6.2 to 6.9,” says Linda Brewer, a soil scientist with the Oregon State University Extension Service. Brewed coffee is acidic with a pH of 4.8 to 5.1, which may be the source of the belief that coffee grounds can lower soil pH. Brewer says the acidity washes out of the coffee beans when they are ground and brewed. They do not have enough acidity left in them after brewing to affect soil pH, she says.

Would putting coffee grounds out daily for months eventually lower the soil pH? “No,” Brewer says. “That’s not realistic. A dose of coffee grounds large enough to adjust soil pH would not support plant growth because there would be more coffee than soil,” she says.

Too many coffee grounds could adversely impact the soil food web, which refers to the complex ecosystem of microorganisms, including bacteria, fungi, protozoa, and nematodes, that live in the soil and play a key role in plant health, she says. “Too much of a good thing is a bad thing,” Brewer says.

The bottom line: “Coffee grounds do not affect soil pH,” Brewer says. If coffee grounds don’t affect soil pH, they won’t change hydrangea bloom colors.

Better Ways to Change Hydrangea Color

Is It Worth Reusing Coffee Grounds in Gardens?

There's nothing wrong with seeking more environmentally friendly ways to garden and to reduce waste. Brewer thinks that's why coffee grounds are being recommended for everything from organic fertilizer to pest control to a way to change hydrangea bloom color. But how effective the grounds are for these various claims is another matter. In most cases you're better off simply composting the grounds.

And while living and gardening more sustainably is a commendable goal, Brewer points out that, “The production of coffee has significant social, environmental, and economic impacts for those who produce coffee and those who consume it—not just the drinkers, but also the importers, wholesalers, roasters, retailers.” Brewer says. Because of all that goes into the environmental footprint of coffee, she points out that using the grounds in your garden isn't going to make much of an impact. "I’m afraid that the only meaningful way to do that is to reduce our lifestyle," Brewer adds.