If you’ve got a pink-flowered bigleaf hydrangea that you want to turn blue, or perhaps vice versa, then a little chemistry can help you achieve the hue you desire.

First, it’s only the bigleaf types that can change color, so if that’s not what you have, nothing will have an effect on the flower color. If you do have a bigleaf hydrangea, measure your soil’s pH with a pH meter. A reading between 4.5 and 5.5 is the acidity level necessary to keep blue hues flowing on your hydrangea. That’s because a pH level under 5.5 makes aluminum in the soil available to the plants, and this element is what turns the flowers blue. Higher pH levels than this, and your hydrangea flowers will look purple or pink, depending on how alkaline your soil is.

To increase soil acidity so you can enjoy blue hydrangea flowers, apply garden sulfur near the base of the plant according to package directions. You may have heard rusty nails, copper pennies, or used coffee grounds do the trick, too. However, as previously noted, it’s aluminum, not iron or copper, that bigleaf hydrangeas need to turn blue.

Used coffee grounds have a neutral pH, so they won’t help acidify the soil. However, fresh grounds and brewed coffee are acidic, but you would need to apply them regularly throughout the growing season to affect pH.

If you want to have pretty pink hydrangea blooms, use garden lime around the plant to raise the pH, so aluminum in the soil stops being available to the plant. It can take months, no matter which method you use to change your soil pH, to see differently colored flowers than what you have, so remember to be patient.