Peter Atkins and Associates.,LLC

Why aren’t my Hydrangeas blue?

You may have heard that you can change the color of a hydrangea's flowers by adjusting soil ph. But there's a little more to it than that.

First, not all hydrangeas produce blue flowers. Hydrangeas with white or cream flowers, such as Annabelle hydrangeas, oakleaf hydrangeas and members of the Pee Gee family, can only produce white or cream flowers. Sometimes their blooms take on a pink tinge at the end of the season, but

that's about as colorful as they get.

Hydrangeas with bloom colors that range from pink through blue and purple usually belong to the hydrangea cultivars known as mopheads and lace caps. These types of hydrangeas have the interesting ability to change the color of their blooms based on the chemistry of the soil. When grown in alkaline soil, the bloom colors are pinker. When grown in acidic soil, the bloom colors are bluer.

All About the Chemistr

Because it's the soil chemistry that determines the bloom color, the variety names given to these types of hydrangeas means very little when it comes to bloom color. For instance, Nikko Blue, Pretty in Pink, Forever Pink and Blue Deckle, all have an almost equal chance of blooming pink or blue,

depending on the soil they are planted in.

So, remember that even if you purchase a hydrangea in bloom, you cannot be sure the plant will produce the same color flowers once it's growing in your garden.

This blog is about hydrangea macrophylia, often called Mophead due to their large rounded blooms. These are the “bush form” hydrangea, as opposed to tree form, which typically have white to light green blooms.  The most common variety of Mophead hydrangea, Endless Summer, is easily found in local nurseries and is the most popular due to the claim that the plant will continuously bloom all season. There is a second hydrangea macrophylia, Lace caps that stay white.

So, remember that even if you purchase a hydrangea in bloom, you cannot be sure the plant will produce the same color flowers once it's growing in your garden.

It all comes down to the soil Chemistry

Because it's the soil chemistry that determines the bloom color, the variety names given to these types of hydrangeas means very little when it comes to bloom color. For instance, Nikko Blue, Pretty in Pink, Forever Pink and Blue Deckle, all have an almost equal chance of blooming pink or blue,

depending on the soil they are planted in.

So, remember that even if you purchase a hydrangea in bloom, you cannot be sure the plant will produce the same color flowers once it's growing in your garden.

Start by testing the pH of your soil. This will give you an idea how much of an uphill battle you'll be waging. Be mindful that the health of the plant should be your priority. It's quite difficult to make a dramatic change in soil pH and doing so can adversely affect the overall health of the plant.

Conditions for Blue Hydrangeas. To encourage blue hydrangea flowers, grow the plant in soil that has a pH of 5.2-5.5. If your soil is more alkaline, you can lower the pH by applying Soil Acidifier at the rate specified on the package. Soil pH can also be lowered (more gradually) by applying an acidic organic mulch, such as pine needles or pine bark.

If the pH of your soil is naturally quite high (alkaline) it will be very difficult to get blue flowers — even if there's plenty of aluminum in the soil. Alkaline soil tends to "lock up" the aluminum, making it unavailable to the plant. (However, you can grow fabulous pink hydrangeas!)

If you are trying to maintain a certain pH level, you should test your soil each year. This is how Peter Atkins and Associates does it each year. The effect of adding materials to raise or lower the pH may not be immediately apparent. You should also expect that over time, the pH will revert to its original level,

which is dictated by the native soil.

Home pH Soil Test:

First, get a handful of dirt.  It only takes a small amount of soil to test the pH level. Next, place the soil sample in a container.  Pour distilled white vinegar over it.  If the solution fizzes, the pH level is high,and your soil is alkaline.  (If it doesn’t fizz, then the soil is neutral or acidic; you will not have to amend it as much to make your hydrangeas blue.)

Let’s assume the soil is alkaline and needs to have the pH lowered while adding Aluminum.  There are a variety of products available at your local nursery that address this issue.  The desired products include soil acidifiers, ammonium sulfate and aluminum sulfate, all are readily available. 

Soil pH can also be lowered by applying acidic organic mulch, such as pine bark or pine needles, orange peels or coffee grounds, but these will work slowly.  Simply follow the instructions on the purchased product and you will be on your way to beautiful blue blooms!  It should be noted that the change back to blue will be slow regardless of method, and it is often the case that the feeding of the plant/soil will have to be repeated a few times each season to maintain the blue color.  Hydrangeas planted near concrete sidewalks or concrete foundation are more likely to produce pink blossoms.  Lime can leach out of the concrete and keep the soil pH too high to produce blue blooms. 

Soil pH can also be lowered by applying acidic organic mulch, such as pine bark or pine needles, orange peels or coffee grounds, but these will work slowly.  Simply follow the instructions on the purchased product and you will be on your way to beautiful blue blooms!  It should be noted that the change back to blue will be slow regardless of method, and it is often the case that the feeding of the plant/soil will have to be repeated a few times each season to maintain the blue color.