One of the most curious things about Bigleaf Hydrangeas (Hydrangea macrophylla) is their chameleon-like ability to change color. That striking pink-flowered hydrangea you just planted from your local garden center may have blue flowers by the time next year rolls around. Likewise, that stunning blue-flowered hydrangea you just transplanted from your mother’s house may have pink flowers next growing season. But how can this be?! What makes a hydrangea change color, and how can you be sure that it will stay a certain color?
Photo: Bigleaf Hydrangeas (H. macrophylla) come in a rainbow of pinks and blues
The answer to this perplexing plant phenomenon has to do with aluminum in the soil. Aluminum is the most abundant metal in the Earth’s crust, but it is toxic to almost all plants. There are very few plants that can actually accumulate aluminum internally without any ill effects. One of these rare aluminum-tolerant plants is Bigleaf Hydrangea. Hydrangea uses organic acids to detoxify aluminum, forming aluminum-citrate. While inside the plant, aluminum also reacts with the plant pigment anthocyanin, producing a blue hue in the flowers. As the concentration of aluminum in a hydrangea increases, the intensity of the blue color also increases.
Thankfully for all the non-aluminum-tolerant plants, this metal cannot typically be absorbed by plant roots in its normal form. Only when the soil is very acidic and the pH drops below 5.5, does aluminum change and become the soluble ion Al3+ which can be absorbed by plant roots. So even if aluminum is present in the soil (which it almost always is), it may not be available to your hydrangea because the soil is not acidic enough.
If you want to change the color of a Hydrangea that is planted outside, focus on adjusting the soil pH. Always take a soil pH test before doing anything. Based upon the results of your pH test, you can add elemental sulfur to the soil which will slowly lower the pH over time, eventually resulting in bluer hydrangeas. Or you can add lime to the soil which will more quickly increase the pH, resulting in pinker hydrangeas. Still, changes in soil pH typically take several months, so patience is key. Also, be aware that soils near concrete walkways and house foundations as well as turf areas that have been heavily limed will be more alkaline, and work better for growing pink hydrangeas.
Photo: Pink flowered hydrangeas show their colors in more alkaline or basic soils - those with a higher soil pH
On the other hand, if you are growing a hydrangea in a container and have used potting mix rather than mineral soil, there is a good chance that aluminum is missing. If you’re looking to grow a potted hydrangea with bright blue flowers, you might need to add some aluminum sulfate. This specialty soil amendment supplies aluminum while lowering the pH. The tricky part though, is knowing how much aluminum sulfate to apply. At too high aluminum concentrations, even aluminum-tolerant Hydrangea can be dwarfed and killed. Even the great plant-God, Michael Dirr has admitted to killing Hydrangeas with too much aluminum sulfate. For potting mix that has no soil in it (and therefore no existing aluminum), Dirr recommends that 0.75 – 1.5 ounces (21 – 42 grams) of aluminum sulfate be applied to the surface of a 3 gallon container. Multiple applications at this rate may be needed if the pH remains higher than 5.6. Make sure to read all label directions carefully and follow application rates carefully! Some hydrangea growers will apply aluminum sulfate at the end of the season in August or September, and then again during the forcing stage the following spring. It is important to water thoroughly after application to ensure movement into the root zone and to not apply when the potting mix is dry.
Photo: Before adding any soil amendments, be sure to always take a soil test.
Photo: Before adding any soil amendments, be sure to always take a soil test.
Lastly, the type of fertilizer you use can also affect hydrangea flower color. Nitrate nitrogen fertilizers increase the pH, and ammonium nitrogen fertilizers lower the pH. High levels of phosphorus can decrease the uptake of aluminum, while high levels of potassium can increase the uptake of aluminum.
In summary, if you want blue or pink Hydrangeas:
· Blue: high Al, pH 5.0 – 5.5, very low P, high K, and moderate ammonium N
· Pink: low Al, pH 6.0 – 6.5, high P, low K, and high nitrate N
Hopefully that clears up some of the mystery behind Hydrangea coloring, a topic that is almost as confusing as Hydrangea pruning! If you’d like to learn more about hydrangeas, a great resource is Hydrangeas for American Gardens by Michael Dirr from 2004, published by Timber Press.
