Hydrangeas, always a favorite for garden enthusiasts, are unique in that the color of their flowers change dramatically.
First, let’s clarify that the hydrangea in question are the 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, Lacecaps that stay white.
The color question is likely the most frequent query addressed to the gardeners from Terrapin Landscapes. The question typically revolves around the fact that the blooms are a vibrant blue upon purchase, but it’s often noted that they start to turn pink as the plant gets older. So why the color change, and what can be done about it?
Hardy in zones 6 through 10, hydrangeas tend to be a bit fussy about where they live. The color is in the soil. So, what is it that allows us, as gardeners, to alter the color of our Hydrangea blooms? Many people will quickly respond, it is the pH of the soil. On one hand they are correct, soil pH definitely plays a role in flower color. However, it is the availability of Aluminum (AL) to the plant that effects flower color. Aluminum is a naturally occurring mineral that is found in most of our native soils.
Just because a mineral (or nutrient) is in the soil doesn’t mean it is available to the plant for use. This is where soil pH plays a role – plants are able to absorb (or take up) Aluminum when the soil pH is between 4.5-5.8 with the best range being between 5.0-5.5. When the pH moves to 6.0 and above then the plants will not be able to absorb the Aluminum although it is present in the soil. This variation in the soil often leads to some interesting flower shades such as pinks, mauve, purple and combinations of each.
So what to do? Most of us are not going to test the soil (see below for a simple home test), so 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. Some examples:
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.
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.)