If you take a walk in your neighbourhood this spring and summer, you are almost guaranteed to see at least a few hydrangea bushes in someone’s garden. These ubiquitous plants are easily recognizable by their distinct “snowball”-shaped blooms, that can range in color from white, green, or blue, to pink or red, with every shade and intensity in between.
You may not give these seemingly ordinary plants much thought, but there is more to the common backyard garden staple than you might initially think. There is some interesting science behind where hydrangeas get their variety of colors, and with that knowledge in your back pocket, you can actually change the color of the bushes in your yard.
Where do Hydrangeas Get Their Colour?
You may be surprised to know that hydrangea blooms are not actually true flowers. The majority of the bloom is made up of sepals, or modified leaves, which almost completely overshadow the small, floral portions in the center .
What truly makes hydrangeas different from other flowers, however, is that their colors are not caused by a variety of different pigments, as is the case with roses or tulips, but instead behave in the same manner as litmus paper.
In case you’re a little hazy on your high school chemistry knowledge, litmus paper is chemically-treated paper that is used to determine whether or not a solution is acidic or basic. If you dip the paper into an acidic solution (one that has a pH less than seven), it will turn red. Likewise, if you dip red litmus paper into a basic solution (one that has a pH greater than seven), it will turn blue.
Hydrangea bushes act in a reverse-fashion to litmus paper, and are a natural indicator for the pH of the soil in which the plant grows. In acidic soil, the bush’s blooms will be blue, and in basic soil, the blooms will be red .
The Aluminum Factor
While the pH of the soil is an important factor in changing the color of a hydrangea bush, there is one more key component that is required: aluminum ions.
In acidic soil, aluminum ions are very mobile because there are other ions that are readily available for them to react with. These ions can be taken up from the soil into the bloom where they interact with the pigment, which is normally red.
In neutral or basic soil, aluminum ions combine with hydroxide ions to form aluminum hydroxide, which is not mobile. In this case the ions are not taken up by the blooms, and so they remain red .
How to Change the Color of Your Hydrangeas
So now you know the two necessary ingredients to change the color of your hydrangea bushes: acidic or basic soil, and aluminum ions- but how do you apply this to your garden?
Before you try to change the color of your hydrangea bushes, it is important to note that only some of the Bigleaf hydrangeas (H. macrophylla), in particular the Mophead Lacecap varieties, and the serrata variety, change color based on the pH of the soil .
White hydrangea bushes are not affected by soil pH, therefore the color will not change regardless of the soil conditions. They do, however, prefer alkaline or neutral soil. Green blooms are usually an indication of age, and the color change cannot be reversed .
If you want to change the blooms to blue you must increase the acidity of the soil. To do this, you can apply a solution of one-quarter ounce aluminum sulfate per gallon of water to the soil three times per year. Aluminum sulfate can be purchased at any garden center.
For the first application, you must soak the ground with the solution after the plant begins to grow in the spring, and then repeat this two more times every three to four weeks. Additionally, you should apply a 25-5-30 fertilizer to the soil once per year, as per the manufacturer’s instructions .
If you want to change the blooms to red you need to make the soil more basic, in other words, increase the alkalinity. To do this, you can spread ground limestone on the soil at a ratio of four pounds per hundred square feet, and water it well. Then, in the spring or fall, apply a 25-10-10 fertilizer, also following the manufacturer’s directions.
It is important that you water the soil very well after applying the limestone because if the soil is too basic the leaves will turn yellow, a process called chlorosis .
Other Factors to Consider
The color of your hydrangea bushes will not change right away. The process can sometimes take weeks or even months before you notice any difference. It is also easier to change blue flowers to pink than the other way around.
Garden experts also recommend waiting until the plant is at least two years old before attempting to change the color, to give it time to recover from the shock of its original planting.
Finally, hard water, which is water with high mineral content, can also turn blue flowers to a more pinkish hue, so whenever possible, use rainwater to water your hydrangeas .
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