How to Manipulate Hydrangea Color
Manipulating the color of your hydrangea blooms through use of specific elements is one of the most intriguing aspects of hydrangea gardening. Although seemingly magical, the chemistry of your soil gives a somewhat predictable measure of hydrangea color change.
Hydrangea flower clusters can vary in color in shades of blue hydrageas, pink hydrangeas, reddish-wine hydrangeas, purple hydrangeas, mauve hydrangeas, and several shades in between. Flower clusters of the hydrangea macrophylla are available in two forms: the garden hortensias or “mop-headed” hydrangea types and the lacecap hydrangea with flat flower heads. The hortensias consist of almost entirely sterile flowers that are arranged into pom-pom like hydrangea balls. Lacecap hydrangeas, on the other hand, have a flat center made up of fertile flowers and larger, colorful sterile hydrangea flowers on the outer edge.
The names of many hydrangeas (e.g. ‘Nikko blue’ and ‘Forever Pink’) can be somewhat deceiving. A blue hydrangea given certain conditions will produce pink or mauve hydrangea flowers, while a pink hydrangea given the right nutrients will be blue or purple hydrangea.
There are two factors that affect flower color of hydrangeas: the amount of light the plant receives and the pH of the soil. Although hydrangea macrophylla can tolerate full sun, they will prefer and thrive in partial shade. Shade helps maintain the individual hydragea flower petals, keeping them from burning in the hot afternoon sun. If grown in sufficient shade, the hydrangea flowers will look good all summer long and will begin changing texture and color in the fall..
It is also interesting to note that varying degrees of shade will also affect flower hydrangea color. Two identical hydrangea plants, planted right next to each other, can have different flower tones relative to the degrees of shade/light they receive.
The main influence that affects flower hydrangea color is soil pH, which is a measure of the soil’s acidity. The pH scale is from 0 to 14. Seven is the midpoint and is neutral. PH levels above 7 indicate alkalinity, while those below 7 represent increasing degrees of acidity. The average garden soil is slightly acidic in the range from 5.5 to 6.5. The pH range of 4.5 to 7.5 is not uncommon in garden soils. Blue hydrangeas require a highly acid soil, while pink hydrangeas are produced in a neutral or slightly acid soil.
Again, even the variety ‘Nikko blue’ hydrangea if planted in a neutral soil will produce pink hydrangea flowers. At a pH of 4.5, the color will be deep, vivid blue hydrangea, but as the pH scale goes upward toward neutral, the color will begin to change from blue to purple, from purple to mauve, and from mauve to pink.
A starting point to manipulating hydrangea flower color is to know exactly what you soil pH is. Your local cooperative extension service or garden center may be able to instruct you on this procedure. The results of this test will let you know exactly how acid or alkaline your soil is. If you’re growing hydrangeas in two different soils, you may want to check each soil type for varying pH levels.
If you have a blue hydrangea, and would it to have lavender to pink hydrangea flowers, you need to raise the pH or alkalinity in your soil. Adding lime to your soil can do this. Aluminum is the element that is responsible for the blue hydrangea color. Aluminum, though, becomes more unavailable to the plant as the soil pH becomes closer to neutral. By adding lime to your soil, your soil becomes more alkaline, and hence the color of your hydrangea can change from blue to varying degrees of pink. In general, 4 ounces of lime around the base of your hydrangea plant can change the pH of your soil by one point.
You can add lime in four-ounce increments until you reach the color of pink you desire. You need to try and raise your pH to 6.5. Depending on your soil texture and how acid your soil was to begin with, this can be a long process, taking up to 2 growing seasons if you have an extremely acid soil high in organic matter. Adding nitrogen and phosphorus also aids in preventing the absorption of aluminum and gives a good, clear pink hydrangea color.
Conversely, if you have a pink hydrangea and would like to make it blue or purple, you need to lower the pH to make the soil more acid. This can be done by adding aluminum sulfate to the soil around the base of your plant. Aluminum sulfate is available at most garden centers. Follow the label directions carefully and don’t overdo it. In addition, you do not want to add nitrogen or phosphorus to your soil, as these elements help promote pink colors. Since many general-purpose fertilizers contain these elements in large amounts, it is better to add single-element fertilizers such as muriate and potash.
Remember that hydrangea color changes will not occur overnight, and are sometimes not predictable. The plants genetics and soil both play a part in manipulating its flower color. It can be very exciting watching the results of your soil amendment and the degrees of color change.