So, what makes big leaf hydrangea turn blue, pink or a shade in between? The primary contributor is soil pH. Soil pH influences the pH within the plant cells.
The vacuoles within the bract cells or otherwise known as the small true flowers that look like petals are responsible for how light reflects to our eyes from the pigment anthocyanin. This pigment is a plant pigment that ranges in color usually between blue and pink and sometimes can appear cream colored or colorless when the bract cells are moving from a blue to a pink color.
The color also depends on some other factors less known but still amazingly responsible for the presence and its association with metal ions such as aluminum. Different anthocyanins have slightly different properties and color ranges. Different plant species and even cultivars within a particular species can have different properties and color range and concentrations.
Basically, big leaf hydrangeas that are blue have a larger quantity or range when cellular pH is more alkaline while they are pinker when the cellular pH is more acidic or lower.
This doesn't seem quite right since we have to add products to lower the pH of the soil with elemental sulfur or aluminum sulfate to make the bract cells turn blue and the opposite, using lime or gypsum to raise the soil pH to turn the bracts pink in color. This is because within the plant itself, the inside of the petal cells move in the opposite direction as the pH of the soil. So, in order to raise the pH within the cells of the bracts, and make that hydrangea blue, we actually lower the pH of the soil.
Lowering soil pH (soil acidification): For most plants, soil acidification is unnecessary, but lowering soil pH is frequently required to grow blueberries, azaleas, and rhododendrons successfully. These plants require an acid soil with a pH between 4.5 and 5.2.
If soil pH is above this range, it is usually desirable to lower soil pH to 4.5 prior to planting.
Over time, pH will tend to increase, especially if the water supply used for irrigation is alkaline as it is in many parts of Minnesota and in most municipal water supplies.
Elemental sulfur, iron sulfate, and aluminum sulfate are amendments that can be used to lower pH.
We can also use peat moss, which may take longer depending on the situation of the plant and the garden.
Hydrangeas are just the beginning of the flower color puzzle. Plant geneticists have been working for many years discovering how to make different colors. Using petunias for research, most researchers can now show us that a combination of flavonoids and cell acidity or pH can be explained.
Three pigments - chlorophyll, flavonoids and cartenoids - which are mixed in different proportions with each other, produce flower color.
The biochemistry of flower color is truly amazing since just one tiny change in color will make a blue petunia turn red. This information is being used for those certain colors of plants such as a true blue rose that seem so hard to achieve by simply crossbreeding plants.
The main drawback is the cost that is involved with discovering the hidden color key.
For more information about gardening, you can email me at Stephanie@starpoint.net