What Color Should the Hydrangea Flower Be?
Dick Bir
NC State University
Hundreds of cultivars of bigleaf hydrangeas exist. Each was selected because it had a unique set of characteristics which often include flower form and color. Whether a cultivar was selected for large flowers or an abundance of small slowers, fringed, flat or cupped sepals, as a mop head or a lace cap, if it is a bigleaf hydrangea then flower color is determined by the same set of rules.
In order to have color, pigment must be present. If a cultivar possesses no pigment it will be white. If it contains very little pigment, then the color is likely to be a pale pink or a pale blue but that depends on aluminum. This is why you may see a common cultivar like 'Nikko Blue' for sale in a garden center with pink or lavendar flowers. There is no aluminum in the potting soil but, if conditions are right, by the second year after establishing this same plant in your landscape you will have blue flowers foreverafter.
The pigment involved is the same in pink and blue hydrangea infloresces. It an anthocyanin named delphinidin 3-monoglucoside. When aluminum is present in the sepals . . . what we usually call hydrangea petals . . . it binds with the pigment and a co-pigment. The sepal color changes to blue when aluminum is present to bind with the pigment and co-pigment. The intensity of the color depends upon how much pigment the particular cultivar contains as well as how much aluminum is available to the plant. More pigment and more aluminum means a deeper blue flower like the ones on 'Mathilda Gutges.'
Seems like it should be simple, right? However, if you garden in an area with abundant natural aluminum in soils like I do, once the hydrangea roots are in the soil they are taking up the aluminum and cultivars purchased to be red or pink will flower blue, lavendar or purple. Therefore, you need to either not plant these hydrangeas into the native soil or find some way to limit the availability of aluminum to the plant.
The most common ways to limit aluminum availability in soils is to increase soil phosphorus levels or to lime to increase pH. If the available aluminum is below 15 ppm, plants are not likely to absorb enough aluminum to keep them from being pink. However, applying excessive levels of phosphorus to soils can limit the availabilty of other essential nutrients like iron. That's unfortunate because liming to raise the pH also limits the availabilty of iron so we often see chlorotic bigleaf hydrangeas. This is because they are growing where soil pH is too high either naturally or because soils have been overlimed.
Explanations beyond this get even more complicated. However, bigleaf hydrangeas are relatively easy to grow and have flower the color you want if you know the rules.
Pink Rule 1: If you have lots of aluminum in your soils, which most Easterners do, do not plant cultivars you want to be red UNLESS you lime and add the same amount of superphosphate or rock phosphate you would when planting a hybrid rhododendron.
Pink Rule 2: If Rule 1 does not work, grow your plants in containers or in beds of organic potting soil and sand.
Pink Rule 3: Be ready to apply extra iron if new leaves turn yellow. Older leaves turn clear yellow with age on many cultivars and are nothing to worry about.
Blue Rule 1: Use organic amendments like pine bark when you plant. If you have plenty of aluminum in your soil, flowers will be blue regardless of whether the cultivar label picture was pink or blue.
Blue Rule 2: If your bigleaf hydrangea flowers pink the second year the plant roots are in the ground (after roots have grown into your soil), and the bush looks otherwise healthy, you need aluminum and may need to lower the soil pH. Before trying to lower soil pH apply an aluminum solution. The most common solution is 1/2 ounce of aluminum sulfate in a gallon of water that is applied from the time the flower buds form until they are fully open. Simply water the plant with the solution. How often you need to do this will vary with every garden and cultivar but after a few weekly applications, wait to see how the plant responds. You can get too much of a good thing. Aluminum sulfate will acidify soils as well as providing aluminum so be careful. Some medicines need to be taken over a long period of time in low doses not to harm the patient.
Return to Richard E. Bir homepage
North Carolina State University