David C. Zlesak and Gail Soens, University of Minnesota Extension Educator and Bailey Nurseries New Variety Coordinator / Section Grower Bud and Bloom Hydrangeas & Roses
With so few true blue flowering shrubs for our landscape, it is no wonder so many of us are drawn to the beauty of blue hydrangeas! Only one species of hydrangea we commonly see for sale and in our northern landscapes include cultivars that can be coaxed to bloom a true blue or, if desired, a pure pink. It is Hydrangea macroplylla which is also known as the bigleafed hydrangea. Endless Summer® is the most common cultivar of this hydrangea we see for sale in the north as it is able to bloom off of both old and new wood. This is unlike most other H. macrophylla cultivars which bloom on only old wood. Since this species is marginally stem hardy in zone 4, having the ability to bloom on new wood allows it to still flower in our climate even if the plant dies to the ground and needs to regrow from the base.
So, what makes bigleaf hydrangea turn blue , pink, or a shade in between? The primary contributor is soil pH. Soil pH ultimately influences the pH within the plant cells. The critical part of the plant cells for pH and color are the large watery vacuoles within the bract cells (colorful portion around the small true flowers that look like petals). The pH in the vacuoles influences the configuration and how light reflects to our eyes from the pigment anthocyanin. Anthocyanin is a plant pigment that ranges in color typically between blue and pink and can appear cream or colorless in the transition between blue and pink. The color the tissue appears depends on the pH of the cellular fluid and also other factors including presence and association of metal ions such as aluminum. There are multiple anthocyanins and they all share a common core chemical structure and are distinquished by placement of side chains and sugars connected to the core structure. Different anthocyanins have slightly different properties and color ranges. Different plant species and even cultivars within a species can have a different types of anthocyanins and different relative concentrations. Anthocyanins generally appear more towards the blue end of their range when the cellular pH is more alkaline (or higher in number) and more pink when the cellular pH is more acidic or lower.
What??? Doesn't this seem counterintuitive and contradictory? We have been told to lower the pH of the soil we have our hydrangeas planted in with elemental sulfur or aluminum sulfate to make the bracts turn blue and the opposite with lime or gypsum to make them turn pink. Interestingly, the pH within the vacuoles of petal cells moves the opposite direction as the pH of the soil. So, in order to raise the pH within the vacuoles of bract cells and make our hydrangeas turn blue, we actually lower the pH of the soil around our plants and visa versa for pink.
Bailey Nurseries, the introducers of Endless Summer® produces tens of thousands each year and most of them are grown and sold with blue flowers. There are actually three different cultivars in the series now and they all share the ability to bloom on both old and new wood. There is Endless Summer® original, Endless Summer® Blushing Bride, and now the latest addition, Endless Summer® Twist and Shout. For the Endless Summer® original being coaxed to turn blue, the flowers first appear white and then turn blue from the outside of the large colorful bract inward towards their middle until the whole bract is blue. A question arose if it would be possible to make the petals color up and turn blue sooner (being white for less time). Doing so would reduce production time. In order to work towards answering this important question, we started by doing a series of bract pH tests to understand if bract pH is consistent during expansion and coloration or if it changes and is the trigger or somehow associated with when the bract can turn blue.
We collected mature and immature heads of Endless Summer ® original from plants grown to be pink and other plants being grown to turn blue. We also collected heads of Endless Summer® Blushing Bride (it is a cultivar that has a little bit of blush to it and is stays primarily white). Throughout our bract pH tests, we took a uniform amount of bract tissue and put it in distilled, deionized water. We spun it at 14,000 rpm in a centrifuge. This very fast motion broke the cells and allowed the liquid content within cells to mix with the purified water. After spinning in the centrifuge it was amazing to see the bracts holding their form, but appearing clear or white no matter what color they started as. The solution then appeared the color of the bract before we spun it. We tested the pH of the solution with a standard pH meter.
We sampled fully expanded pink and blue bracts of Endless Summer® original. The solution from the blue bracts had a pH of 6.10 and the pink bracts 5.94. We also took immature florets of Endless Summer® original that were white at the base and their tips were starting to turn blue. We cut a number of small florets separating the white bases from the blue tips and measured the pH of bract tissue of these different colors separately. The white bases were a pH of 5.59 and the blue tips were 5.84. Therefore, for the Endless Summer® original grown to be blue with low soil pH, we saw a clear transition in pH from 5.59 for the white base of an immature bract to 6.1 when the bract was fully blue and mature. We didn't explore all the same stages and color transitions for Endless Summer® original grown to be pink. This is something worth pursuing in the future. It is interesting the pH of a mature pink floret was 5.94. This strangely happens to be a higher pH than that of the blue portion of a bract as it first began to turn blue (5.84).
We also decided to look at the pH in Endless Summer® Blushing Bride. This hydrangea does not turn blue or pink even when pH is altered. It likely produces very little anthocyanin in general and therefore basically remains cream to white regardless of pH. The sample taken from young expanding bracts of Endless Summer® Blushing Bride were a pH of 5.82 and the fully expanded florets were 5.96. A trend of an increase in bract pH was also observed in Endless Summer® Blushing Bride as it matured. The pH of a mature bract of Endless Summer® Blushing Bride was intermediate of that of mature bracts of pink (5.96) and blue (6.1) Endless Summer® original.
In the end this initial experiment helped us learn that there is a strong and clear association with blue bract color and bract pH in Endless Summer® original. The blue color seems to go hand in hand with an elevated pH as it starts out white, begins to turn blue, and then matures as a stronger blue. A subsequent question we would like to answer is if anthocyanin is already present in the white bract tissue and cannot turn from white to blue until the petal pH is further elevated, or if anthocyanin is just not present in the white tissue yet and is synthesized at the point in bract development when we typically begin seeing blue coloration. If the latter is true, it is likely that we cannot speed up blue coloration of bract tissue in Endless Summer® original. The limiting factor in that case won't be the pH, but when anthocyanins are synthesized.
The next steps in this study would be to directly test for anthocyanin pigment in different aged bracts to learn when it is synthesized as well as to possibly do some simultaneous tests growing Endless Summer® original hydrangeas in a range of different low soil pH's. Growing plants in a range of different low soil pH's would be the most direct way to see the effect on coloring rate of florets based on soil pH and also to learn the effect of such pH levels on overall plant growth.