From Michael Dirr: Strategies for growing blue hydrangea — past, present and future

Plant (2007 to 2015) and Premier Introductions, Inc. (2020 to present) (PII), bred and introduced numerous H. macrophylla, including the well-known Endless Summer series. The plants are grown in bark medium which contains no aluminum; hence, all seedlings develop pink flowers (see photo 1). When making selections, there was no way to assess the degree of blue/purple in the initial year. This had to wait until the second year after aluminum treatment.

Consistently inconsistent

Through the years, PII used commercial bluing agents like Blue Max (slow-release aluminum product) and aluminum sulfate (granules). These are applied to the surface of the growing medium or soil and thoroughly watered. Material is applied in the dormant season and to plants in various stages of active growth, including up to flower bud formation. To date, none of the above strategies provided consistent results. Photos 2 and 3 (taken June 6, 2022) show the results of Blue Max (70 grams) on 2021-22 selections treated in January. The only consistency was the inconsistency of bluing. Several selections remained deep rose or red.

PII has applied aluminum sulfate with reasonable results although “complete” bluing took considerable time. Hydrangea macrophylla Azure Skies liners were transplanted July 27, 2018, 2 liners per container, 5 replicates per treatment, cut back in January 2019, treated with either 60 grams Blue Max + 42 grams aluminum sulfate (left row, photo 4) or 60 grams Blue Max (right row, photo 4). I thought the additional aluminum sulfate plus the Blue Max would accelerate/increase/unify bluing. The 42-gram rate was based on Dr. Jim Midcap’s successful studies bluing remontant H. macrophylla cultivars at the Center for Applied Nursery Research, McCorkle Nurseries, Dearing, Georgia. The PII plants were similar in quality and flower color. No phytotoxicity occurred with the combination treatment. Azure Skies with either treatment was not uniformly blue (photo 5).

Unraveling the mystery

Dr. Henry Schreiber and graduate students at Virginia Military Institute, Lexington, Virginia, unraveled and clarified the mystery of H. macrophylla pigment chemistry. One of the research findings forever embedded in my brain is the following. Cultivars contain different concentrations of delphinidin-3-glucoside, the pigment that complexes with aluminum to blue sepals. Eventually sufficient aluminum is absorbed, translocated to the sepals (throughout the entire plant), and complexes with the pigment. Increasing the aluminum in the sepals will not make them darker blue once the threshold concentration (available delphinidin-3-glucoside) is reached/complexed. Schreiber reported that cultivars like Endless Summer The Original, David Ramsey’, and ‘All Summer Beauty’ require less aluminum for maximum bluing. ‘Marechal Foch’, Hamburg’, and ‘Kardinal’ require more to produce the deep blue/purple color. At PII, Rock-n-Roll was treated the with similar bluing agents, rates and times as Azure Skies discussed above. Rock-n-Roll never reached maximum bluing and sepal coloration was variable with both treatments.

For a deep dive into the mystifying intricacies of Hydrangea macrophylla pigment chemistry, see Schreiber’s research papers in American Scientist 102: 444-451 (2014) and BioMetals 24(6): 1005-1015 (2012).

Incorporating new research

In early January 2020, 40 and H. serrata cultivars, each with four replicate plants, were pruned to 6 to 8 inches. Two were untreated and two treated with 60 grams Blue Max on Jan. 12, 2020. The photos of the cultivars were taken between June 1 to 15, 2020. All cultivars reacted to the aluminum sulfate in Blue Max with degrees of blue/purple. ‘Mme. Emile Mouillere’, the best legacy white mophead, showed the slightest tinge of blue in the sepals, indicating a micro concentration of the delphinidin-3-glucoside. This bluing is clearly manifested in the stamens in the center of the sepals when aluminum is present.

In 2020, the new Premier Introductions, Inc. (PII), after a 5-year hiatus, was back in the breeding business. The first seeds were sown on Jan. 1, 2020, using open-pollinated material of Hydrangea and other genera. From the H. macrophylla seedlings, 20 remontant/reblooming selections were made in summer-fall. Again, all were pink as discussed since plants were grown in pine bark medium without aluminum.

In 2021, the 20 hydrangeas were forced (brought inside Dec. 28, 2020; flowered late April-early May) in the greenhouse, treated with 70 grams Blue Max, with variable degrees of blue/purple. We wanted to determine the quality of blued flowers with an eye to taking cuttings and increasing numbers of the best. However, we were never convinced the plants reached their full genetic blue/purple coloration.

Could a solution be the solution?

The true colors were never revealed so from semi-desperation, a new strategy spontaneously surfaced. Back to Dr. Schreiber’s work to reconstitute an aluminum solution. He demonstrated how to change the sepals blue by soaking cut flower stems in an aluminum-based solution. The solution with four major components (440 ppm aluminum) and balanced for pH was sprayed on ~50 to 75% developed inflorescences. Within five days, sepals were fully blue/purple. A beautiful white selection (Twist-n-Shout-14-20) offered promise, but when sprayed with the solution, it turned light blue. Could this be the transformational aluminum-based material to reliably blue hydrangeas? PII progressed into the 2021 growing season with ~10,000 plants on the ground and the idea of commercially developing the spray solution was abandoned. Hydrangea macrophylla and H. serrata hybrids constituted 3,745 of that number. From these, ~45 tier 1 and ~45 tier 2 were selected. Both were treated with Blue Max as discussed in the introduction. Photos 14 and 15 show the “blued” tier 1 plants.

Several of the tier 1 plants are now in commerce or under evaluation/trial. Rock-n-Roll × Azure Skies-01-22 (Blushing Blues) is among the most reliable and prolific rebloomers with habit and foliage similar to Endless Summer Original. On October 9, 2024, ragged, leftover plants used for cutting stock were still flowering. PII decided to rethink the spray bluing study but with a simplified/pragmatic, potentially commercial approach. Plants were moved into the greenhouse on Nov. 11, 2024. There were five 3-gallon plants per treatment, control, aluminum sulfate + proprietary material, or aluminum sulfate. The granules were applied to the surface and watered into the media. Within two days slight bluing was evident. After four days the entire flower was blue. There was no color change with the aluminum sulfate treated plants. Within two weeks the combination treated plants were all blue, even the developing flower buds showing blue. No blue color yet on the aluminum sulfate treated plants. The take home inference is that the combination treatment accelerated the uptake of aluminum, otherwise the rapid bluing would not have occurred.

To date, through trial, error, and measures of success, PII learned that bluing Hydrangea macrophylla and H. serrata is convoluted and never absolute, no matter the material utilized. The absolute requirement is the absorption of aluminum via the roots and translocation to the sepals. Based on the 2024 study, the proprietary material combined with aluminum sulfate, when surface applied, is absorbed quickly resulting in effective bluing. Cultivar responses, application rates, time of year, stage of plant growth, and phytotoxic effects need to be determined.

For more than 40 years, horticulturist, breeder and author Michael A. Dirr has impacted the green industry through research, teaching, books and plant introductions.