The Story of Reblooming Hydrangeas: An Interview with Michael Dirr, Ph.D.

This article appeared in Garden Gateways, Spring 2019. The text appears below the article in a more readable form.

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The Story of Reblooming Hydrangeas: An Interview with Michael Dirr, Ph.D.

The Watkinsville Garden Club's Annual Plant Sale will feature 15 different varieties of hydrangeas grown in collaboration with Dr. Michael Dirr.  The sale will be held on May 4, 2019, during the Oconee Resource Council's Garden Tour.  The home garden of Michael and Bonnie Dirr will be one of the featured gardens.  More information can be found at www.watkinsvillegardenclub.org.

Hydrangeas are one of the most beautiful and perhaps one of the most recognized flowers you can find. Dr. Michael Dirr, professor emeritus, University of Georgia, knows hydrangeas. Although his work in all areas of horticulture has won him well-deserved admiration, his work with hydrangeas has particularly impacted the hydrangea nursery industry.

“People buy hydrangeas when they are in flower, and then a year or two later, they don’t flower,” Dr. Dirr explained. “Largely this is because of the cold. Hydrangea macrophylla develops flower buds in late summer and fall. Low temperatures in winter or spring freezes often kill the flower buds. The genetics of these French/heirloom cultivars do not allow for rebloom on new growth of the season.”

Dr. Dirr grew up in Cincinnati, got his undergraduate and graduate degrees at Massachusetts and Ohio State, and taught in Illinois. “In these colder climates, flower production was erratic to nil because of low temperatures. When I came to Georgia, I had this wild idea that by doing the research I could find Hydrangea macrophylla that would rebloom on new growth. We started in the ‘90s assembling everything we could get our hands on. In the literature, we found references to remontant or reblooming hydrangeas. So we procured them, about 250 total. Unfortunately, there wasn’t much that rebloomed in Georgia.”

In 1998 on a September lecture tour in Minnesota, Dr. Dirr visited Bailey Nurseries. And that’s where Endless Summer® The Original® was found. “It was flowering, new flower buds, pink flowers, and some old spent inflorescences,” said Dr. Dirr. “When I asked Bailey what it was, they said they didn’t know. They had found it 13-15 years before in a St. Paul garden, and had no plans to do anything with it. Obviously, it had reblooming capability. I said I think you have the mother lode. You’ve got the genetics we need to breed with.”

Bailey Nurseries gave Dr. Dirr cuttings. He went back to Georgia where he and a master’s student, Jeff Atkins, worked with the plants.  “We couldn’t stop it from flowering. We kept it under long days, which is supposed to suppress flower formation. We ran all kinds of temperature fluctuation variations. It still flowered. We knew we had something.”

Bailey Nurseries took the hydrangea back and continued to work with it. They patented it and commercially released it in 2004.The impact of this single trait, reblooming, has changed the macrophylla market forever. Bailey has sold over 27 million of the Endless Summer® brand since 2004.

Dr. Dirr continued to think about what could be possible. “What if we cross this new discovery (Endless Summer®) with traditional cultivars? Can we experience gene flow from the reblooming into the non-reblooming varieties and into the progeny so that we can produce hybrids with reblooming capability?

After years of hundreds, perhaps thousands, of trials with what seems an infinite number of combinations, Dr. Dirr and his team introduced 15 new cultivars, including the reblooming ‘Blushing Bride,’ Twist-n-Shout®, BloomStruck® and Summer Crush®. “It was a matter of good luck that I stayed another day in Minnesota to visit Bailey Nurseries. They were going to chuck the plant. Another set of eyes saw it and the mother lode was discovered.”