Rhododendron renaissance: Holden Forests & Gardens using research to improve commercial plants

© Photos courtesy of Holden Forests & Gardens

Rhododendrons have been a mainstay in gardens and landscapes for centuries across the globe, with thousands of commercially available varieties on the market.

They’ve also been the subject of decades of research at Northeast Ohio’s Holden Forests & Gardens (HF&G), one of the nation’s largest arboreta and botanical gardens.

HF&G has a deep connection to rhododendrons, having curated a collection of plants since 1940. The Holden Arboretum is also home to the David G. Leach Research Station, named for the horticulturist and internationally renowned breeder of hybridized rhododendrons. The station houses hundreds of rhododendrons developed in its breeding program.

“Rhododendrons are one of the most important plants in the history of humans,” said Juliana Medeiros, Ph.D., lead scientist of the plant ecophysiology lab at HF&G.

In places where rhododendrons occur naturally, communities have developed ways to make use of all parts of the plant in medicine and woodworking. Culturally, the flowers hold symbolic significance, sometimes appearing in folklore.

Some varieties are the first plants to bloom in spring, becoming an important food source for pollinators. Medeiros added that a good reason to grow them in a garden is simply that they’re interesting and beautiful.

Beauty has been the goal of many rhododendron breeding programs, including some work completed by Leach.

“The breeding program at the Leach Research Station started with David Leach trying to get diversity, colors and forms of rhododendrons that can’t grow in a USDA Zone 5 climate. He was very successful in doing that — we have basically every color of bloom except blue and green that exist in a garden plant that can survive the cold of our climate,” said Connor Ryan, rhododendron collections manager at HF&G. “But I think David Leach missed out on other adaptive traits.”

Ongoing research projects at the Leach Research Station aim to understand the adaptive characteristics that help rhododendrons survive and thrive in a changing climate using cutting-edge technologies like high-throughput spectral imaging.

With an infrared camera, lab staff can estimate how much water leaves use when they open their stomata to gain access to carbon dioxide. The process of capturing an image takes a fraction of a second and can be automated through the use of a robotic arm. Medeiros said that hundreds or even thousands of samples can be collected using this technique, and photos can be saved and analyzed later.

Normally, using classical plant physiology techniques, estimating how much water leaves use would take 20 minutes and would have to occur in real time, meaning that researchers are limited in how many plants they can evaluate per day.

Multispectral imaging is also utilized in the lab, a technique that can quantify plant pigments. Medeiros said there is evidence that rhododendron pigments play a role in their climate tolerance, something that is covered in three currently unpublished papers by the team.

“These types of tools are sorely needed if we want to understand variation in physiology, and breeders need to know about variation in traits to see which ones would be best for breeding, so it supports learning more information than old-school techniques,” she added.

H 

Ryan said that one of the best ways to find adaptive traits is to simply grow a lot of different plants.

“We are constantly acquiring new rhododendrons to evaluate. We have a good sense of our environment here in Ohio and how well-adapted rhododendrons grow here. So, if we have new plants, which could be wild occurring species, named horticultural selections, et cetera, and they grow well here, that’s a clue that something about the plant makes it adapted to this climate,” he said.

This investigative work can take inspiration from many sources. For example, former Leach Research Station director Stephen Krebs, Ph.D., took inspiration from professors at The Ohio State University, who began screening for resistance to Phytophthora in the 1970s, and a veterinarian in southern Louisiana who was successfully growing rhododendrons. Krebs and other researchers had a hunch that the plants’ success in the South was due to a resistance to root rot disease.

“By repeatedly exposing (the veterinarian’s) plants and our hybrids of similar pedigree to the root rot disease pathogen, as well as growing them in more adverse conditions here and comparing them to commercial standard rhododendrons, we can show that the trait is of value,” Ryan said. “(Krebs) was also able to identify other more cold-hardy plants with resistance to root rot disease by doing similar work with our huge collection here at Holden.”

The result of HF&G’s breeding program and work by Krebs is the recently released rhododendron hybrids that demonstrate the balance between beauty and resilience.

Holden’s Fuchsia, Peach, Pink, Pink Flare, Raspberry Ruffles and Red plants are hardy to Zone 5, maintain a compact habit at 3 to 4 feet tall and wide, display flashy blooms and, importantly, have improved resistance to root rot disease.

The new line of Phytophthora cinnamomi-resistant hardy rhododendron was on display at MANTS 2025. Growers can contact Briggs Nursery for purchasing information.

“This is the first introduction of rhododendrons that have some ability to survive this disease,” Ryan said. “Most or all commercially grown rhododendrons are susceptible (to root rot disease), and this is a step forward to having some plants that can survive it over your standard rhododendrons.”

Ryan added that work is still ongoing to combine the “super, super tough” Louisiana rhododendrons with Holden plants. Additionally, HF&G is exploring resistance to the rhododendron lace bug and plants that display sun tolerance.

Atmospheric drought intolerance is a new line of work for the Leach Research Station, with an experiment slated for this summer.  Previous work suggests that low relative humidity caused by drought and heatwaves will be the most critical threat to rhododendrons under climate change, Medeiros said.

“We believe this could arise due to their shallow rooting behavior combined with low water transport wood, so the problem is not just one trait, but how all the traits of the plant work together. This idea is the crux of our work — realizing that a plant can have a very nice leaf that looks pretty and is well-suited to climate change, but if we can’t get the other traits of the plant to change and support this nice leaf, the plant will just be dead because selection acts at the level of the whole plant,” Medeiros said. “This is relevant for breeders because it helps us understand how suites of traits are correlated and the possibility that you could breed for a particular type of climate tolerance.”

Rhododendrons that demonstrate resistance to disease and have other resiliency factors are important for growers and consumers alike. Growers are less likely to lose plants in production and use fewer resources to combat challenges in the face of environmental change. Growers can also appeal to consumers who are interested in buying a more reliable product with unique flowers.

“There are a lot of people who have soured on rhododendrons because they’re hard (to grow and maintain), and I think this is opening up the idea of a new renaissance for people to try again,” Medeiros said. “These are plants you can have for your entire lifetime and pass them on to your children because they’re that long-lived. You don’t want to lose a plant that lives a hundred years. I think Connor’s program is really opening it up for rhododendrons to be popular again.”

Hayley Lalchand is a freelance writer for Garden Center magazine.