Heat resistant hydrangea introductions give gardeners the confidence to push forward

The historic heat dome of June 2021 left homeowners panicked about their crispy hydrangeas and wondering how the plant can thrive as climate change continues to bring unprecedented weather events. Even before last year’s record-breaking temperatures, breeders saw the effects of climate change and began selecting for better heat and
cold tolerance.

“More extreme weather events and fluctuations are what often hurts plants,” said DeVonne Friesen, a founder of Bloomin’ Easy and vice president of Van Belle Nursery in British Columbia. “We’re looking for plants that can handle big swings. If they wilt in intense sun and their leaves dry up, we won’t select those plants. We know the homeowner who doesn’t know a lot about plants will face those experiences and lose interest.”

Even with some disappointments, it would be hard to erase hydrangeas from the top of customers’ shopping lists. The most popular shrub worldwide is either hydrangeas or roses, depending on who you talk to. Regardless of who wins, people love their hydrangeas.

“A lot of it is nostalgia,” said Kristin VanHoose, owner of Hydrangeas Plus^® and Amethyst Hill Nursery. “Hydrangeas remind people of their grandma and grandpa. They remember their grandma in Iowa who had a big blue hydrangea in the front yard. It’s very heartfelt to grow a hydrangea. And these plants are easy to care for. You water them, cut them back in fall. Maybe fertilize; maybe not. Put them in the right location with some shade and they perform June through October.”

It’s also one of the most versatile shrubs, taking a place in almost any style of garden in myriad ways, including in mixed borders, as hedges or specimens and in containers. According to VanHoose, maintenance is minimal, they take full sun, and there’s one for every region.

“It’s no wonder hydrangeas are one of the most popular genera,” said Georgia Clay, new plants manager for Monrovia. “Few plants can grow from California to Florida and up to Canada. It has a huge range.”

The new breed 

The story of modern hydrangea breeding began with a chance discovery by Dr. Michael Dirr, a well-known woody plant breeder and professor emeritus at the University of Georgia. It has been told many times, but bears repeating in an article about hydrangeas.

Dirr was riding around Bailey Nurseries’ growing grounds in Minnesota in 1998, when his well-trained eye spied a hydrangea that was flowering on new buds. It was September, not the time for new blooms. 

“He stopped the car and said, ‘What is this?’” said Alec Charais, chief marketing and product development officer for Bailey. “Before you know it, the Endless Summer^®  brand was born.”

Hydrangea macorphylla ‘Bailmer’, known as ‘Endless Summer’, was the first repeat-blooming bigleaf hydrangea that would change the world. In 2004, the famous blue-potted hydrangea was introduced by Bailey, which can brag of selling over 27 million plants. Four additional varieties —– BloomStruck^®, Twist-n-Shout^®, Blushing Bride^® and Summer Crush^® —– have joined the Endless Summer line. 

Unlike other mophead hydrangeas, ‘Endless Summer The Original’ flowered on old and new wood, stretching the bloom season substantially. Martha Stewart caught wind of the plant, put it on TV and the world quickly caught on. And so did breeders. Since then, most new H. macrophylla introductions are rebloomers.

The five hydrangeas that rank as the most popular in the 50-species genus are: H. macrophylla (mophead or bigleaf), followed by H. serrata (mountain), H. paniculata (panicle or peegee), H. quercifolia (oakleaf) and H. arborescens (smooth). H. macrophylla is the top seller by far,  perhaps because they’re in bloom when people are most likely to be at the garden center, according to Clay.

H. paniculata withstands cold better than H. macrophylla, usually down to USDA Hardiness Zone 3. Bigleaf hydrangeas hover at Zone 5, though hardier varieties keep appearing. Panicle hydrangeas also deal with heat better than other hydrangeas. They send out robust, cone-shaped inflorescences ranging from 6 to 12 inches that continue to bloom into the dog days of summer when many other shrubs have long given up. 

There are other good traits: The shrubs flower on new or current season’s growth, so late-winter pruning doesn’t affect blooming. Butterflies and bees are bountiful pollinators.

“The one that will survive everywhere is paniculata,” said VanHoose, who grows 340 hydrangea varieties. “It’s hardier than other hydrangeas and deals with heat better. Plus, it’s low maintenance.”

The most famous H. paniculata cultivar has to be ‘Limelight’ (H.p. ‘Limelight’ PP12874), a sturdy, heat-tolerant shrub introduced to the market by Proven Winners in 1988 to great fanfare. With a statuesque size (6–8 feet tall and wide), exquisite green flower color and tough-as-nails reputation, ‘Limelight’ stands up to its hype and is one of the most popular hydrangeas in the world.

“It’s a banner hydrangea,” said Natalie Carmolli, media and public relations specialist for Spring Meadow Nursery. “‘Limelight’ is like the Q-tip^® or Kleenex^® of the hydrangea world. It’s a household name. People love those great big green blooms.”

Smaller versions, also adapted to hot sun, have come down the pike at Spring Meadow, including Little Lime (H.p. ‘Jane’ PP22330), Limelight Prime (H.p. ‘SMNHPPH’ PP32511), and Little Lime Punch (H.p. ‘SMNHPH’ PP33207), all topping out under 6 feet with Little Lime Punch growing just 3-5 feet. All have the same stop-in-your-tracks green flowers that age to varying colors of pink and red.

Bobo (H.p. ‘ILVOBO’ PP22782), a Spring Meadow introduction that joined ‘Limelight’ as a winner of the prestigious Pennsylvania Horticultural Society Gold Medal of the Year in 2021, is another compact panicle growing to only 2½–3 feet tall and wide. Hardiness is ranked at Zone 3, and it deals well in the heat. 

“Climate change is causing us to think more about working on finding plants better adjusted to warm climates or arid climates,” Carmolli said. “And more fire-wise plants that are less likely to ignite.”

A hybrid hydrangea, Fairytrail Bride^® Cascade Hydrangea^®, (Hydrangea × ‘USHYD0405’ PP31120), is quite heat- tolerant and deserves special attention, she said with enthusiasm. It’s another award winner, receiving the coveted Plant of the Year at the Chelsea Flower Show in 2018, and showing its value with long, trailing stems with bunches of white, lacy florets at every leaf node for blooming top to bottom. The lacy-looking blooms age to pink as fall arrives. Not so cold tolerant, though. It’s a Zone 6 plant. ‘New on the market this year, it was one of only eight hydrangeas to earn top rating in the Chicago Botanic Garden H. paniculata 10-year trial.

Though H. paniculata is sturdier and hardier than H. macrophaylla, there’s plenty of breeding going on to improve macrophayllas, too. ‘Bloomstruck’ and ‘Summer Crush’ are two from Bailey. 

“Both have nice, thick, waxy foliage that allows them to tolerate the oppressive heat we see in summer,” Charais said. “They were standing up nice and perky in 90-degree heat. I have firsthand experience with these plants and understand what the consumer wants., These plants are absolutely my favorites from hands-on experience.”

Elizabeth Ashley^™ (Hydrangea macropyhlla ‘Hokomarore’ PP31264) is a H. macrophylla bred in Europe and named after the late daughter of a breeder friend of Tom Foley, director of new plants and production for Everde Growers. Foley described the plant as having “great big rich, dark pink flowers with a big petal count on a reblooming 3-by-3 shrub. 

“We’re always looking for that size.” Foley said. “It’s perfect for the yard and it does well in Southern California, as well as the Northwest. It’s fantastic.”

As for H. paniculatas, Foley spoke up for ‘Sweet Summer’ (H.p. ‘Bokrathirteen’ PP21778), coming to the market in 2023. The shrub is very upright to 5 feet with heavy blooming clear white flowers that age to a soft, shell pink in late summer and fall. Stiff stems ably hold up the elegant 12-inch-long blooms.

Charais of Bailey has his favorite H. paniculatas, too. His list starts with Little Hottie^® (H.p. ‘Bailpanone’ PP32549), a brand-new compact variety withstands temperatures down to Zone 4 and maybe 3, and White Diamonds^®, (H.p. ‘HYPMAD I’ PP19082), an underappreciated hydrangea that starts out green and then turns bright white. Large flowers stand up beautifully on both shrubs and they both handle heat well as demonstrated in Bailey’s Georgia trials, which were done specifically to rate heat tolerance. Across the county, ‘White Diamonds’ did incredibly well in both heat tolerance and cold tolerance. 

Wintering well 

While flower size, color and sturdy, compact plants are key for breeders, winter hardiness ranks as one of the top-tier traits breeders consider. As climate change nudges temperatures up there’s the risk the plants won’t get the cold that brings on the vernalization that allows flowering.

“Hydrangeas require vernalization — 600 hours of cooling,” Clay of Monrovia said. “Warmer winters are an issue of climate change. You can mitigate drought with water and shading, but cooling you can’t fake.” 

At Monrovia, excitement surrounds the Seaside Serenade^® collection. The plants are tetraploids —– with thicker leathery foliage that stands up to heat better —– and their compact size makes them appropriate for today’s smaller spaces. Eleven hydrangeas make up the collection, including pink-and-white Fire Island (H.m. ‘HORTFIRE’ PP29058)’, dark pink ‘Martha’s Vineyard (H.m. ‘HORTMAVI’ PPAF)’ and the newest, Cape Cod (H.m. ‘HMUPSI’ PP28974), a classic blue hydrangea. 

“Because we’re coming out with new ones, it’s fueling growth,” Clay said. “Hydrangeas are very versatile and they’re only getting better and better. People are having more success. They are only growing in popularity.”

The Bloomin’ Easy brand added three hydrangeas to the collection this year: Torch^™ (H.p. ‘HPOPR018’ PP32972), Flare^™ (H.p. ‘Kolmavesu’ PP26928) and Candelabra^™ (H.p. ‘Hpopr013’ PP27472). Friesen from Van Belle said easy-care paniculatas hold their flowers up on stout stems emerging from compact plants that need little pruning.

“What I love about these plants is that without knowing how to prune, you can cut them back a foot every year and they grow into a perfect vase,” Friesen said. “No matter the weather, they stand up straight. It grows in a very behaved manner.”

Friesen couldn’t hang up without mentioning Pink Dynamo^™ (H. serrata ‘JPD01’ USPP33412), a high-performing hybrid with both H. paniculata and H. serrata in its parentage. Its lineage contributes to its hardiness and has incredible garden performance. Harsh weather didn’t stop it —– the plant came through with flying colors. At only 2–-3 feet tall and wide, Pink Dynamo^™, a Zone 5 plant, really stands out in a mixed border, container or in mass plantings to show off flowers edged in a pure pink with white and yellow accents near the center. 

VanHoose suggested heat-tolerant H. macrophylla ‘Miss Saori’, winner of the highly sought-after Chelsea Flower Show Plant of the Year in 2014, as well. ‘Miss Saori’ is clothed in dark burgundy new foliage and flaunts unique double white blooms with bright pink petal edges on a 3-by-4-foot shrub that’s hardy to Zone 5.

‘Limelight’, another top performer in the Chicago Botanic Garden, may be the hydrangea most associated with Spring Meadow, but there are so many more. The huge blooms of ‘Incrediball^®’ and ‘Invincibelle^®’ hydrangeas have been incredibly successful and spawned several offspring, including Invincibelle Lace^® (H.a. subsp. radiata ‘SMNHRLL’ PP33290), a new H. arborescens with the first plum-purple lacecap flowers. Because it’s a native, this 4-by-5-foot shrub is especially tough and thrives down to Zone 3. Also in the collection, Invicibelle Spirit^® (H.a. ‘NCHA1’ PP20765), which supports the fight against breast cancer with $1 per sale going toward conquering the disease, is an improvement over the original with darker foliage, stiffer stems and richer pink flowers. 

Carmolli, Spring Meadows’ public relations specialist, is partial to the new Let’s Dance^® series, hydrangeas crossed between macrophylla and serrata. Let’s Dance ¡Arriba!^® (H. × ‘SMNHSC’ PP33206) takes after it’s mophead parentage with purple or pink flowers depending on soil pH, while Let’s Dance Can Do^® (H.s. ‘SMNHSI’ PP32548)!’ looks more like its serrata side. 

Since there’s serrata in the breeding, the Let’s Dance collection is a tough hydrangea, growing down to Zone 4. Carmolli considers them super reliable. They bloom from top to bottom so they’re just a ball
of flowers.

“It’s an exciting time in the world of plants, in the world of hydrangeas,” Friesen said. “Every time a person has a good experience with a hydrangea, it’s good for the whole industry.”