By Don Lyman
The Boston Globe August 13, 2020
BU research technician Matt Rothendler in front of rose mallow plants growing in the Charles River in Dedham in August 2019, during a survey of color forms. RICHARD PRIMACK
On a warm, humid August morning I followed Richard Primack, a biology professor and plant ecologist at Boston University, down a steep, brushy trail, then across a flat muddy area adjacent to the Charles River in Dedham.
As I looked down at my previously white sneakers sinking into the mud, Primack stopped and suggested we pause for a moment to take in the beauty of the scene that unfolded in front of us: hundreds of wild rose mallows (Hibiscus moscheutos), 6 to 7 feet tall, with broad, heart-shaped leaves, covered with large pink and white flowers up to 6 inches wide.
“Look at that flower!” Primack exclaimed, pointing out a large white blossom with a bright red center. “This is to die for.”
The big, colorful, tropical-looking blooms seemed almost out of place in suburban Boston. In fact, many members of the genus Hibiscus are tropical flowers, but some, like the rose mallow, occur in temperate areas of North America.
“It looks like a garden,” said Primack, “but it’s wild. Gorgeous flowers, the envy of any garden, but they’re growing here along the river. You won’t see a more spectacular display of wildflowers anywhere than along the Charles River right now.”
The rose mallow, also known as the swamp rose mallow, is a native plant that occurs in eastern North America from southern Ontario to Texas, east to Florida, and north to southern New England. It is one of the largest wildflowers in North America. In Massachusetts, it occurs primarily in the eastern part of the state in salt and freshwater marshes, and along the edges of other wetlands, like rivers. When water levels are high, rose mallows can grow right out of the water, Primack said.
Rose mallows are making a comeback along some areas of the Charles River, which flows 80 miles through numerous suburbs from its source in Hopkinton to where it empties into Boston Harbor. Primack said he has seen large populations growing along the river in areas of Newton, Needham, and Dedham. One of the biggest populations of rose mallows that he knows of is along the Motley Pond section of the river in Dedham.
“I don’t remember seeing them so commonly along the river,” said Primack. “The habitats along the edges of the Charles River used to be dominated by purple loosestrife.”
Purple loosestrife is an exotic invasive species from Europe that can outcompete native wetland plants like rose mallow, Primack explained. But beetles that feed on purple loosestrife were introduced as biological controls in some areas and have knocked back the loosestrife along various sections of the Charles River.
Primack said beavers also have played a role in the rose mallow’s resurgence by removing birches and other woody vegetation, so there’s more open exposed areas, which is the perfect habitat for rose mallows.
Rose mallow flowers have five petals and vary in size and color, from white to light pink to medium pink to dark pink, Primack said.
Around 30 percent of rose mallows have a red spot in the center of the flower. Those flowers are often referred to as crimson-eyed rose mallow, said Primack.
The red markings are probably a target for pollinators, which include honey bees, butterflies, and hummingbirds, Primack explained. There’s even a bee that specializes on rose mallows and other flowers in the genus Hibiscus.
Because of the dramatic genetic variation in the flowers, rose mallows are popular among plant breeders.
“They can be bred for color, size, forward petals or wide open petals, etc.,” said Primack. “They’re usually bred for the red spot. People tend to like big, white rose mallow flowers with a red spot in the center.”
Rose mallows flower throughout August, said Primack, but have their strongest displays in the middle of August.
The fruits of the rose mallow are a capsule about the size of a grape, Primack said. Each capsule contains about a hundred seeds, which fall out when the capsule splits open. The seeds are dispersed by water.
Rose mallows are perennials, which die back in the cold weather and regrow in the spring. They can live for several decades, said Primack.
Interestingly, the sap of a relative of the rose mallow, the marsh mallow native to Europe, West Asia, and North Africa, was originally combined with sugar to produce marshmallow candy, said Primack. But today marshmallows are made with corn syrup, sugar, corn starch, and gelatin.
After exploring the Dedham rose mallow site, I followed Primack to the home of Chuck Cossaboom and Diane Kemsley in Newton’s Auburndale section. Their home is located next to the Charles River and has several rose mallows that were planted from wild cuttings by Cossaboom’s mother, Monica, in the 1990s.
Several acres of purple loosestrife cover the bank on the other side of the river from their home. Just two white rose mallow flowers are visible in a sea of loosestrife, a vivid illustration of the inhibitory effect of the invasive species.
Kemsley said the appeal of rose mallows for her is their natural beauty and the family tradition started by Cossaboom’s mother. She said they sometimes see hummingbirds visiting the mallows.
Cossaboom likes the size of their flowers and the fact that they’re native plants.
“Rose mallows are among the most beautiful wildflowers in the US,” said Primack. “And they occur right here along our Charles River.”
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