Showy and straight-growing, resistant to heat, humidity, drought and disease — the newly planted elm trees seem custom-fit for the Septima P. Clark Parkway.

But no way are they native. They’re cultivars.

That makes the 89 Everclear Lacebark Elms an awkward addition to the peninsula landscape. They add to the plant diversity. They take away from the environment, at least one biologist says.

The trees were planted recently as part of a $154 million project designed to upgrade the expressway and improve drainage along the flood-prone route, an old creek bed.

The road also is known as the Crosstown. It is U.S. Highway 17 across the Charleston peninsula.

Construction, dust and noise have disturbed residents in adjoining neighborhoods, some of whom say the city would have been better off just fixing the drainage. Lane closures have bottlenecked traffic.

The trees cost about $19,000. Cultivars are plants selected and bred because they have sought-after traits, like heat tolerance, that the native species don’t. These trees were bred in Georgia from a type of elm native to China.

City officials selected the elms after the S.C. Department of Transportation identified them as not needing much maintenance and not compromising road safety, said Pete Poore, DOT spokesman.

The narrow-canopy elms also need little room to grow, so they fit in the road’s thin median, said Laura Cabiness, Charleston public service director.

“There’s just really not much room (for popular native trees like the live oak or crepe myrtle) out there,” she said.

City arborists have wanted more diversity, worrying that a disease could wipe out one of Charleston’s widely planted native species, such as the live oak or crepe myrtle.

American elms, ironically, were devastated by Dutch elm disease in the mid 20th century. Resistant cultivars like the Everclear elm do combat it, but aren’t immune.

Biodiversity is better achieved with native plants, and the Lowcountry has any number of native species that could have worked well along the Crosstown, said Jean Everett, College of Charleston biology instructor.

Research has shown that native plants provide habitat for native insects and birds that other types of the plants sometimes don’t. And newly introduced plants can bring their own problems.

“I’m completely dismayed that so many people think that if a landscape is green, it’s ecologically healthy, because nothing could be further from the truth,” she said. The elms “will just sterilize the Crosstown, rather than making it a more welcoming and ecologically viable road. I certainly think it was a terrible choice.”

Reach Bo Petersen at 937-5744, @bopete on Twitter or Bo Petersen Reporting on Facebook.

Science and environment reporter. Author of Washing Our Hands in the Clouds.