For shade, for stature, for sheer physical grace in the tree world, nothing beats an American elm. The first colonists in New England took notice of this native hardwood tree’s qualities and brought it forth from the woods to grace their streets and town greens. That idea caught on, and elms - from the Liberty Tree of Boston (our first symbol of national independence) to “Herbie” (planted in 1775 in Yarmouth, Maine, and thought to be the oldest living elm in the Northeast) - have been an integral part of Main Street USA ever since.
But this most popular of shade trees has fallen on hard times. A fungus that entered the U.S. in the 1930s has devastated over three-quarters of our wild and cultivated elms in the last 77 years. For a time, it looked like as though the thousands of Elm Streets across the nation would stand in somber memoriam to this tree’s remarkable heyday. Not so. The steady work of researchers has forged a new generation of elms that hold the promise of an American elm revival.
The fungus that started it all is called Dutch Elm Disease (DED). It is transported primarily by the elm bark beetle, and it clogs the vascular systems of elms, preventing the transport of water and nutrients from root to leaf, causing wilt, dieback, and death. Asian elms, having co-evolved with the disease, are highly resistant to it.
By the time DED arrived in the U.S., it had devastated populations of many European elm species. So when the stately elms lining America’s boulevards began to die en masse, tree breeders looked for help from across the Atlantic, where scientists had been at work creating disease-resistant elm varieties for decades. Some of these proven European varieties have been planted to replace dying American elms, as have Asian elms and European-Asian hybrids.
The problem is that these foreign elms, while they may resist DED, cannot approximate the size and grace of form that come so naturally to the American elm. And the life-support measures many towns and campuses employ to preserve standing specimens - injecting chemicals directly into the vascular systems of affected trees - are expensive, costly, and toxic. Creating a pure, shapely American elm with high resistance to DED has therefore been somewhat of a holy grail of tree breeding since DED first appeared.
A collaborative effort to breed these ideal elms began in 1933, when Cornell University started gathering elm seedlings in the wild from across the Northeast and screening them for their natural resistance to DED. A few different sets of researchers crossed the survivors with wild American elms that had natural resistance to DED and so generated various lines of super-elms.
One group of breeders, from the Elm Research Institute (ERI) in Keene, New Hampshire, came out with a set of six varieties under the umbrella name of “American Liberty.” At least one of the varieties is patented and thus illegal to propagate; however, ERI will replace your American Liberty elm if it succumbs to DED. The Institute has distributed 250,000 elms, of which it claims less than a tenth has been infected with DED.
The U.S. National Arboretum has come up with a different set of American elm offerings. Their first significant variety, named Valley Forge, was discovered through DED-resistance trials. They claim it is the most disease-resistant of all the American elm varieties, on par with Asian elms. Their second, named New Harmony, was discovered in the wild. They also offer Princeton (in cultivation since the 1920s and found to have natural DED resistance) and Jefferson, which is said to have outstanding resistance. Other crosses are being developed.
Time is the crux of the problem of determining which elm variety is truly superior. It’s impossible to know an elm’s long-term disease-resistance until it reaches maturity, at 50 years. And each variety has other weaknesses, including vulnerability to wind damage, elm leaf beetles, or elm yellows.
Also, it’s very difficult and time-consuming to propagate (and therefore intensively test) elm varieties: they must be generated from softwood cuttings that are rooted, since grafting onto American elm rootstock (the fastest method) would subject them to DED infection through the roots. Only time will tell, but making sure a large number of each elm variety is in circulation at any one time will give us our best chance of outwitting the always evolving Dutch Elm Disease.
If you want to bring the American elm back to your yard, choose one or a few of these varieties and take a leap of faith. With a little effort and luck, you’ll be rewarded with a lovely, cooling shade tree that can reach 25-30 feet tall in just 8-10 years. And you’ll be a part of the collective effort to bring a little piece of Americana - and ecological complexity - back to the natural landscape of the Northeast.