Elms are some of the most statuesque, sturdy, adaptable and potentially long-lived shade trees growing in and around Napa Valley. Though they are non-native trees here, they thrive as long as they avoid Dutch Elm Disease. We have a few American and English elms more than 100 years old still remaining after Dutch Elm Disease started killing them in Napa County in the 1970s. Disease pressure built up as elms died and elm bark beetles multiplied in dying and dead trees and infected increasing numbers of elms.
Aside from Dutch Elm Disease, American and English elms are susceptible to severe leaf damage by Elm Leaf Beetles. This pest does not kill the trees, but it can render them unsightly when the beetle larvae feed on and “skeletonize’” the leaves, chewing away all of the soft green leaf tissue between the veins.
Like many other favorite shade trees, elms are susceptible to various pests, diseases and decay, but Dutch Elm Disease and Elm Leaf Beetles are the real deal-breakers for elms here.
When it became obvious that Dutch Elm Disease was making American and English elms obsolete as shade tree choices for landscapes, researchers got busy looking at other elm species and hybrids. A partial list includes Accolade, Liberty, Frontier, Homestead, New Horizon, Triumph, Princeton, Valley Forge, Pioneer, Prospector, and Emerald Sunshine.
Some were planted in a test arboretum at Holbrook-Palmer Park in Atherton, California. I visited there in 2012 and wrote a column on it. In general, the new varieties were showing good resistance to Dutch Elm Disease except for one Liberty elm, and good resistance to Elm Leaf Beetles with no insecticide applications.
Aside from the pest and disease resistance, tree structure is critical. We want to see stately upright elms that resemble, if not match, the stature of the American elm. At its mature height and spread up to about 120 feet most gardens cannot accommodate such a big tree. One selection with similar proportions but on a smaller scale is Frontier.
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A hybrid of Ulmus carpinifoia and Ulmus parvifolia, it grows to about 40 feet. An article in “Pests and Diseases of Trees and Shrubs (UCANR Publication 3359) reporting on elms shows it as “Resistant” to Dutch Elm Disease and “Moderately Resistant” to Elm Leaf Beetles, with variability. You can see some young specimens growing in the “Tunnel of Elms” along Highway 29, just north of St. Helena. They look promising as replacements for dying trees.
The one I am most interested in at the moment is Emerald Sunshine. This one is a selection of Ulmus propinqua, a species from China. Like Frontier, the Chinese heritage endows resistance because the disease originated there and the elms co-evolved with it.
I have had the opportunity to see Emerald Sunshine up close in a garden setting over the past two years. I am aware that experience with only one specimen is only anecdotal, but it is confirming the characteristics stated in grower’s descriptions. It shows a nice tendency for “sturdy upright” structure, fast growth, and deep green glossy foliage. J. Frank Schmidt nursery states it grows to 35 feet by 25 feet. I believe they give this as an estimate of expected stature 10 years after planting a small tree.
As to elm leaf beetles, last year this specimen had a couple of leaves with Elm Leaf Beetles egg masses, which are laid in the undersides. This year, there are none. The only sign of Elm Leaf Beetles is a few small holes chewed in a few leaves. In their life cycle, elm leaf beetle adults seek the new, fully expanded elm leaves in spring. They feed on leaves, chewing out roundish holes and then lay egg masses on the undersides of the leaves.
This spring, on the specimen I am watching, I found a few holes typical of adult Elm Leaf Beetles feeding, but no egg masses and no larvae. Apparently, the adults recognized the tree as a potential host but after tasting the leaves, they reject it and move on.