In the past, some species were the favorite shade trees, which is why many cities have Elm Streets. The American Elm is a small to medium (to very large) tree that at maturity has extended branches that form a wide fan-shaped crown. There are few stately American elms left in Missouri and many of them are treated annually with a preventive fungicide to keep the disease at bay. Several elm populations have proven to be resistant, which has led to their spread as cultivars, with “Princeton” and “Valley Forge” being two of the most common.
However, to quote Michael Dirr, “people somehow have the idea that all American elms were destroyed. Streets that were formerly lined with American elm trees were planted with monocultures of green ash (Fraxinus pennsylvanica) or white ash (Fraxinus americana). With their arched branches, American elms were the preferred shade tree for city squares and streets, and have been featured in many historic American events. Since its introduction in 1928 until today, more than 75% of North American elms have died from it.
Disease-resistant strains are being developed and individual trees can be treated with fungicide injections every three years. Upon detecting their presence, the tree begins to clog its vascular system with tyloses to contain the spread. The fungus enters the sapwood and blocks the flow of water to the tree, causing the branches to wither and die, killing the tree before it can reach a large size. Elm trees can live for hundreds of years, so it's been traumatic for Americans to lose trees that were essentially historical markers.
The pumila, an ogre that invades a tree, plagued by diseases, short lived and that can easily grow several feet in a year, grows very slowly and can thrive on rocky soils, limestone cliffs or heavy clay. The rock elm, Ulmus thomasii, is rare in Missouri forests and hasn't been sold in nurseries for more than 100 years. At that time, Native American elms (Ulmus americana) grew in large numbers in the forested landscape and in the urban core. The American elm proved to be tolerant to the challenges of city life and became the preferred tree to be planted on park strips along the streets.
American elms (of several species) still tend to grow from seeds in wild areas and often reach a very impressive size, especially when they are far from human homes.
