National Elm Trial

Executive Summary

The National Elm Trial includes 15-18 DED-resistant commercially available elm cultivars planted at 17 evaluation sites in 16 states. These elm cultivars were evaluated over a wide range of growing conditions and hardiness zones. This nation-wide study has one coordinating and reporting system that is based at Colorado State University. This web site reports the performance data for trees at all sites. Even though many DED-resistant elm cultivars exist and are available in the nursery trade, much of the public is hesitant to purchase and plant any elm tree. While the DED-resistant elm cultivars could enhance the diversity as well as the beauty of urban forests, these cultivars do vary in their response to other biotic and abiotic stresses. Thus, currently available elm cultivars were planted in large replicated trials in a wide range of environmental conditions across the United States so that their growth and performance could be evaluated.

Problem Statement

Due to the increasing threat that exotic pathogens and insects can have on the health of shade trees, it is essential to maximize genetic diversity within the nation’s urban forests. The graceful American elm that once dominated urban forests across the United States essentially disappeared from urban landscapes after the introduction of Dutch elm disease (DED) into North America. While DED-resistant elm cultivars have been planted in trials in various locations around the nation over the years, so often the trials lacked sufficient replication and/or cultivar performance data were never published. Scientific data on growth, form, and pest resistance for existing DED-resistant elm cultivars are essential in order to promote interest in planting these trees. The national trial of commercially available elm cultivars encompasses evaluation sites that represent a wide range of growing conditions and will be sufficiently replicated so that the data can be statistically analyzed.

Study Objectives:

Determine the growth and horticultural performance of commercially available DED-resistant elm cultivars in various climate regimes in the United States.
Determine the relative disease, insect, and abiotic stress tolerance of these cultivars.
Promote the propagation and use of elms through local, regional, and national reporting of the trial results to wholesale tree propagators and growers, retail nursery and garden center operators, landscaper designers, arborists, and the general public.

Methods

In 2005, fourteen to eighteen elm cultivars were planted in various locations in the United States. Each cultivar is represented by one tree in each of five blocks in a randomized complete block. The elm cultivars represented a range of commercially available hybrids and species of Ulmus. The trial was conducted over a period of 10 years. Using a standard format, annual assessment of each tree included: height, diameter, and crown characteristics. At appropriate times during each year, each tree was assessed for response to vascular, canker, and foliar diseases, scale insect infestations, foliar-feeding insect infestations, bark beetle infestations, and abiotic damages (frost/freeze, wind, winter dieback, sunscald, and insufficient soil moisture).