An Evaluation of Holden Arboretum’s Shade Tree Collection

Materials and Methods

This study was located at the Holden Arboretum, Kirtland, Ohio. Founded in 1931 and comprising an area of 800 hectares, the Arboretum contains more than 8,000 different species, varieties, and cultivars of mature and introduced woody plants. The Arboretum is located in USDA hardiness zone 5 (4), with an annual average precipitation of 111 cm. The topography is flat to gently rolling. The predominant soil type is a heavy clay loam.

A universal search of the literature was conducted to disclose tree species recommended for city use (3). “City use” was broadly defined as including streets, parks, cemetries, and other city land. All of these areas are within the city setting but no attempt was made to divide the lists into specific categories. If an author suggested a tree for use in any part of a city, it was noted.

The second phase of this project, a preliminary screening of new urban plant materials, was to evaluate the performance of these and additional trees in the Arboretum. Acquisition source, planting date, survival, records of insects and diseases were recorded and evaluated, etc. The proportion of surviving plants was calculated. Each tree was photographed, and measured in the field for height, diameter and crown spread.

The height of the specimen divided by its age gave the average annual growth rate. Expected mature heights were recorded from the literature. A sixty percent survival rate or a growth rate greater than 10 cm/yr was established as minimally acceptable.

Results

The literature search (3) produced a list of 211 urban use trees from 23 authors comprising 41 genera, the most common of which were: Acer (21 species, 53 cultivars)Carpinus (6 species, 12 cultivars)Celtis (6 species)Crataegus (11 species, 14 cultivars)Fraxinus (7 species, 10 cultivars)Ginkgo (1 species, 6 cultivars)Malus (6 cultivars)Quercus (11 species)Sorbus (5 species, 6 cultivars)Tilia (8 species, 13 cultivars)

Examination of the trees growing at the Holden Arboretum revealed 33 trees which were not mentioned in the literature search as potentially suitable for urban use. Twenty-six of these trees, their growth habit, percentage of survival, and growth rates are presented in Table 1. The performance of the following 7 species were encouraging, but were not included in Table 1 because observations were based on a single tree: Acer macrophyllum, A. saccharum ‘Sweet Shadow’, Fraxinus excelsior, Malus baccata Columnaris’, Malus ‘Donald Wyman’, Malus sylvestris plena, and Sorbus Aria gigantia.

Some of the trees performed poorly at the Holden Arboretum. Fifty percent survival was attained with Acer ginnala, A. truncatum, Crataegus mollis, Prunus sargentii, and Tilia americana gastigiata. Additionally, growth rates of less than 10 cm per year were attained by Acer circinatum, Stewartia Koreana, and Oxydendrum arboreum.

Discussion

Thirty three trees not previously identified in the literature as suitable for urban use, have been identified through their performance at the Holden Arboretum. The 60% survival rate was considered somewhat liberal. Use in street environments might well be more harsh, in which case poorer survival would be expected. On the other hand, many off-street urban sites may be less harsh. A terminal growth rate of 10 cm per year was also considered liberal since others consider growth of less than 30 cm per year as slow (2). However, in many urban uses slow growth does not necessarily indicate an inferior plant.

The identification of these trees then, suggests only that the plant materials are worthy of further trials in city spaces. This list is not final, nor should it be. It is hoped, however, that the materials reviewed here may be an initial step in finding new and better materials for use in the trade.