May 15, 2011
The urban landscape in Ontario is changing. Promoting the production and use of a broad selection of city-tolerant trees is becoming an important issue in the quest of greening our cities.

Meanwhile, emerald ash borer is eating its way through white and green ash (Fraxinus americana and F. pennsylvanica), while Norway maples (Acer platanoides) are being discouraged because of reseeding into wild areas. Landscape Ontario's Growers' Sector Group obtained Farm Innovation Program funding to undertake a research project.

One goal of the project was to develop a list (see below) of urban-tolerant trees that could be offered to the Ontario landscape industry. The project team is led by Thelma Kessel of Lacewing Horticulture, and includes Sean Fox, assistant manager, University of Guelph Arboretum, Jennifer Llewellyn, OMAFRA nursery crops specialist and Dr. Glen Lumis, Professor Emeritus, University of Guelph.

Since urban tree planting sites are quite variable and often extreme, the committee felt it was important to carefully match tree tolerances to site conditions. Factors such as availability, response to transplanting, invasive potential, growth rate, genetic diversity and aesthetics also influenced the choices. Factsheets featuring the trees on this list are being prepared for publication and to appear on the Landscape Ontario website. Look for that detailed information soon.

The committee's plan is to have the tree factsheets on the LO website, as well as in future publications in Horticulture Review and other magazines. Common names will be clearly listed on the factsheets

The list of urban tolerant trees is the result of extensive searches in the international literature as well as consultations with nursery growers, landscape contractors, urban foresters and landscape architects in Ontario.

The trees listed below are possible replacements for Acer platanoides, where it and its cultivars have the potential to become invasive, and for Fraxinus americana and F. pennsylvanica, where the tree and its cultivars are threatened by emerald ash borer. Trees were chosen based on tolerance to urban situations, importance to the urban forest, aesthetics and relative freedom from pests and diseases.

"By choosing from this broad list, we can look forward to a vigorous, beautiful and more biodiverse Ontario urban landscape," stated the committee.
Acer campestre
Acer x freemanii
Acer ginnala (A. tataricum subsp. ginnala)
Acer platanoides only cvs.:  'Columnare', 'Crimson King', 'Globosum', 'Royal Red'
Acer triflorum
Aesculus flava (octandra)
Amelanchier spp.
Carya cordiformis
Carya ovata
Celtis occidentalis

Fraxinus quadrangulata
Ginkgo biloba
Gleditsia triacanthos var. inermis cvs.
Gymnocladus dioica
Maclura pomifera
Malus spp./cvs.

Ostrya virginiana
Phellodendron amurensis
and/or sachalinense
Pyrus calleryana cvs.
Quercus bicolor
Quercus ellipsoidalis
Quercus macrocarpa
Quercus muehlenbergii
Quercus shumardii
Syringa reticulata and/or pekinensis spp./cvs.
Taxodium distichum
Tilia americana
Tilia cordata
Ulmus americana