What Are The Alternatives to Ash?

March-April 2011

SPECIFICATIONS FOR TREES SPECIES can be puzzling these days what with the near-moratorium on planting ash (Fraxinus sp.) and Norway maple (Acer platanoides) in several jurisdictions. Callery pear (Pyrus calleryana) and honeylocust (Gleditsia tricanthos var. inermis) are still good choices but most plans are calling for more species biodiversity. So what else should we be planting, especially in tough urban sites?

On the other side of the fence, we have the nursery growers. They are asking themselves and their customers, what should we plant? It takes anywhere from 5-7 years to propagate and grow trees on to a salable size. Having to destroy these trees because they are no longer desirable has been taking its toll on an industry that has been hit hard by the economic downturn.

Landscape Ontario recently acquired funding and assembled a study group to take a closer look at trees species for tough urban sites, alternatives to ash and Norway maples. A group of horticulturalists are consulting the nursery industry, urban foresters, landscape architects and planners, designers and arborists to try and determine an agreed-upon list of species that should be grown and used in the Ontario urban landscape. By providing a list of several trees and educational material on their merits and shortcomings, the green industry will have access to the information and rationale that are needed to support greater biodiversity in our urban tree plantings.

The Landscape Ontario Alternatives to Ash study group assembled a list of 29 tree species that were known to be hardy and easily grown in Ontario. Among the list were some classic urban trees as well as some difficult to find or relatively unknown species. Research literature, extension publications and reference texts were consulted to determine which of the original 29 were most suitable to tough urban sites in the Ontario landscape. The group was looking for species that are relatively free of significant pests, easy to grow, as well as tolerant of poor, compacted soils, dry conditions and heat.

The next phase to the Alternatives to Ash Project is to provide a photographic, information package that will make it easier for decision-makers to choose a variety of tree species for the Ontario urban landscape. The master list of alternatives will be larger than a “Top 10 Tough Species” list. There are many different growing conditions within the Ontario landscape. The tree chosen for a large lawn does not need to have the same tolerances as the one to be planted in a parking lot island. The gardening public will also enjoy seeing more of the lesser known, worthy of our attention, woody plants that can grow in Ontario. This information will be published on the Landscape Ontario website.

Trees For Tough Urban Sites

1. Ironwood, Hophornbeam
Ostrya virginiana. One of the few trees we grow that is shade tolerant, hophornbeams are also very tolerant to dry, exposed locations with poor soils. The bark is interesting with the long, slightly-exfoliating plates. It is a slow grower so you’ll need to plant it as a smaller caliper tree and have a little patience. So far it doesn’t seem to have any major pests in Ontario.

2. Ornamental Crabapple
Malus species & cultivars. Probably the most sure sign that spring has arrived is when crabapples trees are in full bloom. The autumn fruit display can be almost as stunning. There are many bloom colours, fruit colours and tree forms from which to choose. With carefully chosen cultivars, crabapples will not suffer greatly from fireblight or apple scab. Ornamental crabapples can handle a lot of tough urban sites without taking up too much space.

3. Baldcypress
Taxodium distichum. This deciduous conifer provides bright green foliage in spring and sage tones in summer that mellow to a russet in the fall. The interesting bark, urban tolerance and few pests make this a tree to considered for a location where its slender pyramidal shape can be enjoyed. Virtually mess-free, the tiny needles blow away soon after they drop in the fall.

4. Yellow Buckeye
Aesculus flava. This eastern North American native is a lovely, large tree considered well-suited to locations where its shape can be more fully appreciated. It is quite showy in bloom, attracting pollinators. The rich dark green leaves rarely suffer from leaf scorch and end their growing season with shades of pumpkin, salmon pink or red. The range of soil and moisture tolerances combined with the showy flowers make this an excellent urban candidate. A great alternative to the other buckeyes.

5. Amur Corktree
Phellodendron amurense. A medium sized tree with interesting cork-like bark and is virtually pest-free. The broad spreading crown provides wonderful shade. Use male cultivars if you are worried about the messy black fruit. When given adequate soil volume, this tree will tolerate most urban sites.

6. Kentucky Coffee Tree
Gymnocladus dioicus. Although it doesn’t look like much when its young and is one of the last trees to leaf out in the spring, this tree has handsome foliar texture and is gorgeous as a mature specimen. It grows moderately and is one tough tree in the urban landscape as it can tolerate high pH, and dry, compacted soils. On the downside, the long seed pods are considered to be messy by some homeowners.

Alternatives to ash aren’t an original idea; our study group cannot claim any credit for identifying any of these species. It is a collection of species and cultivars that have been tried and tested at various sites throughout Canada and the United States. At the beginning of our consultation, I think some of us thought we would be raising some eyebrows with the likes of Kentucky coffee tree and amur corktree. When we ran through our preliminary list with the nursery industry, we were met with knowing smiles and murmuring. “OK, so what’s the joke?” we asked. “We’ve been growing those for years and we couldn’t sell them. No one knew what they were so we had to destroy them,” they said.

Contact Information

This column is written by Jen Llewellyn, Ontario Ministry of Agriculture and Food and Rural Affairs (OMAFRA) Nursery Crops Specialist

Email any questions you have directly to Jen and we'll publish her response.

P: 519-824-4120 ext. 52671 • F: 519-767-0755

OMAF website: www.ontario.ca/crops

Nursery Landscape Agriphone: 1-888-290-4441

Our mission is to enhance and promote the care and benefit of trees for present and future generations in Ontario through education, research and awareness.