By Tony DiGiovanni
Executive Director, Landscape Ontario

As a rule, I stay away from controversy and conflict. However, there are a number of related and increasingly popular movements that need to be examined and challenged. These are native-only planting policies, seed zone restrictions and banning certain trees because of their “invasive” qualities.

In my view, these ideas and policies are costing the public millions of dollars in failed landscapes. They are also causing serious damage to the environment and reducing bio-diversity. On the other hand, perhaps I need to become more open and aware of differing perspectives.
 

The Norway maple debate

This spring, a number of volunteers were busy planting 150 trees at the beginning of the Highway of Heroes on Hwy. 401 in Trenton, Ont. Highway sites are unforgiving. Poor soils, no access to water, salt spray, compaction and exposed, windy locations makes it challenging for trees to survive. Yet there they were — an oasis of large, beautiful, majestic Norway maple trees, thriving within this harsh climate. The birds were enjoying the shade, (so were the volunteers) and squirrels could be seen hopping from branch to branch. Amidst the hot asphalt, endless roar of vehicles and unceasing release of dust, diesel, smoke and exhaust, the paradise of Norway maples made the highway tolerable. It would not be out of place to enjoy a picnic under the stately maples despite the severe surroundings.

Hidden from view, the extensive root system reaches into the soil, relieving the compaction and creating a reservoir of life-enhancing water and air. The leaves silently accept the carbon dioxide from the exhaust and magically and mysteriously convert it into wood fibre, forever trapping the harmful gasses implicated for warming the planet. In addition, the air is cleansed and fine particles of tire dust are entrapped. All these benefits, and yet the Ministry of Transportation will not allow Norway maples to be planted near highways because they are “invasive” and not native. Many municipalities have the same unfortunate policies. The emerald ash borer is taking an overwhelming toll on the ubiquitous ash tree. Native-only policies also take a toll. Many native trees do not do well in unnatural conditions found in the urban environment.

It is time to reconsider the policies that discriminate against trees that deserve to adorn our landscapes. All trees have their place; native or not, invasive or not. It depends on the site and the context. We need to increase diversity in our urban forest. More importantly we need to plant trees that survive.

It is time for municipal arborists, landscape architects and others who specify street trees to reexamine “native-only” and seed zone restriction policies. It is also time to evaluate rules that limit the use of so called invasive trees such as Norway maple. Invasiveness is neither good nor bad. It is a trait that many plants (native or not) have. Sugar maples are invasive too. The native forest is filled with millions of them. Yet they struggle on highways and in urban conditions — where Norway maples thrive. It makes no sense to eliminate a tree from the planting palette where it does well. We should plant more Norway maples on highways and in cities because success is better than failure.

Please don’t get me wrong. I am also a strong advocate for the use of natives. I am on the board of Maple Leaves Forever because I believe in promoting the use of native maples. A number of years ago we were involved in a “Champion Tree Hunt” in the Kitchener/Waterloo area. The public was asked to go out and measure the largest trees in their neighbourhood. There were almost 400 entries and it bothered me that most of the winning entries were native trees that are not used by the commercial trade anymore. It was eye opening. As a profession we have moved away from many interesting native species. The current trend toward natives must be applauded, however “native only” policies should be discouraged. There is no reason to limit our choice. Tree survival should be the priority.
 

Seed zone restrictions

Another policy that does not make sense to me is “seed zones.” The idea is that only plants propagated from a specific seed zone should be planted within that zone. Perhaps this makes sense in forestry, but it is difficult to see the benefit in an urban setting — especially when most of the plants have come from other locations. I would love to see the science supporting this approach. Let’s start the debate.

One defining characteristic of nature is movement. Plant seeds have evolved ingenious ways of spreading, including hitchhiking on humans and animals in a grand effort to cover as much ground as possible. Why do we, as humans, believe we should assign arbitrary zones and constrict how plants spread? Even without artificial seed zones, plants have managed to spread way beyond their original boundaries. The book Ginkgo Biloba A Global Treasure: From Biology to Medicine chronicles the existence of ginkgo fossils in northern Alberta. This probably means that ginkgo existed thousands of years ago in a location that can no longer support any tree cover.

The horticulture industry is based on spreading plants as far as they can go. This has resulted in a rich diversity of plants enhancing lives and the environment. Some introduced plants have caused economic and environmental damage. Most have not. I am an equal opportunity plants person. Native plants should be encouraged, so should non-natives. Invasive plants should be discouraged except in areas where nothing else will grow.

There is a popular saying in horticultural circles, “right plant, right place.” In my view, this is the right principle when choosing plants.

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Tony DiGiovanni may be reached at tony@landscapeontario.com

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