Caveat Emptor, You Who Will Buy Trees to Plant!

Peter Duinker, Halifax Tree Project, 2025-06-28

A couple of years ago, I dropped into a prominent nursery/garden-centre just outside Wolfville and checked out the kinds of trees they had for sale. I was surprised to see that Norway maples (Acer platanoides) were on offer. Why surprised? Most urban foresters in Canada (and probably the USA too) have sworn off Norway maple. It has many issues (see our article on this species in the Street Tree series of the Blog section of the website). The main issue for me is its invasiveness – pretty much every homeowner in HRM who has gardens on their property has experienced each year the joys of pulling out hundreds if not thousands of volunteer Norway seedlings.

In recent years, pretty much every Canadian province has established an invasive species council. Ours, as you would expect, is called the Nova Scotia Invasive Species Council (https://nsinvasives.ca). Norway maple is profiled as one of a few invasive alien tree species. You can find out what “invasive” and “alien” mean on the website, which also shares the fact that two USA states have banned Norway maple from being planted in their territories on account of its invasiveness.

Why does the native range of a tree species matter when one considers buying a tree to plant on an urban property? Besides the invasiveness problem (but not all non-native tree species are invasive here in NS), there is the biodiversity question. People the world over are voicing concerns about two major global issues – devastating climate change and continued impoverishment of biodiversity. Every tree planted in town or city can contribute, in its own small way, to addressing these two global problems. Every tree that grows to maturity stores a lot of carbon (that came out of the atmosphere), casts shade that cools the surfaces beneath it, and transpires water that cools the air around it. That’s all good.

But not every tree contributes equally to the conservation of native biodiversity. Consider a red oak tree and a Ginkgo tree in Halifax. Both can be purchased locally at garden centres. The red oak species (Quercus rubra) has evolved in our landscape for many thousands of years along with the evolution of the hundreds of insect species that live in and feed on red oak. In very general terms, where you have insects, you have birds. A Ginkgo tree supports no birds because it feeds no insects. Ginkgo biloba, an Asian species, did not evolve in the context of the native insect diversity of eastern North America, and our insects avoid it. So, while we do get some ecosystem services from a Ginkgo tree, like cooling as well as carbon uptake and storage, we don’t get any contribution to the conservation of native biodiversity.

Some would say, in defence of Ginkgo, that a good reason to plant it in the city is because it has no known insect and disease pests that could kill it and it performs well in the harsh streetside environment. Granted, but by the same token it feeds no insect species that could support our native bird populations. By the way, Ginkgo is not invasive because we plant only male trees in Halifax so there are no Ginkgo seeds to propagate the species.

Douglas Tallamy is a professor of forest entomology at University of Delaware and author of books such as “Bringing Nature Home: How Native Plants Sustain Wildlife in Our Gardens”, “The Nature of Oaks: The Rich Ecology of Our Most Essential Native Trees”, and “How Can I Help? Saving Nature With Your Yard”. He makes the message simple and forceful – to conserve native biodiversity, we need flourishing native habitats. Plant choices are a vital component of cultivating and perpetuating such habitats for a diverse fauna.

The matter of what is native and what is non-native is not so simple. By my count, there are 42 native tree species in Nova Scotia (some references say 32, but I include a lot of small trees that are sometimes considered shrubs). Some are ubiquitous, like red maple, and others have a limited natural distribution in the province, like eastern white cedar and American elm. The latter can grow in Halifax if we plant them (witness our elm-lined streets), but they are not here by nature. But lots of tree species will grow in Halifax if we plant them – one estimate I take seriously is that we probably have more than 150 tree species in the city!

In this era of climate change, it is now sensible, in my view, to consider what are called near-native tree species. Twenty millennia ago, there were no trees in Nova Scotia. The land was under a thick sheet of ice. How did we get all the native tree species we have today? As the icesheets retreated northward, the tree species migrated, mostly by seed movements, north from the parts of the eastern seaboard that were south of the glaciers. We might imagine that, with or without contemporary climate change, such migratory behaviour might continue over the next millennia. However, WITH climate change as we now experience it and anticipate it to develop, our NS climate (from a temperature point of view) is becoming increasingly hospitable to many tree species the northern range extent of which is now somewhere in New England and deep southern Ontario. In many respects, the insect and bird diversity of these forests is rather similar to our current diversity, in contrast to the insect and bird diversity of, say, southern and western USA. In view of that, I do not deem it irresponsible to plant any tree species from the full range of so-called Carolinian species in Halifax. Doing this may also help us keep some semblance of forest integrity as we cope with what is likely to be a perpetual onslaught of invasive alien pest species such as emerald ash borer, hemlock woolly adelgid, and the organisms responsible for Dutch elm disease and beech bark disease. A crude estimate of the number of such tree species, above and beyond those native to NS, would be about fifty (for reference, see Waldron’s (2003) “Trees of the Carolinian Forest”).

Now, onto the theme of buying a potted tree to plant on your urban property. You could drop into any one of many garden centres around HRM – some have a few trees, some have many. Or, you could make the trip out of town to a tree nursery – two examples are Baldwin Nurseries near Falmouth and Charlie the Tree Guy in Old Barns. Despite the distance you need to go to shop for a tree, I strongly recommend visiting a real nursery. You will get professional advice on tree-species options and, more likely than not, be able to focus on species native and near-native to NS. You may also get products grown from seeds collected in the province by the nursery manager.

To further my understanding of what the garden centres have to offer, in mid-June I undertook a small informal investigation handy to where I was that day. In Bayer’s Lake and Hammonds Plains, I did a quick tour of each of four centres’ tree offerings. I’m sure I didn’t get them all, but here is what I found (I’m using common names here, a theme to which I shall return later). In the table I have included information about native range of the species/cultivar in the following terms: (a) native means grows by nature in NS; (b) near-native means it grows naturally in northeastern North America; and (c) non-native means it grows naturally far away from NS.

In the four garden centres, I found a total of 27 tree species/cultivars. I was a bit surprised at how many of the specimens were crosses between two species, or special breedings of species (hence cultivars). 14 species were native or near-native, whereas 13 were non-native.

Based on my musings above about conservation of native biodiversity and the information provided in the table, I have the following suggestions for tree purchasers:

1_ Consider putting conservation of native biodiversity at the top of your criteria for choosing what kind of tree to purchase and plant. If this is not important to you, perhaps you have other good criteria for species choice.

2_ Consider going to a nursery rather than a garden centre for your purchase. Wherever you shop, try to discover how much the sales agents know about the tree choices on offer. Often they know nothing about them other than what the info cards on the trees say.

3_ Shopping around is a good idea. Try not to buy a tree on your first visit. Consider using your phone to photograph the info card – front and back – hanging from a tree of potential interest, and then do some internet-based research on the info provided, particularly the tree genus, species, and cultivar. I saw one card that highlighted the name “Crimson King Maple”. Nowhere on the card was there information that this is a cultivar of Norway maple and therefore not the best choice of maple.

4_ This brings me to the issue of tree naming. Each tree type has a Latin name, which is authoritative, and an English name (or several). Don’t rely on the English name to provide authoritative information. Sure, if you see “Sugar Maple” on the card or tag, you can feel confident that it is Acer saccharum, our native sugar maple. But I check the leaf shape immediately for confirmation. The Latin name has at least two parts, sometimes three: (a) the genus (e.g., Acer); (b) the species (e.g., platanoides); and (c) sometimes the cultivar “e.g., Crimson King”). Just giving you the genus is not enough because there are native/near-native as well as non-native species in many genera. For example, horse chestnut is Aesculus hippocastanum (non-native), and Ohio buckeye is Aesculus glabra (near-native).

In conclusion, I urge people to plant more trees on their properties, and to choose native or near-native tree species, preferably of local provenance. It pays to become a shrewd and knowledgeable shopper in the context of garden centres where tree advice is often sketchy. A good species choice, well situated and planted on your property, will give so much joy, as well as pay huge dividends in terms of ecosystem services and property values, for many decades.