The Norway Maple: Not a good tree

These days, the Norway Maple doesn’t show up on lists of the best tree to plant in your back yard. Arborists don’t recommend it. In fact, most positively discourage it.

“It’s a ‘bad guy’ - number one in our black book,” says Tobias Effinger, owner of Arboreal Tree Care, a Thornbury-based arboriculture firm.

But the tree – which goes by the botanical name Acer platanoides - is widely available from nurseries and is popular with homeowners for its tolerance of urban conditions and the attractive foliage, from deep purple to variegated, of many of its cultivars.

What else is in the black book? I ask him as we wander through the Collingwood Arboretum, a lovely lakefront park where he is the arborist in charge.
Nothing, he replies.

In fact, Effinger is a man who has a good word for almost any tree, even those like the White Poplar Populus alba, Black Locust Robinia pseudoacacia, or Manitoba Maple Acer negundo that are often dismissed as invasive or weed trees.

But not for the Norway Maple.

“It’s a wolf in sheep’s clothing,” he says. “Take it down.” On occasion his firm will do so for free. The only situation in which he doesn’t recommend immediate drastic action is where these are the only trees on the property, in which case he advises a gradual approach to phasing them out, by pruning judiciously to reduce the canopy while replacements are planted, with the ultimate goal of removal.

“It’s still a tree,” he notes. It does many of the jobs we need trees to perform – storing carbon, cleansing air and groundwater, recycling nutrients, providing shade and temperature moderation, and even nesting habitat for some birds (although it does not provide much food to feed the chicks).

So what makes it so bad?

-It’s seriously invasive, with abundant crops of seeds annually, unlike many trees that only produce seed every two or more years. The seeds can spread aggressively, either blowing into adjacent areas or being washed through storm sewers and other drainage into ravines or waterbodies from which they can spread.

-It leafs out earlier in the spring and goes into dormancy later in the fall, giving it a competitive advantage. The Wikipedia entry notes: “It is one of the few introduced species that can successfully invade and colonize a virgin forest. By comparison, in its native range, Norway maple is rarely a dominant species and instead occurs mostly as a scattered understory tree.”

-Biodiversity is reduced because plants won’t grow under it.

The Norway Maple has been said to be allelopathic, exuding chemicals from its roots and foliage that inhibit the growth of other species. But Effinger says that’s not the case, and neither is a similar charge levelled against the native Black Walnut Juglans nigra. “It’s a myth,” he says. The reason plants don’t grow under the Norway Maple is its very dense canopy, testimony to its origin at a more northerly latitude where light levels are lower. In addition, its shallow root system allows it to out-compete nearby plants for nutrient uptake.

So, should homeowners do their bit for maintaining Canada’s most iconic species by choosing a Sugar Maple Acer saccharum, for their front lawn?

Not necessarily, says Effinger. “The Sugar Maple is overplanted,” he explains. It can live 400 to 600 years in a forest along with other Sugar Maples, but life is considerably curtailed when it’s out of its element as an isolated specimen tree, competing with grass, its roots exposed to the sun, its leaves not left when they fall.

However, he notes, some yards may well be suitable - there are situations when the Sugar Maple is indeed an ideal option.

In contrast, the Norway Maple can cope with many difficult conditions.  It’s tolerant of salt, air pollution and soil compaction - which is what made it such a great choice for urban forestry until the downside became clear.

Other trees that have evolved to grow in open spaces would be more appropriate, though Effinger’s answer when I ask what tree he would recommend as a front-yard specimen is “great question!” And then he won’t elaborate. The variables that would make a tree successful in any spot are too many – exposure, soil type, drainage, micro-climate and so on – the answer has to be site-specific. “There’s a process to choosing a tree.”

On the other hand, if a tree has survived for decades in the “wrong” place, Effinger is committed to making it comfortable for many more years. An aging Sugar Maple, for instance will benefit from removal of the sod around it, addition of a layer of compost and then a layer of mulch on top of that. This will come close to replicating decades of natural decomposition in the forest. At the arboretum, the wood chip mulch is finer than is generally available, having been double-ground so it will break down quickly. And when it does, the compost/mulch treatment is repeated.

“We also work with the tree structurally, we reduce certain parts of the tree to encourage others,” Effinger adds. Identification of portions of the tree where there’s declining vitality permits a targeted reduction, something that trees do for themselves as they age, dropping branches and shrinking in size. “The tallest trees in the forest are not the oldest.”

On the other hand, older does not necessarily mean weaker, even when there’s rot on the inside. “What’s inside has very little to do with the strength of the tree,” he says. The taper at the base of an old tree offers stability, and the structure is sustained by the outside of the trunk, not the mass of the interior. It’s not unusual for an older tree to weather a storm that might uproot its younger relatives.

Tree preservation is the primary focus of Arboreal, which is why the firm is the appointed Tree Trust arborist for Meaford and Town of the Blue Mountains. Tree Trusts, which have been established in seven Ontario communities since 2019, grew out of residents’ desire to preserve legacy trees (also known as heritage or veteran trees).

But the time may come when a tree has to be removed. That’s when its life can continue as a snag – a dead or dying tree or a portion of one that remains upright to decompose naturally. Trimmed if necessary, it can remain in place to provide cavity nesting for birds and mammals and a place for insects and other life to grow.

New technology and creative thinking allow for better ways of determining best management for trees. Effinger describes sonic tomography, a method for assessing decay and general health that’s less intrusive than the traditional core sampling. And he’s working with a colleague on a classification system for cavities that can be created in habitat snags. ““How many and what?” How many cavities could there be, of what size and spacing to accommodate which species? For instance, “a tree swallow and a merlin, that’s not going to work out.”

Eventually, the snag will fall to the ground and become a log, in its turn equally important habitat and a potential “nurse log” for tree seedlings. It’s estimated that removal of dead material from forests can mean a loss of habitat for up to one-fifth of the animals in the ecosystem.

Understand the cycle – that’s the message. Trees are really good at taking care of themselves as well as all the creatures and organisms that live with them – including people. We just have to work with them. And one of the lessons is, leave the Norway Maple to Scandinavia where it has a role to play in the forest; don’t plant it here where it is hugely disruptive.

Postscript - How to tell a Norway Maple. If you pull a leaf off a Norway Maple, the broken stem will emit a milky sap. The sap flows clear on a native maple.