When you consider all of the trees that are grown for street plantings in the urban landscape, you will quickly come to realize that the number of possibilities is quite small. Our streets used to be lined with gorgeous, vase-shaped elms…until Dutch elm disease came along. The ash tree was a popular and drought resistant species…until emerald ash borer came along. Of course it was a mistake to plant so many of one species (or one cultivar). We weren’t thinking about how easy we were making it for pests to spread and infest trees along these long, continuous rows.
The Norway maple (Acer platanoides) has been a successful street tree, lining several streets and providing shade and habitat in several cities in Canada. The problem is that the uncultivated (original) species produces large amounts of seed and those seedlings are very shade tolerant and therefore present a competition problem when planted near forested areas. For decades, North American nurseries have been offering a wide range of cultivars of Acer platanoides, and these cultivars differ significantly from the original species. As you know, the Norway maple species has become labeled as an invasive species in some parts of North America. There are some groups that feel it should be illegal to grow and plant Norway maples here in Ontario. Most recently, the Invasive Species Act being proposed is designed to stop the production and sale of several commonly used ornamentals in Ontario. There is concern that there is insufficient scientific criteria used to classify “invasive species” from several stakeholder groups.
We used to be able to plant some native maples…until residential properties became so small and were graded with such poor soils that many native species will not survive, or will never reach their full potential and often become a hazard. And what do homeowners want to buy when it comes to planting trees in their yards? It probably won’t surprise you to know that one of the most popular trees purchased are those “red maples”. Yes, many homeowners are seeking out those “beautiful, red leaved maple trees” and I don’t mean the ones that turn red in the fall. They want red-leaved cultivars of Norway maple (e.g. ‘Crimson King’).
The cultivars of Norway maple differ in flower production, seed quantity, germination time, and seed viability. These factors can provide insight to the cultivars which provide the lowest impact to natural ecosystems. Research conducted at Penn State University evaluated several cultivars of Norway maple and the original species. They found that some cultivars of Norway maple pose a much smaller risk to our natural areas. Norway maple, the original species, produces copious amounts of seed and a large proportion (75%) of that seed is able to germinate and produce another tree if it finds suitable conditions. But there are some cultivars (e.g. ‘Crimson King’) that will produce < 10% the amount of seed that the original species does – and that seed is quite low in viability (10%). That means that ‘Crimson King’ has less than 1% viable seed than the original species does.
|Peak Flower Production
|Peak Seed Production
|Acer platanoides (original species)||≈1600||≈1200||0||≈75|
Of the many different cultivars of Acer platanoides, research indicates that there are some that are less invasive. ‘Crimson King’, ‘Faasen’s Black’ and ‘Globosum’ all produce the fewest amounts of flowers and therefore the least number of seeds. Not only do ‘Crimson King’ and ‘Faasen’s Black’ produce lower numbers of seeds, the seed also has the lowest rates of germination. “Crimson King’ and ‘Globosum’ also have a significantly smaller canopy than the species.
Norway maple cultivars with lower germination, seed number and flower production may be worth investing in if the site is a stressful urban one. By planting them away from naturalized areas and using them in plantings that are diverse in species, perhaps some cultivars of Norway maple can help build our urban % forest cover while posing very little invasive threat.
Conklin, J. and J. Sellmer. 2009. Flower and seed production of Norway Maple cultivars. Hort. Tech. 19: 91-95.
Conklin J. and J. Sellmer. 2009. Germination and seed viability of Norway Maple cultivars, hybrids, and species. Hort. Tech. 19: 120-126.