Norway Maples. Can We Use Them Wisely?

Issue: 
July-August 2009

WHEN YOU CONSIDER ALL OF THE TREES that are grown for the urban landscape, you will quickly come to realize that the number of possibilities is dwindling. Our streets used to be lined with majestic, vase-shaped elms – until Dutch elm disease came along. The ash tree was a popular and drought resistant species – until emerald ash borer surfaced. 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. The Norway maple (Acer platanoides) has been a successfully introduced street tree, lining residential streets and providing shade and habitat in several cities in Canada. Of course the problem is that the original species produces large amounts of seed and those seedlings are very shade tolerant and therefore highly competitive in our forested areas.

As you probably know, Norway maple is considered an invasive species in some parts of the United States (e.g. Connecticut). There are some groups that feel Norway maples should be prohibited from being grown and planted here in Ontario. The Ontario Invasive Plant Council (OIPC) is a new organization that is working to address the threat of invasive plants in the province. OIPC has been meeting over the last year or so to discuss this topic and try to determine lists of plants that are considered invasive for Ontario. Visit their website at www.ontarioinvasiveplants.ca. Here you will find a link to a “List of Invasive Plant Information Documents.”

What trees should nurseries be growing to forest today’s stressful urban sites? Nursery growers grow what their customers want to buy. If they don’t, people will take their business elsewhere. In fact, nurseries spend a lot of time sourcing uncommon plant material from other nurseries across North America in order to completely fill orders. And what do consumers 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 “beautiful red maples.” And I don’t mean the ones that turn red in the fall. They want red-leaved cultivars of Norway maple.

Recent research indicates that some cultivars of Norway maple may be good candidates for street trees because they do not pose significant invasive risks to our natural areas. Norway maple, the original species that is grown from seed, produces copious amounts of seed and a large proportion of that seed will germinate and grow into trees. Conklin and Sellmer (2009a) found that there are some cultivars (e.g. Crimson King) that will produce less than 10% the amount of seed that the species does and that the seed is actually quite low in viability and germination. Even on Connecticut’s Invasive Plant List there is a footnote under Acer platanoides indicating that further research is needed to determine whether the cultivars are potentially invasive.

Conklin and Sellmer (2009a) found that ‘Crimson King,’ ‘Faasen’s Black’ and ‘Globosum’ exhibited significantly lower flower production and number of seeds, producing less than 10% of the seed of Acer platanoides, the species. We have seen these cultivars behave this way in the landscape. The same research group found that ‘Crimson King’ and ‘Faasen’s Black’ seed exhibited lower rates of viability, 20-25% compared to 75% seed viability in the species (2009b). ‘Columnare’ and ‘Royal Red’ also exhibited lower seed viability than the species. Dr. Tom Ranney, North Caroline State University, and Dr. Richard Olsen, US National Arboretum, have been leading the charge to develop sterile cultivars of Norway maple. Because shade trees take so long to become sexually mature, breeding for sterility will take time, patience and a lot of support.

So perhaps some of those red-leaved maples aren’t such a bad idea after all. By planting them away from naturalized areas and using them in plantings that are diverse in species, some cultivars of Norway maple may pose very little threat to our forests. Incidentally, I once had a neighbour who went to the garden centre looking for one of these “red maples.” He came home, planted the tree and invited me over to look at it. I was thrilled to find him standing beside an Acer rubrum. I let him know that the leaves wouldn’t be turning red for a few months but when they did, it was going to knock his socks off. I also gave him a few pointers on how to improve the site conditions. Because it was in the middle of his backyard, he could really nurture it along. I made a big fuss about him choosing a native tree and how special it was. We were both rewarded every October.

References
Conklin, J. and J. Sellmer. 2009a. Flower and seed production of Norway Maple cultivars. Hort. Tech. 19: 91-95.

Conklin J. and J. Sellmer. 2009b. Germination and seed viability of Norway Maple cultivars, hybrids, and species. Hort. Tech. 19: 120-126.

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
jennifer.llewellyn@ontario.ca

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