I see that the “ban all non-natives of all genera” people are at it again! But, is there hope for some reason to prevail yet?
In the January 2010 issue of Landscape Trades magazine, Jennifer Llewellyn, the nursery crops specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs wrote a short item on the topic of specific cultivars of the Norway maple (Acer platanoides) and the facts about why we should definitely not be excluding these trees from urban planting. Here are some excerpts.
“When it comes to survival in nature, most ecologists will tell you it’s a numbers game. Look at the most successful species in nature, and you will find that many of them produce a staggering number of offspring, way more than required to propagate the species. Many won’t make it to maturity, because they serve as a food source to other species. This is true for animals and for plants. Norway maple is one species that produces ample seed, much of which is viable. Although animals eat some, it is not as delectable as our native maple species [Sugar maple—Acer saccharum and Red maple—Acer rubrum for e.g.]. This European species has been used as an urban tree quite successfully over the last several decades [I would say the last half-century at least], and plays an important role in moderating our city climates and providing a refuge for wildlife. It has attractive foli-age and is one of the few species that can tolerate the tough city soils and climates so common in new developments.
“The sheer volume of seed, shade tolerance of the seedlings and stress tolerance has enabled this species to spread, unintentionally, into natural areas. It’s no surprise then that Norway maple (the species) appears on several lists of unwanted plants. But should we paint Norway maple cultivars with the same brush as the species? Cultivars are clones of ‘selections’ of a plant species that have desirable traits, such as superior leaf colour, flower size or form. There are several commercially grown cultivars of Norway maple, many selected for their unusual leaf colour, size or form. Almost everyone recognizes ‘Crimson King’ and ‘Royal Red’ cultivars [virtually the same tree and the two are constantly mixed in the nursery trade], because they have glossy, burgundy leaves all season, and are among the most popular shade trees that consumers request. Something else you’ll notice about these red-leaved cultivars, they don’t have as many flowers as the species and seed number is also quite low.
“The Connecticut Invasive Plant Working Group identifies the species, Norway maple, as an invasive [as do many other professional and technical organizations, but I would be the last to say that those types of organizations should hold sway on this topic!], but concedes that cultivars need to be evaluated for invasiveness.
“A research team from Pennsylvania State University, led by Dr. James Sellmer and PhD candidate Janine Conklin, recognized the need for cultivar evaluation for this important street tree, and undertook a multi-year study of several mature trees in Ohio. This research group evaluated Norway maple cultivars for flower production, seed quantity, germination and seed viability. Their intention was to identify the cultivars providing the lowest impact to natural ecosystems, or in other words, are thought to be non-invasive.
“The researchers found ‘Crimson King’, ‘Faasen’s Black’ and ‘Globosum’ produced the fewest flowers and seeds. In addition, these cultivars showed only a slight tendency to increase seed production over time. Similar to other deciduous trees, there was much variation in seed viability from year to year for the cultivars and species. ‘Crimson King’ and ‘Faasen’s Black’ both exhibited lower germination rates in growth chamber experiments and all other test conditions, such as open landscape and forest germination. Most cultivars showed moderate to low seed viability and significantly lower germination when compared to the species. Not only do ‘Crimson King’ and ‘Faasen’s Black’ produce lower number of seeds, the seeds also have the lowest germination. ‘Fairview’ and ‘Keithsform’ were both found to have moderate-high seed viability and germination.
“There is a great interest in using native tree species in new plantings right now, but we have to think about the right plant for the right place. The trouble is many of our native trees cannot survive on these newly constructed sites because of poor quality soils, poor drainage, exposure issues and small root zone areas. And you can see evidence of this—empty boulevards and front lawns—along our newer streets. Perhaps we should reconsider using some of the cultivars of Norway maple. Conklin and Sellmer indicate that the best choices for non-invasive cultivars are ‘Crimson King’, Faasen’s black’, ‘Globosum’ or ‘Royal Red’. Next in line would be ‘Superform’. Some of these have smaller canopies, but the benefits of their low seed number and viability really make them a desirable choice, especially where site conditions are tough and care is not even an afterthought.”
I thought I should conclude this week’s article, with a couple of comments on other disorders that may be a concern with not only Norway maples, but also with its cultivars as mentioned above.
In wet climates (such as Vancouver) all of these trees may be found to be susceptible to the annoying powdery mildew disease and I have shown it one of the photos included here. While there are fungicides that can be used to prevent and/or kill this disease, it is likely not worth pursuing since in subsequent years with a slightly different climate cycle, the disease would not likely appear.
The same applies to tar spot of maple, which under certain climate conditions will be very prevalent on the leaves of all the maples discussed here as well as others. It resembles 1 to 1.5 cm round black spots on many, many leaves. This generally causes the leaves to fall prematurely. While no application of any fungicide is suggested for this disease either, it is advisable to rake up the diseased leaves and dispose of them other than by composting them in a home compost pile. In fact, leaves that have been covered at all with powdery mildew would also be as well disposed of in other then home compost piles.