The summers of my youth in Eastern North Carolina smelled of Chinese privet (Ligustrum sinense) and Japanese honeysuckle (Lonicera japonica). As a kid, I loved playing with the tiny “berries” of the privet and sucking the nectar from the honeysuckle flowers. Warm memories aside, these two species are landscape plants turned weeds, which escaped cultivation and invaded large areas across the Southeast. As someone who works with the nursery industry and specifically with this issue of weedy or invasive plants, it sometimes feels that folks believe all introduced plants are bad, and we should only grow natives to protect our ecosystems.

We should think, however, about what is it we are asking our landscape plants to do. In the city, we want them to survive stress, even to flourish. We want to punish them with drought, heat, pavement, and poor and compacted soils while still enjoying their shade, beautiful flowers, lovely scent, and fruit. Whether native or introduced, plants that thrive well enough to escape cultivation are doing exactly what we asked of them.

I often hear that we should only plant native plants because they are best adapted to a site or region. If that is the case, how do the non-native and introduced species outcompete them? There also are “native” plants that have become “invasive”: western juniper, for instance, now covers more than 2 million acres of grassland in Oregon, its spread aided by fire suppression. We need plants that do well in our cities. We should care less about their provenance and focus more on their behavior. The problem isn’t trees that flourish, but trees that won’t stay where we put them.

Take the Amur and Norway maples, two resilient species commonly found in our cities. Easy for producers to grow, they thrive where other species may not survive. Amur maple is hardy to USDA Zone 2, fitting the bill for a small urban tree in regions short on options of plants from which to choose. Norway maple is hardy to USDA Zone 4, making it suitable as a medium to large tree in most of the US. Both are relatively free of major pest problems, and transplant well. Norway maple is also incredibly well-adapted to heavy clay and compacted soils, tolerates pollution, and holds up better to drought conditions than sugar maple. Unfortunately, both have done their job too well, and have escaped cultivation to invade native forests and cause real problems in several parts of the country. As an urban tree, however, they fit the bill incredibly well, helping to ameliorate the heat-island effect, manage stormwater, and beautify our paved metropolises. It is not surprising that such resilient trees can outcompete other species.

On Burnside Avenue in downtown Portland, Oregon, just down the street from Powell’s City of Books, there is a planting of Norway maple that separates opposing lanes of traffic. The soil volume is tiny, and tall buildings loom on either side. Yet, these Norway maples are gorgeous; more than 35 feet tall and healthy, they cover most of the five-lane driving surface and cast shade on the sidewalks for pedestrians. Contrast this to urban instances of our native bigleaf maple, such as the large specimen near Valley Library here on the Corvallis Campus, or the majestic tree that greets you as you set out on the trail at Hoyt Arboretum in Portland. These are “easy” sites for trees, with large soil volumes and little compaction. You will not find bigleaf maples adorning streets like Burnside Avenue, however.

We could alter conditions to suit bigleaf maple—redeveloping our cities for more soil volume, less concrete, and less pollution—but that does not seem likely. Alternatively, we could breed more resilient bigleaf maples—a path that is being explored, but likely will take a very long time.

The problem isn’t trees that flourish, but trees that won’t stay where we put them.

My research program is making great progress pursuing a third option: breeding Amur and Norway maples that stay put where we plant them. We want to provide growers, land managers, and the public the utility of resilient trees that are good for cities, but also do not reproduce in sufficient numbers to displace our native flora.

Here, it’s worth mentioning ‘Bradford’ pear. Perhaps the most numerous of the many cultivars of Pyrus calleryana, it has become the poster child for invasive plants. Smelly, weedy, fragile in ice storms, it’s the tree people love to hate. ‘Bradford’ and other pears are self-incompatible, which means they need another genotype to fertilize their ovules and form seeds. Soon—as new cultivars were introduced—these genotypes started cross-pollinating and producing fruit, soon becoming the weed we know today. Pyrus ‘NCPX2’, the Chastity® pear developed by Tom Ranney of North Carolina State University, by contrast, was recently tested for fertility compared to wild-type, and is not merely self-incompatible. Chastity® is a triploid—that is, it has three sets of chromosomes. This odd ploidy (number of chromosome sets) disrupts normal formation of pollen and eggs, resulting in a plant that infrequently or never produces viable seeds. The most famous triploid out there is banana. If you have enjoyed a ‘Cavendish’ dessert banana, then you have enjoyed a delicious fruit rendered seedless through triploidy.

Though there are reported examples of Norway maple exhibiting reduced seed set or seed germination, in my experience these cultivars are perfectly fertile. It is unclear in what contexts the trees have set seed, but these cultivars are not sterile across environments—thus my reluctance to use the word “sterile” in context of seed set. As with most cases in nature, there is a gradient from perfectly fertile wild-type down to complete sterility. As such, I try to stick with “reduced fertility” as the descriptor for cultivars that reproduce at such a low level as to pose no ecological threat.

The first step in the process was to induce chromosome doubling of standard diploid plants (containing two sets of chromosomes) to develop tetraploids (plants with four sets of chromosomes). We planted our tetraploids alongside diploid cultivars at our field in Corvallis and allowed them to open pollinate. We collected seed from the tetraploids, grew seedlings, and tested their ploidy level. Fortunately, most of these seedlings were triploid—they received two sets of chromosomes from their tetraploid female parent and one set from their diploid male parent. Furthermore, these seedlings are not genetic composite (chimeras), but are triploid in all cell layers, and thus highly stable from one generation to the next.

Ten years after starting this project, I published the results of this work in 2020 in the journal Horticulturae. But the work in so many ways is just beginning. To produce the seedless trees we desire, they must be propagated clonally. Traditionally, Norway maples (and Amur, too, in some nurseries) have been chip budded, grafting the cultivar of interest to seedling rootstocks. While this production system speeds up the production and quantities of triploid clones, we need a new tactic. This is because we must avoid at all costs grafting our sterile triploids onto fertile diploid rootstocks—rootstocks that can sometimes send up their own shoots and eventually produce seeds, which happened with callery pear. For Amur maple, this is not a major problem, as it readily roots from stem cuttings. For Norway maple, which does not, we have been working to optimize cutting propagation. We now have triploid genotypes of both species, which we are growing via micropropagation, using sterile culture in vitro to multiply plants in large numbers relatively quickly. This technique is used in many taxa that would otherwise be slow to increase using other methods, such as hazelnuts (Corylus avellana). It also is frequently used in red maple as a means to increase and distribute clean clonal material. Our triploid plants will be ready to come out of micropropagation and harden off to begin production trials during 2022.

Evidence of reduced fertility gives us much reason to hope. Amur maple triploids in our plots have flowered in the presence of pollinators and fertile pollen donors and have produced no viable seeds to date. While this inspires confidence, I am not ready to bet the farm—or rather, to have growers bet theirs. Our next step is to work with nurseries, universities, and public gardens around the country to install replicated tests of our trees to see how they perform in other environments. The stakes are too high not to verify.

Of course, my title is Ornamental Plant Breeder, so the trees resulting from this work should have some aesthetic appeal. To that end, we are working with J. Frank Schmidt and Son Nursery along with Tom Ranney to evaluate seedlings of Amur maple selected at JFS in Boring, NC State in Mills River, and Corvallis, OR. Ten genotypes from each location were propagated during 2021 under production conditions to identify superior forms. Furthermore, the trees from micropropagation will be included in a parallel study. The end goal is to develop and test trees according to the best scientific methods we have, while working with growers to ensure that we are meeting their needs for trees that work in production.

There is no doubt of the need. Industry partners report more than 90% reduction in Norway maple sales, with steep declines in Amur maple as well. Certainly, overplanting of maples has reduced demand, but the invasive issue has also had an impact, and the industry is ready for cultivars of these species that could be sold in longstanding markets such as the upper Midwest and New England.

Evidence indicates the trees I have developed (and those of my colleagues like Dr. Ranney) are “sterile,” or close enough that they present no threat of invasion. The biological side of the problem is largely solved. What remains is the political aspect, which in many ways is more difficult. The story of ‘Bradford,’ damaging in its lack of nuance, has spread effectively, and plants like Norway maple may prove difficult to reintroduce as a result. Already it is illegal to plant A. platanoides in Massachusetts, and many other states—a rule which leaves no room for reduced-fertility cultivar exemptions.

We need a national conversation on this topic in the Green Industry, to collectively establish the framework for reintroduction of sterile versions of weedy species. The specifics of individual plants are highly regional, and thresholds should be determined at a state level, but the issue is a national one. The shade of that tree you’re enjoying on the east coast may have gotten its start here in Oregon. As such, the rules enacted in Massachusetts have wide-ranging impact. The need for education, collaboration, and nuanced regulation will only grow, so long as cities remain, and climate change increases the demand for resilient trees.

Ryan Contreras is Professor and Associate Head of Horticulture at Oregon State University.

From “free” to “friend”…

Established in 1911 as the Bulletin of Popular Information, Arnoldia has long been a definitive forum for conversations about temperate woody plants and their landscapes. In 2022, we rolled out a new vision for the magazine as a vigorous forum for tales of plant exploration, behind-the-scenes glimpses of botanical research, and deep dives into the history of gardens, landscapes, and science. The new Arnoldia includes poetry, visual art, and literary essays, following the human imagination wherever it entangles with trees.

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