Real Talk: Space Invaders
Space Invaders: Breeding “Non-invasive” Cultivars
There's nothing like the bloom period of an invasive plant to cue you into its pervasiveness in the landscape. Along the highways, you can spot the bright flowers of forest-eating Asian wisteria, Bradford pear, and sweet autumn olive. As the season progresses, there will be multiflora rose, barberry, and Japanese honeysuckle. These space invaders are everywhere, clogging up our woodlands, along our ditches, at the edge of yards, and seeding into abandoned fields.
How did we get here? What does this mean for the greater landscape?
The presence of invasive plants indicates the quality of habitat conditions. In stable, mature habitats, it is difficult for invasive plants to get a toehold. However, in areas with high disturbance levels, such as along woodland edges and roadsides, bare ground is regularly exposed, allowing weed seeds to germinate. Some argue that because high disturbance areas are the main sites where many invasive species occur, it isn’t a significant concern, relegated to train tracks, byways, and fence lines. If only it were that simple. These out-of-the-way locations with high disturbance are areas where a number of our native plants prefer as well, needing periodic opening of the land between closed-off, mature canopies to expose the ground to dormant seeds. Invasive plants clog up opportune areas for native diversity, blanketing it with a widespread monoculture so thick it chokes out any possible (i.e., the acres of Phragmites outside NYC)
What makes a successful invasive plant?
- The plant has multiple modes of reproduction (seed, rhizome, layering)
- The plant is widely adaptable to conditions
- The plant grows quickly
Some folks argue that better plant breeding allows us to have our cake and eat it. Improved “sterile” selections of nonnative invasive plant materials mean gardeners can have beauty without danger, but is that true? Today, we're not reviewing one space invader—we're looking at a category as a whole—' sterile’ cultivars of nonnative invasive plant material.
How did these plants get here?
The road to hell is paved with good intentions, as the saying goes. Several invasive plants were brought in as horticultural heroes like the Bradford pear or purple loosestrife, which are beautiful, hard-working, adaptable, and easy to grow. Some invasive plants were miscalculated by well-intentioned folks hoping to improve local conditions, like kudzu vine from Japan, coming in as a soil erosion stabilizer, or Johnson grass from Africa as cattle feed. While we had native equivalents that weren't as aggressive, such as Virginia creeper vine for soil stabilization or our native prairie grasses as nutritious animal feed, sometimes we meddle with the best intentions, and cannot control a plant's escape. All living things want to live and grow, and when a plant is given ideal conditions with limited competition or predator checks, these plants run rampant. All it takes is a few decades, and what was once a few plants in the back 40 becomes hundreds of miles suffocated in alien plant material.
What makes a sterile cultivar?
A sterile cultivar has low to no fecundity or the ability to make viable seed that can grow to maturity. Usually, this is done in horticulture using intensive traditional breeding practices such as ploidy manipulation or large-scale outbreeding, but it can include mutation and transgenic breeding programs. Transgenic breeding (genetically-modified organisms or GMOs) is where specific genes, known as transgenes, are introduced to a target genome to improve desired traits, i.e., disease resistance or decreased fertility. Mutation is the application of chemicals or different wavelengths (X-rays or gamma rays) to modify plant cells. Most sterile cultivars result from traditional breeding practices, which still include impressive genetic work. By understanding the ploidy of the target plant (complete sets of chromosomes in a cell), plant breeders cross-breed between species and select plants with attractive features with reduced to nonexistent fertility breeding for triploidy (having three sets of chromosomes). It’s not as simple as Gregor Mendel’s work with pea plants, but plant breeders have many tools in their arsenal. Breeding for sterility is a well-understood process, but there are some hiccups along the way.
What's the harm?
The harm around the release of sterile cultivars can come from miscommunications between the breeders and the layperson, such as the one around Bradford pears and purple loosestrife, as well as the fine print of what makes a “sterile” cultivar. We’ll give slack to the plant breeders for the misunderstanding around the invasion of Bradford pears. It is commonly reported nowadays that the original plant was advertised as “sterile” and planted widely for its ornamental value. However, the plant was always “self-incompatible”. In the literature back in the 1930s, plant breeders understood it to be reproductively viable when outcrossed with other Pyrus calleryana specimens, but unable to self-fertilize. In a bad game of telephone, “self-incompatible” became understood as “sterile,” and catalogs began touting the plant's sterility, not knowing its true nature. As Bradford pears became ubiquitous in the landscape, cross-pollination occurred, fruit was produced and consumed by birds, and over time, an attractive, multi-season landscape tree became the scourge of open lands along I-95 from Georgia to Connecticut. Between miscommunication and overplanting, a garden hero soon became one of America’s most despised horticultural blunders.
Another misunderstanding can occur when breeding efforts go into producing “noninvasive” cultivars. “Noninvasive” cultivars are when companies expend the resources to breed “sterile” or “nearly sterile” cultivars of a known invasive plant. Some would ask why someone would waste such resources when we already have this plant as a problem child in our ecosystems. Usually, invasive plants are highly adaptive, so they’re rugged and reliable in the landscape, making them attractive plants to continue growing. They also have a lot of “brand” recognition, as non-horticulture individuals have heard about the plant and have come to recognize it and purchase it without needing a lot of marketing efforts. There is an argument that if the ornamental plant is bred to be sterile, and if people like it, what’s the problem? Firstly, non-native invasive plants displace our native plants in the ecosystem, whether along roadsides or in someone’s garden. Every time people default to a non-native plant choice, it takes away an opportunity for a native plant to be planted in its place that can support wildlife and the greater ecosystem in the same spot. It’s an opportunity cost, being chosen again and again and again and again. Secondly, in plant breeding terms, “non-invasive” cultivars don’t mean “it will never, ever, in a million years, produce viable seed.” There is room in the definition. Generally, “non-invasive” cultivars mean it’s not 100% sterile; it just means it will produce 5% or less viable seedlings while in testing. Fewer seedlings are not zero seedlings. Sometimes, the amount of viable seed a plant can set doesn’t happen in the first 1-3 years of review and plant trials. It starts 7-10 years after the plant is grown in the landscape, with other varieties or wild plants grown nearby, which is what happens in the landscape. When there is pressure to make a return on an investment, i.e., breeding a new cultivar, time constraints are placed on evaluation programs. It hastens the review period, limiting the thorough process of years of trials in multiple test plots established in different regions of the country before introducing the new cultivar. This exact scenario was found by a recent evaluation of a “noninvasive” barberry cultivar, where in the first 5 years of trial, the plant was found to be near-sterile. Still, by year 10, the same plant cultivar had increased fruit production by over 1000%. Uh-oh. In another case, a supposedly reduced sterility hybrid buddleia variety outcrossed with wild populations and seedlings returned to full fecundity when trialed in one region versus another. It seems like we keep hoping to outsmart Mother Nature, and frequently, when a plant is tested in a new way or given enough time to reach maturity, the plant returns to reproductive viability. As Dr. Malcolm said in Jurassic Park, “Life finds a way.”
Be Skeptical
Some scientists argue that the public’s mistrust of “sterile” cultivars is a misunderstanding that can be fixed through improved testing and better plant breeding. However, our clogged roadsides with acres of Miscanthus, or the barberry taking over the understory layer of Eastern woodlands, or the Bradford pear blooming and colonizing roadways as you drive hours and hours up I-95 in spring, feel like we’ve been burned pretty bad by “miscommunication”. A “miscommunication” that results in millions, if not billions, of dollars in cleanup efforts and restoration. So, while these “non-invasive” cultivars are available on the market, we argue that wasting space and money on something that can potentially cause havoc in the ecosystem is unnecessary. We’ve got endemic plants that are just as pretty while supporting our local ecosystem. As our parents used to say to us when we were kids begging to eat out, “We have food at home.”
When you look at some of the most famous cases of non-native, invasive plants that have escaped into the wild, the shocking part is that a native plant alternative was waiting in the wings, hoping to be chosen for someone’s garden. If our native plants spread around, it would be a blessing. Imagine if we planted wildlife-supporting serviceberry everywhere instead of Bradford pear, and our migrating songbirds had an essential food source that provides adequate sugars and fats to supply their arduous journey south along I-95. Imagine all the swallowtail butterflies that would flock to stands of Joe-pye weed and the nesting bees that could hibernate in the standing, hollow stems in our ditches instead of purple loosestrife. We’d love it if plant selections went off autopilot and if folks gave native plant beauties a try. Folks forget how much marketing went into first introducing horticultural plants that were later found to be invasive. Consider it a sunk cost and move on- we think it's high time we transition from plants that cause environmental havoc and embrace the native beauties that have been waiting all along.
Give us a try - This Instead of That
Marsh marigold instead of lesser celandine
Creek sedge instead of Liriope
Goat’s beard instead of Japanese knotweed
Joe Pye weed instead of purple loosestrife
Allegheny spurge instead of Japanese pachysandra or English ivy
Coral honeysuckle instead of Japanese honeysuckle
Switchgrass or yellow prairie grass instead of silvergrass
Old man’s beard instead of sweet autumn clematis
Further Reading:
M. Brand. (2015) Sterile cultivars (or close to it) ― is this a viable option for the nursery industry? International Plant Propagators Society, Department of Plant Science and Landscape Architecture, University of Connecticut.
Ranney, T.G., Touchell, D.H., Eaker, T.A., Lynch, N.P., Mowrey, J.A., & Smith, J.C. (2006).
Breeding Non-Invasive Nursery Crops. Department of Horticultural Science, Mountain Horticultural Crops Research and Extension Center, North Carolina State University, Fletcher, North Carolina.
Contreras, Ryan. (2022) Breeding for Non-Invasive Nursery Crops: Status of Cultivars and Regulation. Department of Horticultural, Oregon State University.
