BioScience

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Horticulture’s Weedy Introductions in a Changing Climate

In case I need a reminder that the horticulture industry has a history of introducing weedy plants to natural areas, I get one each time I bike to work. Riding along the Boise River Greenbelt, a trail that for much of its length is flanked by cultivated landscapes on one side and a highly modified but largely naturalized river bank on the other, I see a mixture of both native and introduced plants. Of the introduced plants, many are horticultural species that have escaped cultivation and established themselves on the bank of the river. There are catalpa and black locust trees brought in from the other side of the country, St. John’s wort and chicory from Eurasia, honeysuckles primarily from Asia, and a few different cherry species and hybrids with varied provenances. And this is just a small sample of what can be found along my three and a half mile bike ride.

St. John’s wort (Hypericum perforatum) on the banks of the Boise River

This is certainly not a new concern. We have been aware of the role that horticulture plays in introducing invasive species for quite some time now. Several years back, while doing a deep dive into the topic of invasive species, I wrote about this issue right here on this very blog. According to a study published in Frontiers in Ecology and the Environment (2021), out of 1285 plant species identified as invasive, 61% are currently sold in nurseries. If that’s not concern enough, an additional factor to consider is climate change. Plants that were less likely to escape cultivation and head for the wild, may take the opportunity to do so in a changing climate. Plus, horticultural plants that are already problems in certain areas could expand their range as climates become more favorable in new locations, especially if these plants continue to be sold in nearby nurseries.

These concerns and more are the topic of a paper published in BioScience (2023). Evelyn M. Beaury, et al. looked at nurseries across the United States and the plants they sell in order to determine where invasive plants are still being sold in regions where they are invasive. Additionally, they looked at plants known to be invasive but that are not currently invasive in the regions they are being sold. Using climate models, they predicted whether or not these plants could become invasive under changing climates.

Plants are being moved around with a lot more ease than they once were, and the sales of problematic plants are increasingly difficult to regulate. For one thing, plants prohibited for sale in one state can be purchased at nurseries in neighboring states and brought back to be planted in regions where those plants are invasive. And while mail order has existed for a long time, online ordering makes the process even simpler; and many online plant vendors are not liscensed nurseries, making them much more difficult to regulate. But even regulation is typically a response to something that has already become a problem, rather than a proactive measure to prevent plants from escaping into natural areas.

Beaury, et al. identified 672 nurseries across the United States, both online and traditional retailers. Each of these nurseries were selling one or more of the 89 plant species that became the focus of their research. These are plant species that are either on federal or state noxious weed lists or that have been identified as invasive by Invasive Plant Atlas. The reach of each nursery was determined by using customer reviews to compute distances that plants might travel after being purchased at nurseries or from online stores. Obviously, not every customer that purchases a plant leaves a review, but this is a good way to get a general idea how far away customers are from nurseries without having access to more detailed records. These geotagged reviews can also be cross-referenced with known distributions of invasive plants. Using climate models and environmental predictor variables, the researchers determined areas of current and potential invasion for each of the 89 plants.

tansy (Tanacetum vulgare) – one of the 89 plant species looked at in the study

The first question was about proximity to current records of plant invasions. Results showed that “49 of the 89 ornamental invasives were sold within 21 kilometers (13 miles) of an observed record of invasion.” When invasive plants are sold and planted near locations where they are already known to be invasive, it gives them the opportunity to add new plants to existing or developing invasions. In ecology, this is known as propagule pressure. When it comes to current and future climate, most species in the study are being sold by nurseries where the climate is either currently favorable for range expansion or may eventually become favorable. Specifically for future climate, 40 of the 89 plants are being sold in regions that are currently suitable for invasion and will continue to be suitable as the climate changes, and 25 of the 89 plants are being sold in regions where the climate is currently unsuitable but will become suitable as temperatures warm.

Particularly for plants being sold in areas that are not yet suitable for invasion, there is time to educate both the nursery industry and the general public and to look for alternatives to these plants. However, as the researchers point out, their analysis “only examined about 10% of the larger pool of U.S. ornamental plants known to be invasive,” and they “sampled only a subset of the nurseries that could be selling invasive species in the United States.” It is highly likely that the results of this study are an underestimation of the problem. Clearly the work of education and finding alternatives to problematic plants is monumental. The hope is that studies like this can help with education and can assist with working out ways to regulate sales of invasive plants.

coltsfoot (Tussilago farfara) – another one of the 89 plant species looked at in the study

Regulating the sale of plants is beyond most of our control, and how much regulation we should be enforcing on nurseries in the first place is a debate we should be having. Outside of those questions, there is a responsibility that we should take as gardeners and as residents of the planet. If we choose to grow plants, it is crucial that we get to know them. We should be taking the time to observe the degree to which they spread and how they are being dispersed. When they do move around our yards, where are they going, and are they able to grow outside of our care? Are they leaving our properties and coming up elsewhere? If we choose to plant non-native species, we should be mindful of how they might affect nearby, wild landscapes if they were to escape our yards and establish themselves in these locations. We should also be aware of where we live in the city. If our gardens are in the middle of a dense urban landscape, perhaps there is less concern that our plants will move beyond the borders of our gardens. But if we garden near natural areas, we should be significantly more selective about the things we plant, and we ought to be more observant as to what those plants are up to.

Nurseries generally sell the plants that gardeners want to buy, which means we can choose not to buy problematic plants and instead demand alternatives to these plants. Seeking out nurseries that sell the types of plants that are better suited for our regions and do not exhibit invasive behaviors can send a message to other growers that they should phase out certain plants and start growing the plants that gardeners are asking for. This may be a simplistic take, and as with most things, it’s complicated. While one of the goals of this research is to help influence regulators, another goal is simply to “[share] information about high-risk ornamental invaders across states and regions, and [work] with horticulture and community members to reduce the escape of ornamental species into natural areas.” This is precisely the area where gardeners can make a difference.

On that note, I will be starting a new series of posts to discuss some of the ornamental species that have gone weedy. By getting to know the plants that find themselves in this predicament, we can be better situated to make informed decisions about what to do about them.

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Horticulture’s Role in the Spread of Invasive Plants

I live in the city of Boise – a bustling metropolis by Idaho’s standards. It is located in the high desert of the Intermountain Northwest in a region called the sagebrush steppe. Our summers are hot and dry, and our native flora reflects this.

When I leave my apartment I am greeted by a flowering quince (Chaenomeles sp.). At this time of year it is in full bloom and looking amazing. It originated in East Asia. To my left I see a tree of heaven (Ailanthus altissima), a common urban tree that came to America from China via Europe. To my right there is a row of Norway maples (Acer platanoides), another popular urban tree. As its common name suggests, it is a European species that is distributed across large portions of eastern and central Europe. None of these plants are native to the sagebrush steppe, nor would they survive the harsh conditions without supplemental irrigation. All are horticultural introductions.

Tree of Heaven (Ailanthus altissima) – photo credit: wikimedia commons

But there is another thing that at least two of these species have in common. Tree of heaven and Norway maple are considered invasive species in North America due to their propensity to spread into natural areas and disrupt native ecosystems. They also have a reputation of being pesky urban weeds.

My experience isn’t unique. Yards across North America are planted largely with species that are not native to this continent, and while most species stay where we plant them, a significant portion of them have leaped out of our tidy landscapes and disseminated themselves across natural areas, earning them the title invasive species.

In a paper published in BioScience (2001), Sarah Hayden Reichard and Peter White discuss the role that horticulture has played in introducing invasive species to the United States. Humans have a long history of moving plants from one part of the world to another for food, fuel, and fiber. However, collecting plants from around the world and organizing them into gardens for aesthetic purposes is, by comparison, a more recent thing. Species used for ornamental horticulture are what Reichard and White are concerned about.

As an introduction, Reichard and White offer a quick history of the beginnings of ornamental horticulture in the United States. This period is summed up well in an article by Richard Mack and Mark Lonsdale in the same issue of BioScience:

As colonists became more secure in their new environments, they began to import ornamental species from their homelands and elsewhere, in simultaneous quests for both familiar and unfamiliar plants. These plant importations sprang from deep-seated or primal aspects of human behavior shared by people in former colonies and homelands alike. … Many needed to be reassured with familiar plants from home, and they also had seemingly antithetical desires to experience novel, exotic ornamental plants.

Today, plant explorations continue throughout the world, often with the goal of introducing new plant species to the horticulture trade, and avid gardeners remain eager to find something new and interesting to add to their yards. There is nothing inherently wrong with this. Nor is there anything inherently wrong with filling our yards with exotic plants. The trouble comes when these plants escape cultivation and cause problems in neighboring ecosystems. Bringing awareness to this darker side of ornamental horticulture is what Reichard and White endeavor to do.

“Thomas Jefferson, an avid horticulturist, also introduced several species. He may have been the first person to introduce Cytisus scoparius (Scotch broom) as an ornamental species; that plant is now an invasive species in many parts of North America.” — Reichard and White (2001) [photo credit: www.eol.org]

Major players involved in the global movement of horticultural specimens include botanical gardens and arboreta, nurseries, garden clubs and horticultural societies, and the seed trade industry. The motives for transporting species vary among the groups, as do their roles in addressing the invasive species issue. Many botanical gardens have extensive plant exploration programs, which today are often more conservation focused than they were in the past; however, some of the species acquired during these explorations are released to the public, often without certainty that they won’t spread.

Even though most nurseries don’t have active plant exploration programs, they may acquire plants from nurseries or other institutions that do. For business reasons, plants may be sold before they have been properly screened for invasive-ness. Some retail nurseries make an effort to not sell plants that are known invasives in their regions. However, there are plenty of mail order nurseries that may not be aware of or may simply ignore the fact that they are shipping plants to regions where they are invasive. Seed exchanges between garden clubs and botanical societies, as well as the seed trade industry, are also responsible for shipping species to areas where they are currently or may become invasive.

“Uninformed people sometimes dump their aquarium water and plants into local water sources, and many of the aquarium plants survive and multiply. Hydrilla verticillata, a very aggressive aquatic weed in the South, was probably introduced to provide a domestic source of this plant for the aquarium trade.” — Reichard and White (2001) [photo credit: wikimedia commons]

Plant exploration will continue, and many new plants will be introduced to the public through the horticulture trade. Rules and regulations help restrict some plant movement, but in a capitalist society such restrictions will ultimately be, as Reichard and White write, “a compromise between ideal invasive plant exclusion and trade facilitation.” Plants can be screened for invasivibility, but it is difficult to know if, when, and where a species may become invasive. Furthermore, given enough time, a species that appeared to stay put can suddenly start to spread (or could have been spreading all along unnoticed).

Reichard and White acknowledge that “the burden of finding a solution to the problems posed by invasive plants does not necessarily fall on the shoulders of [the horticulture] industry.” Various groups from broad disciplines will have to come to together to work towards a solution. Reichard and White offer some suggestions for working together. For example, invasive species biologists can share their research with the horticulture industry which can, in turn, communicate this information to the public through garden writers and speakers. Botanical gardens can take a leadership role by vowing to “first do no harm to plant diversity and natural areas” and by providing public education about the issue.

Efforts can be made to ban the sale of problematic plants and to encourage proper screening of new introductions, but public demand for certain plants may remain. So, “better communication from ecologists to the public about which species are causing problems will discourage people from buying them.” Involving the public in eradication efforts can also help raise awareness, as people can see first hand that plants in their yards have invaded the wild.