Making Decisions with Imperfect or Insufficient Information: Garden Edition
Wherein I wade into the native-introduced garden war
Recently, I was reading through the comments on a gardening article, and one commenter mentioned giving up trying to eradicate lesser celandine as a measure of acceptance. After clutching my pearls and retiring to my fainting couch, I took a minute to examine why I was so triggered (aside from the hours I devote to the years-long process of eradicating exact same thing in my yard every spring).
The commenter justified this decision by referring to an article that points out that introduced species increase biodiversity. This is true. And that we need a more nuanced discussion on introduced species than “introduced = bad, native = good.” I agree. And that our actual, data-based understanding is woefully incomplete. Yep, this is correct.
But, also, biodiversity alone is far from the whole story.
Without further ado, my current lay-enthusiast’s understanding of biodiversity, introduced species, and why more plants isn’t always better:
First, a little background on why there is a strong movement supporting planting native plants rather than those introduced from other parts of the world. While there are tens of thousands of introduced plant species in North America alone, most of which are not overtly problematic, biodiversity (the number of different plants you can count in an area) doesn’t actually tell us much about whether or not that space is a functioning ecosystem that supports a whole web of life.
The plants that are native to an area have grown together with one another and with the local critters for eons. There are a lot of specialist relationships, especially at the foundational levels of the food web, which mean that swapping in an introduced plant in place of a native species will likely cut out a critical layer of the ecosystem with far-reaching effects.
To quote Doug Tallamy, a professor who studies entomology and ecology, discussing the Melaleuca trees introduced to the Florida Everglades:
Melaleuca has transformed the sunny wet grasslands it has invaded into deeply shaded, drier forests dominated by a single species. The grassland birds that breed in the Everglades cannot nest in Melaleuca groves, and they find fewer insects to eat because native insects cannot eat Melaleuca leaves (Costello et al. 1995). Alligators cannot make their wallows or find food in Melaleuca groves, and so they have lost hundreds of thousands of acres of habitat. Butterflies cannot find their host plants, egrets cannot hunt the fish they eat, and hummingbirds cannot find the nectar they need to survive from day to day. Even though the number of plant species present in the area is higher by one, the ecological interactions that drive the Everglades ecosystem have collapsed where Melaleuca has invaded because its dominance contributes nothing to other organisms. Diversity is important, then, only when the species that can be said to create it are contributing members of their ecosystem. This contribution is most likely when species have evolved together over long periods of time. [Emphasis mine.]
The focus on keeping or restoring functioning ecosystems is critical because of the biodiversity crisis that is fueling the mass extinction of species happening now.
NATIVE PLANTS
Native plants are those species that have “evolved together over long periods of time,” as Dr. Tallamy puts it, in your particular location and, therefore, are a functional part of your local ecosystem. By definition (see below), native plants can never be invasive. That does not, however, mean that you want each and every one of them in your yard, or want them where they are attempting to grow.
For instance, I have American elm trees in my yard. Due to their prolific seeding, I pull thousands of elm seedlings every year when weeding. I keep and protect the mature elms I have, but I do not have an appropriate site available for a large tree, so the small ones get plucked. I also get black snakeroot volunteering all over my yard. It is native to this area and beneficial to wildlife. It also has horribly stickery seeds and is abundant elsewhere, so mine ends up in the compost.
Many of the native plants that I have were bought from a nearby nursery that specializes in our native plants, but plenty more have been rounded up from the weeds that grow of their own accord. I like the PlantNet app for identifying plants, so I can remove the invasive ones and transplant or leave alone the natives I would like to have in my yard. Transplanting your “volunteers” can make for a beautiful garden that sustains the local ecology and costs only the effort it takes to move plants to somewhere I’d like them to be.
INVASIVE PLANTS
In the US, for a plant to be considered invasive, it must be an “alien species” (introduced from outside the local area) “whose introduction does or is likely to cause economic or environmental harm or harm to human health.” By definition, only introduced plants can do this because native plants are, well, native. They will push and shove one another depending on how site conditions favor or disfavor particular plants, but they are all evolved to be a functional part of the local ecosystem.
We live in the age of information, so whether or not a plant is invasive in your area can be researched on your phone in a few minutes while you are at the garden center. Just pull up your search engine of choice and type in NAME OF PLANT + INVASIVE + YOUR LOCATION. A state or province is usually the relevant unit of location in North America.
This is the easiest category to find information on. Every state has a registry of plants that are considered noxious weeds or invasive. If the plant you are considering shows up on these lists, don't buy it, don't plant it, and if you have it, look up how to control or eradicate it.
NOTE: Looking up best practices is critical for dealing with invasives or noxious weeds. The way a plant gets on the invasive list tends to involve a combination of prolific reproduction and being nigh-on indestructible. Consult with the eggheads and do as they recommend. This may involve things you don't much care for (such as using herbicides), but I assure you, if the Department of Natural Resources is telling you to spray poison, they definitely tried digging it out and cutting it back first without a good result.
Invasive plants have been growing in their introduced environment long enough that we know they are an environmental disaster. They escape cultivation and make their way into wild spaces and choke out native plants that were growing there. Taking an example from my own yard, buckthorn is a known baddy.
Forests have four above-ground layers: the canopy of big trees, the understory of smaller trees and young saplings that will replace the big trees in time, the shrub layer, and the herbaceous layer of non-woody plants. In forest preserves here, if the land is not carefully and laboriously managed, buckthorn will replace most of the saplings and small trees in the understory—eventually affecting the canopy as there are fewer replacement trees available, replace essentially the entire shrub layer, and shade out much of the herbaceous layer. Thus, we have plenty of data to indicate that we have no business planting buckthorn and a strong case for removing it wherever we do find it.
Lesser celandine is an invasive species that affects the herbaceous layer of my local woodlands. Much of the diversity in the native herbaceous layer in these woodlands consists of spring ephemerals that live most or all of their annual lifecycle before the deciduous trees and shrubs grow their leaves in spring.
In a natural experiment, the house next door to me is slated to be torn down due to decades of neglect, and the current owner gave me permission to rescue any plants I wanted before the bulldozers arrive. The yard has been more neglected than the house, so the laissez-faire landscaping consists of plants native to this area since before it was a subdivision (~70 years) and the invasive species that are displacing them. The backyard of this property is overrun with lesser celandine, which blankets nearly all available ground from before the spring ephemerals emerge through when most have gone completely dormant. The front yard of this property has not yet succumbed to that particular invasion.
From the front yard, I transplanted trillium, spring beauty , common wood sedge, white trout lilies, and cutleaf toothwort. There were several more species in the maple-and-oak dominated front yard that I didn’t bother transplanting.
From the backyard, there were only a few trillium, and just about everything else I could see was lesser celandine or buckthorn.
NOT TECHNICALLY INVASIVE, BUT PROBABLY NOT A GREAT IDEA
This category is fuzzier than the prior one. It includes plants that are both introduced and aggressive spreaders. Often, these are advertised as "low maintenance" plants that will “fill in quickly.”
Definitely be wary if you find them listed as invasive or noxious weeds somewhere, even if not in your exact location. With plants from this category, please do give them a miss. Think about what you are trying to do functionally or aesthetically, and see if there is an alternative option that is less likely to wander out of your yard of its own accord. Often, an internet search for "alternatives to _______" will get you started.
In my yard, yellow archangel is an example from this category. It is not currently listed as invasive or noxious in my area, and I don't tend to find it outside the flower bed a prior owner planted it in, but it is on such lists in the northwestern US and Canada. As such, it is on my list for removal, but it is being cleared out bit by bit as I turn the landscaping over to match my preferences. Also, since it seems like it has the capacity to become a problem, I put yellow archangel into a contractor bag and then send it to the landfill, rather than throwing it in my compost.
PROBABLY OKAY INTRODUCED PLANTS
This category includes plants that have been introduced in your area for at least several decades or so without ending up on any watch lists.
A bunch of food plants fall on this list, as do many decorative landscaping plants. An argument can be made about opportunity costs associated with the plants they are displacing, though in practice, if they are in someone's yard in this day and age, the construction process is when the original ecological sin was most likely committed, and the most probable thing being displaced is turf grass.
In my yard, hostas fall under this category. For all their reputation of prolific spreading, I've never seen a forest preserve overrun with them. I just ripped out a patch of hostas that had been neglected for decades, and while they had certainly reproduced, they hadn't creeped over the boundaries of their original planting.
I also have containers that I plant every year with tomatoes and herbs, all of which are introduced plants, but not invasive, as well as containers of sentimental plants from different parts of the world that wouldn’t survive our winters. I don’t have any compunction about any of them. They feed me and/or bring me aesthetic joy, and they aren’t harming anything.
In my mind, this category falls under aesthetic or functional preference. One is not doing significant ecological harm by planting hostas, basil, or a row of tomatoes. Though if introduced species are the only thing growing in your dirt, that dirt and its inhabitants aren’t going to be well-integrated into the surrounding web of life that sustains life in your local ecosystem.
WE DON’T KNOW
Anything that has yet to be introduced is an unknown quantity. It could be fine, or it could be the next great invasive. How are we supposed to tell?
The most conservative approach is to skip it entirely. Having spent more hours ripping out invasive plants than I care to count, this is my general inclination, but I do understand that my personal stance is more restrictive than many avid gardeners prefer.
That leaves us with looking for proxy information to get at least a solid guess at if it is likely to be a problem or not.
Based on Principles of Ecological Landscape Design:
JUST SAY NO:
This species is already listed as invasive elsewhere and not native to anywhere near you.
It's super fast growing or fast maturing, reproduces vegetatively (sends out shoots, not just seeds), and its seeds will germinate readily, and not from anywhere near you.
BE CAREFUL; FURTHER ANALYSIS/MONITORING NEEDED:
Native to your continent, but not your local area, and has a reputation for being very aggressive or invasive elsewhere.
It is an introduced species with that matures quickly, produces viable seeds, or spreads quickly by vegetative means. Any one of these is enough to put it on the “be careful” list.
PROBABLY OK:
Species is bred to be sterile1 and does not spread quickly by vegetative means.
Species is native to your continent (but not your specific location), does not grow rapidly, does not spread quickly by vegetative means, has seeds that need pretreatment to germinate, AND is not in a family or genus that is already strongly invasive on your continent. All of these conditions need to be met.
TL;DR:
Natives can't be invasive, though they can be aggressive spreaders or inappropriate to your landscape/garden needs.
Invasive plants should not be planted and should be removed when found (trust me, I understand what a total PITA/ongoing battle this can be).
Newly-introduced plants should be treated with some wariness if they produce viable seeds, grow quickly, or spread quickly.
Introduced species with an established reputation of non-violence to other plants probably do have an opportunity cost, ecologically speaking, but are generally fine for yard planting (though maybe don't rip up virgin soil to plant them).
The ‘Bradford’ cultivar of callery pear is a classic example of a failure here. The cultivar was developed to be self-sterile, which meant that it could not reproduce with other ‘Bradford’ callery pear trees. Unfortunately, it could make viable seeds if there was another non-sterile callery pear cultivar nearby. Per the Morton Arboretum, “This tree is under observation and may be listed on official invasive species lists in the near future.” Whoops.
Loved reading your thoughts on this!