True or False: Six questions to ask before you choose native or exotic plants for your garden
Paul Cappiello Ph.D.
Yew Dell Botanical Gardens Executive Director
For our third and final installment of the native plant/exotic plant discussion, it seems we need to get to the crux of the matter. You need to pick a plant to add to your garden. Do you plant a local native or an exotic species?
Now if you’ve read the first two installments of this series you already known the answer . . . it depends!
So in deciding how to approach this entry I turned, of course, to the keeper of all knowledge . . . the internet. I mean who needs rigorously tested hypotheses and impeccably presented discussions of rigorous research results when I have ready access to the online musings of Kim Kardashian, Snoop Dog and “Weird Al” Yankovic?
And my online research resulted in a list of comments I thought I’d present as a bit of a True/False test. See how well you do . . .
1- It’s better to plant natives because they are best adapted to the local growing conditions.
Hmmm. That’s an interesting one. To address this one let’s think about our old friend Amur honeysuckle (Lonicera maackii). Clearly an exotic and by all accounts a wildly invasive exotic. We’ve previously discussed how it moves into both woodlands and fields in many parts of the eastern US and chokes out just about everything in its path. Not sure about you but to me it seems like Amur honeysuckle is much better adapted to the growing conditions than are those natives that are left in the dust. Does that mean that we should run out and replace all our native plants with honeysuckles . . . of course not. But the “plant natives because they’re better adapted locally” thing . . . that dog just don’t hunt.
Score one for the “false” team.
2- In general, native plants are more in sync with local pollinators because they evolved together.
The poster child for this one is the now well documented myth that many tropical orchids are pollinated by only a single species of insect. It is certainly true that some plants are successfully pollinated by a wide range of general pollinators but there are others that have a much shorter list. But single species . . . not so much. But still . . . in general, ecological factors such as reaction to changing temperature profiles, day length changes throughout the seasons and others seem to give the nod to local plant/pollinator pairings as generally being more in sync. Without question there are plenty of well-behaved exotic plants out there that partner quite nicely with local pollinator species. It’s a mixed bag but in general we’ll give the nod to the native partnerships.
We’ll give this one a qualified “true”.
3- All exotic plants are invasive because they are free from pests, diseases and herbivores they have to deal with in their home land.
In the high plateau of Tibet there is a plant that grows in high alpine meadows. Stellera chamaejasme – it doesn’t even have a common name – is a plant that could quite possibly cause me to sell my soul to the evil gods of horticulture . . . just for the chance to successfully grow one single specimen. I’ve tried. I’ve tried. I’ve tried. It is a stunning plant with a fragrance that makes you go weak in the knees and stay that way for good. I know of one planting in North America in the mile high city of Denver. I’ve visited it several times.
You see where I’m going here? Not only isn’t this stunningly beautiful exotic siren-with-cell-walls not invasive, it is virtually impossible to cultivate in most places on the planet other than high alpine environments let alone my Louisville, Kentucky garden. Sure it’s an extreme example but I think you get my point. “Exotic” and “invasive” are not synonymous. In fact, despite the quite obvious handful of seriously bad actors, the vast majority of introduced/exotic plant species are reasonably well behaved plants in the garden.
This one gets a big old “false”.
4- Native plants are better because they have deeper root systems so can better deal with occasional drought conditions.
You know the kid from grade school who picked up the nickname “stinky” (maybe just because he left an egg salad sandwich in his locker too long)? You know how that kind of nick name can stick for a long . . . long . . . long time?
There’s a graphic that has been circulating around the internet and popping up in garden conference Power Point presentations for years. It’s a graphic that shows the supposed profile of a group of native and exotic plant species with drawings of both their tops and bottoms (roots). And quite magically, it clearly shows the exotics with wide spreading and very shallow root systems and the natives with deep, carrot-like roots. Quite convincing. The problem is, it’s entirely fabricated! Sure, you can hand pick a few native species with deep roots and a few exotics with shallow roots. But on the whole, root system architecture is a highly complex product of millennia of evolution. Even in the very same ecosystem you can find locally native plants growing side by side with completely different root system profiles. Indeed, plant communities evolve together to actually exploit all the space and resources available. If all plants in an ecosystem developed the exact same rooting strategy (or pollinator attraction strategy, or any other adaptation for that matter) it would be a rather weak community with very poor resilience.
Nope. This one is a big fat “false”.
5- Introduction of non-native genes into an ecosystem can damage fragile native plant species.
This is a tough one. Short’s goldenrod (Solidago shortii) is a local endemic represented in the wild by only a handful of very small populations. It’s an attractive little summer bloomer found in open meadows and woodland edges. The problem is, several of these little isolated populations are so small and have been effectively isolated from each other for so long that they are exhibiting what we call inbreeding depression. You know about King Tut’s brother and sister parents? Yup. Same thing going on here.
As a result of too shallow a gene pool, some of these little goldenrod populations produce little to no viable seed. So here’s the rub. Do we artificially introduce plants from one population to another population to add a deeper end to the gene pool and give the populations a genetic boost? Is that our place in the grand order of nature? Should we do all we can to keep these unique little populations completely isolated and watch as they march toward inevitable extinction?
The whole maintenance of genetic purity discussion is a broad and complex one. I simply present the story of our little goldenrod as a single example to illustrate how complex it can be.
Answer to this one . . . Hmmmm . . . It’s complicated . . .
6- Don’t plant English ivy and creeping euonymus because they are invasive exotics.
There’s little argument about whether or not these two popular landscape plants are a problem. Certainly they are exotic and without question they are invasive in and around Kentucky. But here’s the question. With what seems like 2 out of every 3 yards sporting some version of one or both of these species, and with both run amok in parks and wild lands all across the land, do we make it any worse by planting another couple of plants around the mailbox?
Personally, I wouldn’t have either in my garden. They’re not only ecologically destructive, they are also ridiculously vigorous plants that want to take over your entire garden. To me, the question isn’t so much whether or not they’re invasive exotics. To me, I won’t have either in my garden because they are a pain. They’re weedy. There are so many better garden plants out there.
At Yew Dell Botanical Gardens, we have adopted the philosophy that we look for and promote what we call Good Garden Citizen plants. If they’re invasive, we don’t grow them. We don’t want to make the invasive plant problem any worse. But if a plant is weedy, overly aggressive or otherwise makes gardening more difficult, we’re not interested whether the species is native or exotic. We avoid planting it.
Canada goldenrod (Solidago canadensis) is native across much of North America. It’s a tough little native meadow plant that blooms in late summer and services all kinds of pollinators. But it’s a terrible garden plant because it is way to vigorous and aggressive in a cultivated setting. Sure it’s native. But on this one I’ll take a pass.
You’ll have to answer this one for yourself!
This article was originally published in the Courier Journal on February 11, 2022.
About the Author
Blurb about Paul here.
