Last week Jeff kicked off a lively discussion about invasive plants. Let me state up front that no one on this blog is promoting invasive plants. But the issues surrounding invasive plants are extremely complex and have profound implications for many groups with whom we work in landscape horticulture and urban and community forestry. It is essential in these discussions that we separate fact from hyperbole. In some quarters, lines have been blurred and people fail to make key distinctions and lump exotic, alien, or non-native species together with invasives. According to the Federal Executive Order on Invasive species “Invasive species” means an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health. All invasives are alien but only a small fraction of alien species are invasive (all humans are mammals but not all mammals are humans). Nevertheless, there is a temptation to ‘hedge all bets’ and promote only native species for horticultural planting since native plants, by definition, cannot be invasive. In addition, there is a ‘feel good’ aura that surrounds native plants – if they’re native they must be good – that clouds some of the logic in the argument.
Some examples:
Natives are more stress tolerant and better adapted than exotics.
Really. If native plants are always better adapted, why do we have invasives? Shouldn’t the “better adapted” natives out-compete them? Stress tolerance and adaption are a function of natural selection pressures of the environment in which a species or population evolves. The world is full of stressful environments and, therefore, lots of stress tolerant species. There is no a priori reason, for example, to believe that a native species needs less water than an exotic. The ability to withstand drought depends on the particular species in question. I’ve done a lot of research on stress physiology of Scots pine – few, if any, native species here in Michigan can match it for drought and cold hardiness. Moreover, as Jeff pointed out, most of our urban and suburban environments no longer reflect native conditions. Urban heat islands can result in temperatures 10-20 deg. F warmer than the native countryside. In our research on heat island effects in downtown Lincoln, NE we logged temperatures in tree canopies in excess of 125 deg. F. These temperatures were coupled that with the usual urban conditions of impervious surfaces and compacted soils – what tree species is native to that ecosystem?
Native restoration? This nurse-log ecosystem is typical of forests in western Oregon & Washington. Trying to keep it alive in downtown Portland requires constant mist irrigation..
Native plants are more pest resistant than exotics. This would be true if native pests were all we had to contend with. But the exotic pest train has already left the station. Emerald ash borer, Dutch elm disease, white pine blister rust, chestnut blight, Asian long horned beetle, and sirex wood wasp are here and here to stay. And their friends are coming. The continued expansion of global trade will almost undoubtedly mean that exotic pests, for which native trees have not evolved resistance, will become more, not less, of a problem in the future. Relying exclusively on native trees means more, not fewer, catastrophic tree failures. Heavy planting of green and white ash, which are both native in Michigan, has resulted in the loss of 30% or more of the urban tree canopy to EAB in some Michigan communities.
Natives increase diversity This presupposes that exotic species do not or cannot fill niches occupied by natives. Exotics can certainly add structural diversity and age class diversity to an urban and community forest. I would also argue that they add to species biodiversity as well. If we consider an urban community such as Lansing or Detroit, there are maybe six or seven native tree species that we could expect to have reasonable longevity as street trees. If we expand our choices to include non-natives we can expand the list to twenty or so. Not a huge number to be sure, but still a better hedge against catastrophic urban tree loss that the ‘native only’ policy.
Where to go from here? We cannot ignore that fact the invasive plants are a huge economic and environmental issue. Presently we do not have models that will accurately predict which exotics will become invasive and which ones won’t. Trees that are demonstrated to be invasive in a given environment need to be dropped from planting programs. Except for the desert Southwest and parts of the Plains, every region of the country has great native trees that can. and should, be an integral part of their urban and community forests. While it’s tempting to play it safe and promote natives only, this policy has significant shortcomings. Urban and community forests provide enormous economic, environmental, and societal benefits. In order for our urban forests to provide these functions over the long term we need as broad an array of trees species as possible, including appropriate exotics.
Nice post, Bert. And it connects well with where discussion ended up last week on Jeff’s post. I got a lot of heat from the local native plant societies years ago when I started challenging the “native plant superiority” mindset (readers can see this column from 2001 at http://www.puyallup.wsu.edu/~Linda%20Chalker-Scott/Horticultural%20Myths_files/Myths/Native%20plants.pdf). But urban environments are not natural, and many native plants do poorly under artificial conditions, regardless of geographic nativity.
Thank you for a concise and thoughtful discussion of this topic.
I’ll add two further considerations:
In an agricultural state like mine (Kansas) the ornamental woody invasives so condemned back east – burning bush, barberry, buddleia – simply aren’t a problem. You don’t get any kind of invasives in a wheat field…
As far as pasture or timberland – well let me put it this way. For several years I drove past a quarter section of neglected pastureland on my way to work. I watched the natural transition as the hedge, locust, and red cedars filled in. Oh, and sunflowers, too!
(A brief aside here – many years ago, the Nebraska legislature proposed that sunflowers be denoted a noxious weed. The governor of Kansas threatened to mobilize the state National Guard and invade – the sunflower is the state flower, after all. I could be wrong about the details, but that is how I remember the incident…)
Anyway, one spring as I passed that pasture, I saw Callery pears blooming there. Well, I guess all those 10’s of thousands of Bradford pears that we all planted 30 years ago do indeed set viable seed! By fall, however, that was no longer a problem – there were 160 new homes on that 160 acres, and nobody had to worry about any kind of species, native or alien…
Brian
Right on, Bert. I was just visiting another gardening blog’s comments regarding a “Low-maintenance” garden. A reader commented [Quote] “I’ve gone native. By replacing “fussy” plants with versions of what likes my Ohio climate and soil, I don’t spend time on coddling. I still weed, prune, and otherwise encourage my garden, but I’m not pushing the rock uphill to keep alive temperamental stars that are borderline hardy, pest/disease prone or otherwise labor intensive extreme.” [End quote]
This kind of vast generalization/thinking baffles me. I have a nice mix of “natives” and “non-natives” in my home garden; same as with the campus garden. We don’t treat one any differently than the other. Ever. Bed prep, mulching, weeding, cutting back, etc. All the same. ???
Thanks, Bert, for a really good post. I’m involved with the New England Wild Flower Society, a great native plant organization, but have no illusions that natives are the sole answer. Natives are well-suited for native habitats; plenty of non-natives are, too. The fallacies that worry me, because they’re being spread with abandon by architects, some landscape architects (yup — I blush to say it, but it’s unfortunately true), engineers, and developers, is that natives need no maintenance and no irrigation. Sheesh. Natives are plants. Non-natives are plants. If you put a plant in conditions where it can thrive, it will thrive. If you put a plant where conditions will kill it, it will die… I’m glad you posted this piece; the more reason brought to the exotic invasives/natives discussion (it seems as if it has to be a discussion that includes both natives and exotic invasives), the better!
I take care of quite a number of mostly or all native gardens in the midwest – both prairie and woodland gardens. They’re not watered much if at all after the first season which is a plus. But they’re definately not lower maintenance, if fact they require just as much maintenance as the non-native perennial garden – lots of pruning, lots of culling the seedlings and thinning out those that spread everywhere. And that’s for aggressive ones. Other natives are not so aggressive and if fact you have to keep their more rogue neighbors from taking them over – the poor royal catchfly, Michigan lilies, prairie smoke, gentian to name a few can be overwhelmed by all the asters, goldenrods, cup plant, joe pye weed and the like. Native or not if you want it to look good and be healthy you have to put some time in.
I like the perspective that this post brings. Since I’m the resident curmudgeon in our group, I like to point out to our Native Plant enthusiasts, that Poison Ivy is a native plant …
nice post!
It seems we’re spending a lot of time trying to freeze the natural world in a moment in time?
Correct me if I’m mistaken, but it’s quite possible many plants (and other critters, too) native to Minnesota and Michigan, for example, went extinct as a result of the series of ice ages that swept over these lands tens of thousands of years ago.
Also, what about the impact of the constant drift of continents across the face of the globe on “native” plants?
“Native” critters have been coming and going since long before humans impacted this planet (think dinosaurs), and will likely continue to come and go well into the future – maybe even including humans?
Thoughts. . . . ?
RSS feed please!
Terry, there are TONS of examples that back up your suspicion (some in link). Personally, I’m just focused on maximizing biodiversity and trying to maintain some of the aesthetic feel of the original ecosystem (e.g. by maintaining notable charismatic natives like redwoods).
Darn, and I thought I was offering a somewhat original thought;-)
Everyone chiming in here should read Matt’s perspective based on his review of a special issue of SCIENCE earlier this year. Simply click on his name to go to his blog. Well said, Matt!
I’m wondering about you statement on diversity, of course increasing the number of tree species increases the diversity of trees, but does it increase the diversity of native wildlife that depend on trees? in his book Bringing Nature Home, Doug Tallamy seems to argue that non-native trees provide little habitat to native insects which native birds depend on, and that is partly why song bird populations are declining. Also, do you know of any municipalities that actually do have a native only policy? I have not run across that here in my part of Virginia.
Gardens are more than just a collection of plants. Gardens with no native plants will support no wildlife. While gardens with a wide diversity of native plants will support a wide diversity of wildlife. That simple piece is what you left out of your argument. It may mean nothing to you, but it means a lot to wildlife.
Carole’s response is one that is heard often. But the simple fact of the matter is that exotic plants can offer habitat and food to native wildlife. Many South African Proteaceous species provide nectar for native honey eaters here in Australia. An exotic weed in Australia, nettles, is the preferred food source of a critically endangered butterfly, and drifts of nettles in some areas are the lone food source and breeding grounds for this endangered and beautiful butterfly. So I think there’s more to the supposed dichotomy that no native plants = no native critters. While I wholly agree with what Bert says in a horticultural sense, on an ecological level, in Australia at least, native revegetation projects are highly important. This has a lot to do with Australia’s isolation – the majority of our native plant taxa are unique to our continent, and since European settlement Australia has lost an alarming number of native floral taxa, and continues to do so. However, native revegetation projects and backyard gardens are two fundamentally different things. I’ve loved many a purely Australian native species garden, but on a horticultural level I’ve never considered myself a natives-only man (but I have a purely native species garden at the moment, go figure!). I just don’t think it makes horticultural sense in urban areas. If I think of a plant that will do well in a certain spot in my garden, I never ask myself if it’s native or not. If it’s the perfect plant for that spot, then it’s going in! On a conservational level, i.e. with large scale revegetation projects, I’m a native purist because, in Australia at least, it helps to preserve an endangered and dwindling list of native taxa that might not be here in 20 years’ time. Some would call me a fence-sitter, but in different contexts I can be a natives-only purist or a sensible urban horticulturist. Great topic!
Lots of great posts and glad we’re stimulating some discussion. To respond to a couple of posts. I’m not aware of communities that have mandated natives only but I have certainly heard this sentiment at presentations that I’ve given. I’m a forester by training and consider myself to be ‘pro-native’. I clearly remember neighborhood tree walks in my undergraduate dendrology class – any tree we couldn’t identify was shrugged off as “some $#*! ornamental”. My main goal in the post was to point out that native advocates often hurt the cause by making sweeping generalizations that don’t hold up to scrutiny. There are many reasons to favor natives and, where appropriate, restore native habitats. But when we consider highly disturbed, urban and suburban environments, exotic plants can provide ecological services and economic benefits such as shading buildings, mitigating storm water run-off, and reducing urban heat island effects. To be sure, there are ecological niches and specific symbioses for which no other species can substitute; but at a broad level, a diverse urban and community forest made up of dozens of species, native and exotic, will be better buffered and function better in the long term than one built on a narrower base of tree species.
Your statement makes a lot of sense, and there are non-natives that I like and use in my work, but you know, you won this argument a long long time ago. Go to any nursery, it’s almost all ornamentals. When I have a client come in, what do they ask for? Non-natives. So I’m skeptical that we’re to the point that we need to provide a counter-point to the use of natives in the landscape. I guess that rather than saying that non-natives may work better on disturbed sites, I’d rather see information on building practices that would be more amenable to natives species.
Great discussion.
About Doug Tallamy, I think he’s misquoted quite a bit because he acknowledges that about half of all wildlife are generalists – happy to feed on nonnative plants. So the notion that nonnatives do nothing for wildlife is debunked by Tallamy himself.
And about whether there are natives-only laws in place – believe it! For government-funded projects, especially. In my county, lawn replacement plantings are subsidized only if they’re natives-only. In Annapolis, MD all new developments are required to have a large number of native plants – then if they want to use more plants, they can use some nonnatives. In D.C., funds are available for rain gardens and school gardens only if they’re all-natives (except for kitchen gardens).
It’s what native-plant advocate Rick Darke calls the “misuse of native plants” – expecting them to perform where they’re not happy – and increasingly, it’s mandated.
It’s sweeping generalizations that drive me crazy, too. One thing it leads to is people who have no intention of maintaining their gardens happily buying native plants – then seeing them die due to neglect. “But they’re native!”
That’s my rant.
I’m curious about your quote about Tallamy. I don’t think that he or other people are really claiming that nonnatives do nothing for wildlife. I believe his claim is that our native insects are not good at feeding on nonnative plants and that these insects are an important source of food for other native animals like songbirds. I also don’t think he is saying there are no exceptions to this theory – he seems to be saying that overall, native insects do better on native plants. And I see from a quick look at his book that he has data that even generalist insect herbivores do considerably worse on nonnatives plants than natives. Dying plants aren’t only a problem only of native species. Any plant, native or not is going to need some care while it’s getting established. Anyone who thinks they won’t be maintaining their yard is bound to be disappointed. You can choose the wrong nonnative just as easily as the wrong native.
I feel most comfortable with how it grows’ posts. I have native and non-native plants in my small garden. Both chosen for their low-maintenance and ability to survive on rainfall most years. I can pick a non-native that does this as well as a native. A gardener won’t make the same choices a newcomer to planting the yard will make. On a blog like this, I expect a choir, and I can’t imagine this post being read by those who would scream “native” after the thing died from inattention.
Invasives should not be lumped in with the native discussion. If invasives are the issue, then let’s list them and keep them out of the garden. So far my lilies, sedum, roses, irises, etc., etc., all non-native, are still fair game my garden.
The biggest point being: Native or non-native in your garden is your choice. But, invasive or not-invasive is a community issue, it goes beyond our garden’s borders.
I suggest reading Tallamy’s book and research articles before spreading more misinformation about the “non-natives are as good as natives” claim. The post by “how it grows” is the best reflection of his data. 35 times more insect biomass on native trees vs. nonnative trees is an enormous difference for biodiversity. Planting more non-natives might increase the number of species in an area, but certainly doesn’t contribute to real biodiversity if it breaks the food web.
The question “Do alien plants reduce insect biomass?” was answered quantitatively only in 2009, see Heleno, Ceia, Ramos and Memmott “Effects of Alien Plants on Insect Abundance and Biomass : a Food-Web Approach”. They found that the food web is not broken, but biomass is reduced only because smaller native insects are selected for. Now, as I understand this, that would create a niche, which would presumably be filled by some large non-native insects. Yikes….!
As the climate changes, we all need to make intelligent choices of what to plant. Insects will find it easy to move about and other than trying to avoid locust plagues, we should be fine. Plants however, are slow movers and many species will require assisted migration to avoid becoming extinct.
Gardeners who plant exotic plants are helping to build up a pool of knowledge about what will survive where, which will be valuable as these inevitable realities are forced upon us.
Our choice is between trying to cling to the comforting security of what we already have, or designing the future by consciously selecting that which will thrive in times to come.
Solid reasoning, certainly. And isn’t it the case that gardening always includes some element of the exotic? I visited a pueblo joven — newly-established shanty town — in Lima, Peru, where the residents were collecting plants from the other side of the mountain to grow in their public square, simply because they were different from what was already there. That’s what gardening is about — we grow showy plants from somewhere else. Whether that somewhere else is the next mountain, the next state, the opposite coast, or across the world, we are importers of plants. It’s just not in the definition of gardening to try to re-establish the ecosystem that was in place at an arbitrary moment in history before we showed up.
The impact of nonnative plants upon animals is a complex issue. While Tallamy’s study may show a decrease in insect species richness (as influence by nonnative plants), other species may be more adaptive. I coauthored a paper several years ago (Reichard, S., L. Chalker-Scott and S. Buchanan. 2001. Interactions among non-native plants and birds, pp. 179-223. In: J. Marzluff, R.A. Bowman, and R. Donnelly (eds.), Avian Ecology and Conservation in an Urbanizing World, Kluwer Academic Publishers, Boston, MA.) In this review article we found that many fruit-eating birds benefited greatly from the fruits of nonnative species. On the other hand, insect- and nectar-feeding birds can be negatively affected by invasive species. (You can find the article on Google books if you’d like to read the first few pages yourself.)
interesting post – i learned some few things. thanks.
I enjoyed reading this post very much. I think many gardeners and nursery professionals have it ingrained in their heads that natives plants are best suited for their environment, but that’s clearly not the case as evidenced by the success that countless plant species have had in non-native environments.
I agree with Ethan above. Gardening should always include a bit of the exotic, else the mundane can start to set in. I love growing certain species of palms here in the Seattle area garden. Some of them do very well in our traditionally wet environment, and I love the texture they add to the natives.
hello
I want to know in your country what new extension methods are using for delivery new information about agriculture?
Hi Leda,
I’m the county forester for Arlington County, and I recently reworked
our tree lists. You can see it here, if you’d like:
http://arlingtonva.s3.amazonaws.com/wp-
content/uploads/sites/13/2014/01/Master-List.pdf
I’m glad people are having this discussion, because there is
definitely more nuance to research. That being said, I think you’re
addressing this issue somewhat cynically, and I’d like to talk about
some of the points you made.
I don’t think people believe that native plants are universally more
stress resistant than non-native plants, bred for toughness. This is
one good reason we need to work with nurseries to see if we can breed
tougher natives. On the other hand, there definitely are native trees
that match up with the non-native plants in toughness. I see just as
many willow oaks thriving as I do Chinese elms and bradford pears,
and I’ve selected my street tree list based on research in my
community on survivability, and many of the native trees do just as
well.
I have a hard time believing someone thinks native plants are more
pest-resistant, but preventing the importation of non-native plants
certainly will reduce pest introduction of future insect
infestations. Except for the Mountain Pine beetle, most of the
insects we’re dealing with were brought in, so maybe we need to be a
little more careful with that. On the flipside, non-native trees are
not meant to resist our pests, and that’s shown with historic
introductions. We’ve found many trees that work in our environment,
and don’t seem to have many pests, but that also means they probably
have near to no wildlife value.
That brings me to the third issue, of wildlife value. While I don’t
disagree that some non-native trees have some wildlife value, and
trees like lindens and willows can be indistinguishable for our
wildlife, it’s a falsehood to assume they can provide the same
benefit. Doug Tallamy’s research shows clearly the great divide in
insects and other wildlife grazing on non-native plants.
What we need to look at is the following:
– Provide the right space for native trees in our urban environment.
All this work on soil volume and proper care is perfect for expanding
our native planting pallette. It’s definitely beneficial for us to be
picking native plants.
– Look to warmer regions of our country for more climate-change
resistant trees, but don’t try to assume you have to go outside of
the continent to find something suitable.
– Work with nurseries to breed more resistant plants. We’ve already
done this with the river birch, for example, with its ‘Dura heat’
cultivar.
– Listen to our naturalists. We had a discussion on removing Chinese
Elm from our lists, because it started having major invasive
properties. Our naturalists provided evidence, and we removed it. As
you can see from my list, I still have non-native plants on there,
for aesthetic and toughness purposes, but none of them are on our
invasive plant list. Just because you like a tree doesn’t mean you
should be making the gamble to hurt our ecosystem.
Thanks,
Vincent Verweij
Arlington County Forester
703-228-1863
Vincent:
Thanks for comments. I have to disagree on a couple of points. First, there are plenty of people out there that believe native plants are inherently more pest resistant and therefore need fewer pesticides (and less water and less fertilizer) than non-native plants. Just google “Why plant natives?” and you’ll see lots of iterations on these claims.
My point is that stress tolerance (pest resistance, drought resistance, need for fertilizer) is a function of species; not native versus exotic.
Thank you for the insightful blog, you brought up some very good points to consider. As a long time gardener I have heard the siren song of exotic plants, growing out of zone and planting just for the fun of seeing if it will survive.
I agree with Vincent regarding the issue of non-native plants not supporting the trophic pyramid to the extent that natives do.
My current interest is in Urban Ecology. I think that if we don’t try to add native species (grass all the way up to woody ornamentals) we will never build any meaningful ecosystem in our cities. Dr. Talamy and Dr. Landis’s research have pretty much made that evident in my mind. Our standard landscape plant pallet is never going away now however that doesn’t mean we can’t enrich the pallet with more native selections. This may mean design and construction standards may need to be amended to overcome some of the limitations that urban conditions impose on native plants.
We grow what we grow in our cities because they are tough, they get fewer bugs and diseases and aesthetically they are bling when compared to the average native plant.