how effective are nativars? with doug tallamy
I’M A GARDENER, someone who loves showy plants in artful arrangements. But in recent years, I’ve been looking less with a collector’s eye when shopping and more from the point of view of an insect. Yes, really.
That means more and more I’m layering native plants into my landscape, but which ones among the ones tagged “native” do the very best job? You’ve probably heard the word “nativar,” as in a cultivar of a native plant, but what does it mean and how effective are these often showier cultivated varieties at supporting wildlife? I asked Doug Tallamy, professor of entomology at University of Delaware and author of “Bringing Nature Home” and “The Living Landscape,” to help me understand more about this important subject.
We talked about what a cultivar change–bigger flowers, maybe, or colorful leaves, or smaller overall stature–actually does to a plant from the point of view of insects. And I learned about a beta version of a new tool for creating powerhouse native-plant lists–the best of the best for supporting healthy food webs–for my specific area, or any area by Zip Code around the nation.
Read along as you listen to the May 28, 2018 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on iTunes or Stitcher (and browse my archive of podcasts here).
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natives and ‘nativars,’ with doug tallamy
Q. I was thinking of you yesterday as I was mowing. Dare I use the “M” word? I was mowing the increasingly smaller areas I do mow any more, partly thanks to you, and noticing some spots that I’ve unmown since our last conversation now are the nurseries for seedling oaks. Something like what I remember happen to you.
A. Oh, yes, yes. [Laughter.]
Q. Isn’t that amazing what happens?
A. It’s not all that amazing. If we leave it alone, plants do come in.
Q. Yes, they do, they do. So I don’t even kind of know where to begin, but tell us: nativars—where should we begin? Should we begin with definitions or ...
A. Sure, that’s a good place.
Q. Probably smart.
A. You know that is the most common question I get. Are cultivars of native plants, and people just call them nativars—although I have learned the horticultural trade does not like that term-
Q. Oops. Too bad.
A. … but are they as good in terms of performing their ecological functions as the straight species from which they were derived?
A. It’s a good question, because when you go to the nursery, most of the time, that’s what’s for sale. It’s some genetic variant of a straight species. Now there are lots of reasons that cultivars are created, most of them do focus on aesthetics, but any genetic variant that is disease-resistant is also a cultivar.
A. Let’s make sure we’re not talking about hybrids. Hybrids are the cross between two different species, and they are offered as well, but it’s not officially a cultivar.
We just finished a study looking at six common cultivar traits in native plants, and seeing how they might have impacted insect use of that plant. Now these are all woody plants, so we did not look at flowering; we did not look at flowers at all. So we didn’t look at the impact on pollinators; other people are doing that.
A. But just in terms of how well leaves support caterpillars and other things that drive food webs. We looked at what happens when you make a green leaf, red or purple. What happens when you change that leaf into a variegated form? What happens when you take a tall plant and make it short, or change the habit in some way? When you enhance fall color? When you increase fruit size? What was the other trait? I think that’s it; there were six traits.
The only thing that consistently deterred insect feeding was taking a green leaf and making it red or purple.
Q. I knew you were going to say that and that makes me sad, but O.K. [Laughter.]
A. Yes. It makes sense because that’s changing the leaf chemistry.
Q. Of course. The chemistry and chlorophyll tastes different from what, the anthocyanin pigments, or something?
A. That’s right; anthocyanins are feeding deterrents.
A. So when you load a leaf with feeding deterrents, of course it deters feeding. [Laughter.] You know that is something that gardeners have been seeking for a long time. They don’t want any insects in their garden. And if our gardens just occupied a little space on this planet, that’d be O.K.
But they don’t, they occupy a huge area. It’s not just our gardens, all the landscapes around us. So not only do we bring in cultivars of native plants, we bring in non-native plants that are also very poor at supporting insects, and then a lot of those escape.
What is it?—85 percent of our woody invasive plants are from our gardens. There are very few habitats that aren’t just choked with plants, typically from Asia. So we’ve converted an awful lot of our land into a landscape with non-native plants that are not supporting the food webs that everything else needs.
Q. Right. So in my area, I’m in the Northeast, in New York State, sort of on the border of Massachusetts and Connecticut. I might see … what is the common name, the red osier dogwood, Cornus sericea. I might see that, I think, and then I might have one in my garden like Cornus sericea ‘Silver and Gold’ [above], a plant I first met at the home of Dick Lighty at Mt. Cuba Center.
A. Oh, O.K.
Q. And he had just introduced it or was getting ready to introduce it a long time ago, and so all these years I’ve been enjoying that plant and I love the pollinators who come to the white flowers and I love the white fruits later. And as a gardener, I like the variegated foliage and the gold twigs and so forth. I love when the birds, especially thrush relatives, come and devour all that fruit when it’s ready in the summer.
But am I not doing a good job? Do you know what I mean? Should I be casting it out, or … what happens here?
A. [Laughter.] No, no, first of all there’s room for compromise here.
A. We shouldn’t look at single plants. We should look at our entire landscape as a unit, and say is our landscape doing the basic things that we all need to do on any privately owned property. We all have to support food webs. We all have to sequester carbon. We have to have plants that are building their bodies out of carbon and then pumping it into the soil. We all have to have plants that manage our watersheds, and we all have to support pollinators. So every single plant on your property, it doesn’t have to do all of those things.
A. Of course another thing our properties have to do is be pleasing to us. But I contend we can do all of those things if we choose our plants carefully.
So you just named a number of ecological services that your Cornus is performing. The Cornus itself, one of the things we’ve learned recently is that there are big differences among native plants and how they support food webs.
There are really just a few genera that are really good at them—they are making most of the food. Most of the other genera are O.K., and some don’t make a whole lot of food at all, even though they’re all natives. So Cornus—it’s pretty far down the list there, in terms of supplying particularly the insects that drive food webs.
It does help pollinators and it does make those berries for the birds in the fall. But adding your Cornus or taking out of your landscape isn’t going to make a huge difference to the ecological value of your landscape.
A. Adding an oak would make a huge difference.
A. Because that is the most powerful plant you can put on your property.
Q. So you know where I live, and I know where you’re located, but wherever anyone is, where can they look to find out what the key plants, would we call it the keystone plants?
A. I call it that, yes. [Laughter.]
Q. Yes, keystone plant. Where would someone research to find out if it’s oak, or if it’s whatever it is?
A. We just happen to have made a very convenient website for that.
Q. Oh! [Laughter.]
A. You can go to National Wildlife Federation website and just put in “native plant finder.” The ranked list of plants, both woody and herbaceous plants, will pop up for your county, wherever you are in the country.
So what used to be a challenge, figuring out which of the best plants, is now easy. Just plug in that Zip Code and it’s ranked in terms of the ones that support the most caterpillars, which are driving these food webs. So just start at the top of the list and go down, and you’ll hit those powerhouses. [The beta version of the new site includes an “About” page that explains how the plants were chosen, from historical records, and how the site will continue to evolve.]
Q. The late Elizabeth Farnsworth of New England Wild Flower Society, whom I admired very much, said this to me last year:
“Nativars can be the gateway drug to native plants for gardeners.”
So in other words, they can help to get us moving toward knowing natives, yes?
A. Sure, I would go along with that. There are two reasons that I wouldn’t turn to a cultivar as a first choice. One is that most of them are propagated clonally, which means there’s zero genetic variability. In the age of climate change, in particular—we’ve always needed a lot of genetic variability in our plants, but in the age of climate change, it’s particularly important. We’ve got all these wild swings. And putting plants with no genetic variability out in the environment is just not a good idea.
The other thing that bothers me, is if the only thing that we sell in nurseries are designed for aesthetics only, it perpetuates the idea that plants are just decorations.
A. Of course, they are decorations, and they are beautiful, and we do want beautiful plants in our yard. But we also want to think about those vital functions that they must be performing in our yard.
So I would just love to see straight species sold right along with those cultivars, so if the homeowner is more interested in restoration or getting the maximum value out of that particular species, they have the option of buying it. That’s what I would like to see.
Q. Right, so this sort of tension between ornamental and the sort of functional, the two things—instead of a tension, like it’s either-or—that both should be sold, and they both have a place.
So maybe right near my front walkway where I have that Cornus ‘Silver and Gold,’ I might choose the variegated one, but in my upper field area, it’s a more wild area, I might choose to have an island of the straight one, do you know what I mean?
A. Sure, sure.
Q. Mix it up. Yes.
A. You know close to the house you probably want smaller plants.
A. There are a lot of cultivars that are smaller in stature or more compact. Those are essentially functional traits that enhance our ability to use those plants in a domestic landscape. So yes, that’s all fine. Remember a lot of these cultivars are genetic variants that were simply found in nature. So it was nature that selected them. It wasn’t humans.
A. So bringing them in and planting them and putting a name on them doesn’t automatically make them bad. They’re fine plants when they’re collected that way.
Q. So you’ve mentioned the food web and so forth, and we just mentioned sort of straight species. So this is a story then … In other words, in my layperson’s version, I always tell people when I’m lecturing or doing workshops in my garden: “We’ve got to grow more bugs, we’ve got to grow more bugs. Everything survives because of plants, and then because of bugs.” [Laughter.] That’s my version of science. But it’s a very …
A. That’s a very accurate version.
Q. Fairly accurate, right? That’s a little theatrical and that works in a workshop environment. [Laughter.] Everybody starts laughing, but at any rate:
We’re trying to grow more insects, and those are feeding lots of other creatures and up, and up, and up this food chain. So those insects, these beneficial native insects, and other animals and the straight species of these plants, these not-nativars and especially not hybrids, these straight wild types of these plants, they have a co-evolution story right [with these insects]? They’ve been around together a long time and is that why there’s this importance that you’re talking about. Why their maroon leaves are not as good as the plain green one that nature made?
A. Exactly. Plants, whether native or not, don’t want to be eaten by insects, so they protect themselves chemically. They load their leaves with nasty-tasting compounds. We call them secondary metabolic compounds, that make the leaf either bitter or in many cases downright toxic. So in order to eat these plants, insects have developed adaptations that allow them to get around those defenses.
A. The particular enzymes that can detoxify the compounds or store or excrete them. Their behavioral adaptations that allow them to minimize their exposure to the compounds and life-history adaptations that do the same thing, but an insect can’t develop adaptations to all the compounds that are out there, because there are many thousands.
So what they do is they focus on one plant lineage that’s using one particular type of compound, and they get good at getting around that one. I always use the monarch butterfly as an example-
Q. Of course, yes.
A. … because it is a specialist on milkweed. Now it chose a tough genus to specialize on, because milkweeds are very well protected with compounds called cardiac glycosides, and also that milky latex sap. Most insects cannot eat milkweeds. But Monarchs have gotten good at detoxifying the cardiac glycosides and also avoiding the sticky latex sap by how they chew through the midrib of that leaf, and that blocks the flow of this sap down to where they’re eating.
Q. Clever. [Above, monarch on milkweed; photo by Cornell monarch researcher Anurag Agrawal from this interview.]
A. But in developing all those adaptations, they have not spent any time developing adaptations to get around the tannins that are in oaks, or the cucurbitacins in cucurbits, or the nicotine in tobacco, or the cyanide in cherry, and on and on. All plants have compounds, and if they don’t have the adaptation to get around them, the insect can’t eat it.
So they find their host plant by chemistry. It smells right, it tastes right. And if we change that leaf chemistry, they probably don’t recognize it, or they don’t have the adaptations to be able to deal with that chemical change.
For this same reason, when we bring in a plant from Asia, if we put Ginkgo in our yard, our insects don’t have a clue how to eat Ginkgo. They don’t have any of the adaptations that allow them to process that food material.
Q. A paper you sent me to read was very interesting—a very recent paper that you did—I think you cite that the fact that 90 percent of insect herbivores are diet specialists, is that right? Do I have that number right?
A. That’s right.
Q. So they have a specific relationship with a plant, like what you just said about the monarch. They’re not generalists feeding on any old thing.
A. Right. Now 10 percent are generalists and we say, “Oh they can eat anything,” but that’s not at all true. It just means they could eat several things.
A. But if you look at the range, if I think of the number of plant genera in my yard, I don’t know how many it is, but it’s got to be close to 1,000. I look at the most generalized insects. Actually I’ve got the white-marked tussock moth in my yard. It’s the most generalized insect in the whole country, and it eats 110 genera of plants.
A. But that leaves 900 genera that it can’t eat. So even the generalists are fairly focused compared to what is available out there.
Often they are regionally even more focused. So luna moths [above], for example, can eat, I think it’s 25 genera of plants. But if I take a luna moth from the Eastern Shore of Maryland, where it’s all sweet gum—that’s their host plant there—and feed them a host plant that’s listed in the literature, like it’s listed that they eat oaks, but they die there because there’s specialized sweet gum in that one region of the country.
A. Even though they’re considered a generalist.
Q. There’s so much to learn.
A. So we have to be careful with that “generalist” term. Most insects are really focused on particular lineages.
Q. In a recent conversation with George Coombs at Mt. Cuba Center [in Delaware], who does the trial gardens there-
Q. … you’ve done some collaboration University of Delaware and Mt. Cuba Center. Recently he was telling me about his report on the results of their Phlox trials, and the idea that sort of bigger isn’t better. This goes back to the hand of plant breeders, and making things for the garden eye-catching appeal. So we’ve made Phlox flowers bigger, and certain colors, and shapes and whatever—the flowerheads—and it turned out that the biggest ones weren’t necessarily the best for you if you’re a butterfly, right? So that’s another kind of an example. [Above, Mt. Cuba photo of an American painted lady on a Phlox paniculata cultivar.]
A. That’s right, that’s right. I should mention that it was Mt. Cuba Center that funded our study of cultivars so we want to give credit there.
Q. Oh, that’s fantastic. See now, I didn’t understand that. As much as I know them and talk to them regularly, I didn’t know that was the relationship, specifically, but wonderful.
A. Yes, and actually we had a undergraduate defend her senior thesis yesterday, and George Coombs was there. Where again Mt. Cuba supported her study looking at Hydrangea cultivars, particularly focus on Hydrangea arborescens and the very common cultivar that you get.
Q. ‘Annabelle.’ [Photo of it, above, by Giligone from English Wikipedia.]
A. If you go to the store and ask for arborescens, they give you Hydrangea arborescens ‘Annabelle.’
Q. ‘Annabelle,’ sure. [Laughter.]
A. Because they’ve changed the lacecap flower into a mophead flower that is loaded with sterile bracts and that’s what the big mophead is—it’st all these white bracts. So no surprise, what the student did was set up video cameras and look at the flower visitors that came into the flowers. And ‘Annabelle’ was the bottom of the list.
A. Because first of all, it has far fewer fertile flowers, and even the fertile flowers it does have are very low in nectar. The nectar quality is very poor—so we’ve changed it. It’s prettier for us, but you’ve taken a native plant, which actually is a really great pollinator plant when it’s the straight species, and we’ve created a cultivar that’s almost useless to pollinators.
And that’s what people have to realize. What has a cultivar change actually done to your particular plant? There’s no one answer to that. The answer is: It depends.
A. If you have a double flower, those are all sterile. If you have anything that’s really, really showy, be suspicious that energy has gone into enlarging the bracts or changing the color, and it takes energy away from pollen and nectar production. And if you’re planting this plant for pollinators, you do need to know that.
Q. We can go to that great plant reference website that you mentioned that helps us get to the right ones for our area, but what is your general guidance for me then as a gardener, whether I have an established garden or empty beds—what would you like me to do? What does Doug Tallamy want me to do? What should I have in mind when I’m going out shopping?
A. Well if you are the average gardener, I would ask you to consider reducing the area you have in your property that’s in lawn. [Below, an aerial view of Doug Tallamy’s home landscape, where he has eliminated lawn in favor of oaks and other trees and a more layered habitat.]
A. Because out of those things I listed, lawn isn’t accomplishing any of those.
Q. Zero. [Laughter.]
A. So take the area that you take out of lawn, put more plants into your yard.
A. And think about the role of those plants. You want to have those really productive plants making all those caterpillars so that birds have enough food to reproduce.
But you also want to support those pollinators, and those are usually different plants. Pollinators require plants that are blooming throughout the season. Not a single plant—you want a sequence of plants that are blooming, so that you can support a sequence of pollinators. Keep in mind we’re not just talking about the honeybee here. We’re talking about 4,000 species of native bees, almost all of which are in decline right now because we have these sterile landscapes that aren’t producing the flowers that they need.
Also keep in mind, a lot of these pollinators are specialists just like the herbivores are. So there are plants like goldenrod and native willows, and sunflowers, and asters—those are the top ones that support specialized pollinators. Just those, that’s four genera I named, support over 40 species of specialized pollinators.
So if you can have room in your garden for those four species, you’ve just helped an awful lot of pollinators, and the generalist pollinators go to them, too. So you’ve done a good thing. [Above, native willow Salix discolor, from Silk 666 at Wikimedia. Top of page, examples of some other real powerhouses.]
Q. So caterpillars, we’re going to support them. A sequence of offerings through the season for pollinators.
Q. I do think willows are just underutilized, and I’ve been adding more of them, more native willows, like even in fringe areas—like along the roadside, and I’m in a rural area.
Q. You know just as opposed to some other kind of “hedge” I’ve been using more willows, and it’s a place that I wasn’t otherwise cultivating, and boy oh boy, it’s abuzz when those things are flowering.
A. I was just in St. Louis, and there’s a place called Shaw Nature Reserve that is experimenting with prairie willows, making hedges out of those where they only get 3 or 4 feet tall. They’re blooming all at the same time and they don’t need to be in the wet. As a matter of fact, they like it dry. So that’ll be a great landscape plant that’ll support those early season pollinators. Early season is a tough time for pollinators. They need flowering plants right away. And willow is one of the best.
more about nativars and habitat-style gardens
- My previous conversation about habitat gardening with Doug Tallamy
- The location-specific native plant finder on National Wildlife Federation’s website (beta)
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MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its ninth year in March 2018. In 2016, the show won three silver medals for excellence from the Garden Writers Association. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play the May 28, 2018 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes or Stitcher (and browse my archive of podcasts here).