IT’S HARD TO NAME another genus of shrub that’s more beloved by gardeners than Hydrangea, but with the overwhelming popularity of Asian species, like the big blue mopheads and summer into fall Hydrangea paniculata types, where do hydrangeas fit in as more and more gardeners favor native plants? And especially ones that support pollinators and other beneficial insects.
The native plant experts at Mt. Cuba Center in Delaware have just released a report on the results of a five-year trial, that focused mostly on an important native species, Hydrangea arborescens, and what both gardeners and pollinators have to say about its range of cultivars.
The last time Sam Hoadley visited the show, we compared the dizzying range of cultivars and species of Echinacea. Today’s topic is hydrangeas. Sam is the manager of horticultural research at Mt. Cuba Center, a longtime native plant garden and research site, where he trialed 29 species in cultivars. Before joining Mt. Cuba, he was lead horticulturist for Longwood Gardens Hillside Garden, and he received his degree in sustainable horticulture from University of Vermont.
Read along as you listen to the February 7, 2022 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on Apple Podcasts (iTunes) or Spotify or Stitcher (and browse my archive of podcasts here).
hydrangea arborescens and its kin, with sam hoadley
Margaret Roach: How’s the winter been down there in the Mid-Atlantic?
Sam Hoadley: Yeah, it’s been good. We were enjoying some unseasonably warm temperatures for a while, and we just had the reality check in the last two weeks. It is actually winter, the temperatures, but it looks like it’s warming up a little bit to just kind of the usual thaw and freeze.
Margaret: Yeah. So you’ve been busy as always with the research and then the reports, which I always enjoy so much over the years that Mt. Cuba has created from its trial garden program, and the latest being hydrangea.
And maybe first, before we get to the results and so forth, maybe first we need to sort of give the quick overview of this genus, because the ones you trialed may not be the ones that are taking up the most real estate in American gardens, do you know what I mean? So maybe we kind of say, “O.K., you guys maybe growing this, but we looked into these because…” Can we have that sort of backstory?
Sam: Yeah, absolutely. So as you mentioned, there’s kind of a dizzying array of hydrangeas that are available to gardeners today. A lot of times when people think hydrangea, they think of the big macrophylla types that are pink to blue, and a lot of people know that soil acidity can affect that color.
And I think many people know of hydrangea, even if you’re not necessarily interested in horticulture, I think those are the images that come to mind. We are really interested in this kind of smaller range of native species, and the Eastern United States has four native species included. And we have in this hydrangea trial incorporated three that are very closely related, a little bit different. Oh, sorry, go ahead.
Margaret: No, I was saying ah, so you chose ones that are closely related, I see, because I knew there was one you didn’t include that’s a native American.
Sam: Yes. But that is coming up. Spoiler: We are planning an oakleaf hydrangea trial that will be going into the trial garden this spring, so stay tuned for that. That’ll be the fourth and final hydrangea species that we’ll be trialing in the gardens here.
Margaret: Oh, so you started with these ones that are closely related to Hydrangea arborescens, or arborescens itself. And I think that’s sort of Eastern, but not new England. And when I look at the range maps, I mean you look at range maps for a species and it doesn’t mean it’s in literally every spot in every state, but if it’s in the state, they show it as present [laughter]. I didn’t look at county-by-county, but it’s sort of like the Eastern third of the country.
Sam: Yeah. Hydrangea arborescens is actually fairly widespread throughout the Eastern United States. It becomes a little more rare on the fringes of its range. It does extend up into New York State and down into northern Florida, but it’s fairly, I don’t want to say common, but it can be observed throughout the Appalachian Mountains and a little bit into the central United States.
I’ve actually seen this growing in woodlands in Eastern Pennsylvania, fairly locally to us.
And then there were two, they used to be subspecies, so very closely related to Hydrangea arborescens that have now been elevated to their own species status. Which those two species are Hydrangea radiata or silver-leaf hydrangea. And really the big ID feature of that one is that’s the silvery, almost metallic, white backs to the leaves. Smaller native range; it’s really native to mountains in the Southeast United States.
And then there’s Hydrangea cinerea, which again is very similar to Hydrangea arborescens and Hydrangea radiata. But the backs of the leaves have kind of this grayish-silvery pubescence, they’re kind of fuzzy, and more of a Midwestern and Southeast United States plant. The straight species, Hydrangea arborescens, is definitely the most widespread.
Margaret: I remember the first time I saw radiata. It was at a nursery in Connecticut, at Broken Arrow Nursery, and one of the horticulturists there like turned the leaf over. He said, “Oh, let me show you something.” He turned the leaf over. And I was like, “What?” I mean, it was green on top, but oh my goodness, silvery-white below. And when you see it in the garden, it particularly if it was used in the right spot where it’s a little bit elevated, maybe on a slope or something like that, oh my goodness. It’s just like a whole other thing going on besides when it’s in flower. It’s just…
Sam: Exactly. They’re beautiful. And we use them extensively in our naturalistic gardens here. And when there’s kind of a slight breeze going through the woodlands, you get kind of the shimmering effect in the landscape, I think it’s my favorite species that we trialed.
It didn’t do well in the trials, and we can go into why that happened. But in the gardens here, it’s just absolutely spectacular and it is available, Broken Arrow is a good source. I know Quackin’ Grass Nursery in northeastern Connecticut has them available. They’re out there. They’re just not quite as widely available as regular Hydrangea arborescens.
Margaret: Yeah. So arborescens... If one of the most popular questions being a garden writer for 200 years already now, probably that I’m asked year in and year out, day in and day out, is “When do I prune my hydrangeas?”
Margaret: And the answer is different for the big mopheads versus the paniculatas, and has to do with whether they bloom on new wood produced that season or old wood and so on and so forth. And, what about these? How are the arborescens and its kin, where do they fit in?
Sam: So one of the things that’s great about Hydrangea arborescens and cinerea and radiata, you can treat them very similarly. They do the same things as far as where you would want to site them, the landscape, in most cases and pruning. They all share in common that they bloom on new wood, which essentially means that those flower buds that are going produced in June and July, that’ll produce those flowers, those flower buds aren’t produced until that growing season.
So that means you can prune these plants really heavily from fall, even to early spring, and you’re not going to impact the floral display for that year. Something like Hydrangea macrophylla that blooms on old wood, those flower buds are present all winter long, mostly on the terminal buds of those stems. And there’s always the chance that a late frost or fluctuating temperatures will cause those buds to break early and then a frost will kill them.
And with some of the older cultivars of Hydrangea macrophylla, in particular, that will dramatically impact your floral display for the year. But Hydrangea arborescens is super-reliable. You can really abuse these plants and they will come back and bloom for you.
Margaret: And so traditionally, there was one cultivar [laughter], the ubiquitous ‘Annabelle,’ of Hydrangea arborescens, and I think it was from like the 1960s or something.
And it just became such a giant hit. And even here, I’m in rural New York State, and in the front of old houses, that might have done a landscape update again, like in the sixties or seventies or whatever, you’ll see these giant almost like hedges of this thing. Or like foundation planting, like rows of them. And those giant white mop top flowers of ‘Annabelle.’ And she’s a floppy girl.
Sam: [Laughter.] Yes, she is.
Margaret: She’s a floppy girl ,in flower. So it’s a beloved plant or it has been, but then we found out, as we’ve become more aware in recent years, even among native species, when you get to a native plant, when you get to a cultivar level, they’re not all equal. They’re not all created equal in terms of their ecological services that they provide to a habitat.
So what’s the deal? Why is it that we don’t just go out and keep on planting ‘Annabelle’? What’s the deal?
Sam: Yeah. So, ‘Annabelle’ is kind of an interesting story, even where that plant came from. It was a natural mutation that was found, I believe in Illinois, originally, at the beginning, early 1900s, and wasn’t actually introduced into cultivation until the 1960s. But there was a cultivar even earlier than ‘Annabelle’ called ‘Grandiflora’ and was originally called ‘Hills of Snow.’
But this was also a plant that was found in the wild, that exhibited these huge mophead type inflorescences. So these things existed in nature, but there’s definitely drawbacks, as you mentioned, with their ability to attract and very likely support pollinators. With mopheads, in particular, there are very few or fewer fertile flowers inside of those inflorescences. The majority of what you’re actually seeing are sterile flowers, which makes for a very showing inflorescence, gives you that classic look that we’re used to seeing at heirloom gardens and cottage gardens around the United States. But there’s very little there for pollinators, or in theory, very little there for pollinators.
More wild-type plants that are more typical of species that you would see in wild populations, exhibit a lacecap flower inflorescence. So essentially that lacecap inflorescence is much more flat, and the center is comprised primarily of fertile flowers. And these are offering benefits like pollen and nectar to pollinators. And on the perimeter, the outer ring of that inflorescence, you’re seeing some sterile flowers, but relatively few and in some populations, not at all.
But that’s where we expected to see the most pollinator visitation. And when we actually went through our pollinator observations, here in the trial garden, I was very clear cut that pollinators were going to lacecap hydrangeas, not exclusively, but in much, much higher numbers than the mophead selections and cultivars.
Margaret: Right? And, so if people don’t know the sterile versus the fertile flower and so forth, other florets or whatever you want to call them, it’s almost like the flat… If you think of a hydrangea, even the big puffy mopheads, it’s this big ball of these little flat florets, or I think some people call them bracts, I’m not even sure technically what’s what.
And those aren’t providing the nectar and pollen, as you said. But sometimes tucked in between, and especially in the lacecap types, where those flat ones might be around the edge, the perimeter, like you are explaining, there’s these little dots that don’t look like much to a human, right, to a gardener, in the middle—and lots of them—but those are flowers and those are full of yummy stuff, if you’re an insect.
So the insects, maybe they’re guided to it by the showiness of the outer ones. But they’re, “Hey, let’s go over there. I see some.” But what they’re going to dine on is the stuff in the middle or in between [laughter]. The little tiny insignificant-looking flowers.
Sam: Yeah, exactly. For such a large shrub, those flowers are extremely minute, and they produce even smaller seed. And it’s kind of amazing to us to see these teeny, teeny-tiny seeds that eventually will produce an 8-foot-tall by 9-foot-wide shrub in the largest cases in five, seven years.
Margaret: So you did these trials of, I think it was 29 species and cultivars, closely related, many of them are arborescens. And the winners—it’s almost like in this report, there’s the gardener viewpoint of the winners, and then there’s a page that has this chart about the pollinator visits to each trialed cultivar or species. And that’s quite different.
The ones that are on the top in terms of garden performance are not necessarily the ones that are on the top in terms of insect appeal.
Margaret: Yeah. That’s kind of interesting. So, I don’t know, where do we want to start? We’ve just a little bit talked about the ones that have the most insect appeal, without naming names.
I want to talk about… I mean, ‘Annabelle and straight arborescens are white flowers, but not all of them are even white anymore. And I’m seeing more and more of these, from Proven Winners and from North Carolina State University breeding, I’m seeing more and more of these mop-toppy looking ones, some of them are in different colors.
But it surprises me to see more mopheady kind of ones coming out when everybody’s interest seems to be in pollinator plants. So that conflict, that tension between those two goals. So tell me about it from your point of view, because you were looking at both.
Sam: We were definitely looking at both, so we’re always interested and we’re always going to, essentially our goal is to promote plants that work well in the mid-Atlantic region. And we’re looking at plants from two perspectives. We’re looking at them from a horticultural perspective, and this would be coming back to our mission. We want to inspire people about the beauty and value of native plants. It would be the horticultural value, and then the other value be ecological value for insect pollinators, specifically.
We are looking at both sides of the coin there. So there’s always going to be plants that might just be beautiful if they perform well, but we’re also trying to promote plants that are great for pollinators. And in some cases there’s great compromises. So like something like Hydrangea arborescens ‘Haas Halo’ [above], an absolutely striking, beautiful plant, incredible floral display. It is a laccap hydrangea, so lacecap flower type, and it’s attracting tons of pollinators, in addition to being a beautiful kind of classic landscape plant.
And then there are plants that are the mophead types, which are beautiful plants, but they may not be attracting as many.
So essentially what we’re just trying to do is provide you a well-rounded view of this genus, so that people can make informed decisions based on their various garden goals. If you want that classic look of that big mophead hydrangea, we’ve got a hydrangea for you. If you’re more like me and you really are gardening for wildlife, this chart is really where I would be paying attention to.
Margaret: Right [laughter].
Sam: So something like Hydrangea arborescens ‘White Dome,’ even just straight species Hydrangea arborescens. These are plants that are attracting insects in droves, and that’s really something that we do pay attention to.
We’ve actually been, in recent years, been linking our pollinator scores with the horticultural scores. So plants that attract more pollinators are getting a slight bump at the end of the trial in their scores. So something like Hydrangea arborescens ‘Total Eclipse’ [below], that bump might have been just enough for it to reach our top performers list.
So we want to be recommending a well-rounded group of plants, both from horticultural perspectives and from ecological perspectives, but there’s often plants that have great, great features of both.
Margaret: And one of the first native plant cultivars that I planted here a hundred billion years ago—not that I’m feeling old today, Sam, or anything, all these references to previous centuries—but was because it came from a visit to Mt. Cuba a very long time ago, when Dick Lighty was there.
And he had developed a cultivar of a twig dogwood called ‘Silver and Gold,’ Cornus sericea ‘Silver and Gold,’ with gold stems and silver. And, so it’s a cultivar, so it’s a little different, it doesn’t have green leaves. It has white and green variegated leaves, which as we now know means it’s maybe a little less useful to certain insects for certain things, but it still has a lot of the other traits. And I put it right in my front yard. It’s one of the first things you see when you come in.
But then I have the straight species, a lot of them sort of up on the hill, in the back, a whole big group of them. And I keep thinking about that as a way to say how to balance, even if we both want what the gardener likes, showiness, and what the insects like, which might be a little less showy in some ways. Although that ‘Haas Halo’ you said is so gorgeous for both insects and people.
So I kind of think a little bit of like, maybe you can have a few of these showoffs in a prominent spot, but maybe you can also make room for some of the ones that are a little less so, in a little less showy of a spot. Do you know what I mean? Kind of a balance.
Sam: Absolutely, yeah. It’s really about that perfect plant for the perfect place. And we’ve tried to promote plants that a spectrum of people could utilize in their home landscapes. Even if you just have a small patio garden, we try to have recommendations for you. So that even just by adding one native plant to your home landscape, you’re doing some good.
Even if it is a cultivar, many of these cultivars are still supporting some insects. It might not be as many as if you were to plant some of these mopheads. There’s some value, not much, but it’s still very likely more than non-native plants that aren’t from Eastern North America.
We like to call that “conservation by addition,” even by adding one plant to your home landscape, and no matter how you’re doing it, you are moving the needle in a good direction.
Margaret: O.K. I like that. I like that. Conservation by, you said, by addition, is that what you said?
Sam: Correct. Yeah.
Margaret: O.K. I like that. That’s great. That’s a great idea. O.K.
So tell us about some of the “winners.” I mean, because again, they’re not all white either. Not only are they not all lacecaps, but they’re not all white. So tell us about some of the standouts that and why.
Sam: Absolutely. So to me, if you can only have one hydrangea in your home landscape and I mean, you just have one room for one plant and it can be a slightly larger plant, Hydrangea arborescens ‘Haas Halo’ is my number one choice. I saw it in the trial garden. It was kind of blown away by the floral display, even when the plants were in bud. But as soon as they started blooming, there was a frenzy of insect activity on these flowers. And I went out that day and bought one for my home garden.
It’s an incredible, beautiful combination of horticultural value and ecological value. And it is the top performing plant in our trial, from every perspective. We grew these in full sun. We grew them in some shade. We even cut an example of these back for three springs in a row in late March, just to see what they would do. And it did really remarkably well under that treatment as well.
There’s a few plants out there that from Proven Winners, including Hydrangea ‘Lime Rickey’ [above], which is a really cool plant. It’s a very large hydrangea, but it has this beautiful kind of color-changing inflorescence. It is a mophead with less pollinator value. It’s not one that I would choose if you are trying to garden for wildlife, but it is very interesting from a horticultural perspective.
The flowers start off kind of this lime green. Then they fade to more kind of an ivory tone and the fertile flowers actually have some pink in them offering contrast. And then as those flowers fade, they kind of fade back to that lime green again.
And as you mentioned, there’s some beautiful pink cultivars that are coming out there. And that’s really kind of a recent trend in the horticultural world and horticultural breeding with Hydrangea arborescens. And a lot of that’s coming out of North Carolina State University and Dr. Tom Ranney there, really kind of on the cutting edge of breeding in this new color in Hydrangea arborescens.
Essentially how that’s accomplished is by starting with some of these white mopheads and breeding in with some of these lacecap, pink-flowering selections of Hydrangea arborescens, such as… There’s a couple out there, we had ‘Eco Pink Puff,’ which is one of the ones, one of those kind of starter plants.
What you end up with are eventually through a lot of selection is these large pink mopheads, and there were two in our trials that I thought did really remarkably well, Hydrangea arborescens ‘Incrediball Blush’ is one of the best. It’s a smaller plant, about 4 feet tall and wide at its largest after five years. Huge flowerheads, sturdy stems, again, not as great for pollinators, but it gives you that classic hydrangea look in a very reliable plant.
And interestingly, most plants in the full-sun at our trials didn’t do as well, when you were compared to the plants in the shade, that just lasted a little bit longer in the shade, and they tended to not burn as much. But with Hydrangea arborescens ‘Incrediball Blush’ and a similar plant of, well larger, but a pink plant as well, that came out of the same breeding program, ‘Invincibelle Spirit II’ [above], these plants did better in full sun. And that is a trend that we noticed these pink-flowering plants, pink-flowering versions of normally white flowering plants do better with a little bit of extra sun. So those were the pink-flowered ones that we loved.
And then there’s a couple out there that just really great alternatives to ‘Annabelle’ and ‘Grandiflora.’ And those include ‘Incrediball’ and ‘Bounty,’ both big mopheads, but they’re improved versions, very sturdy stems, resistant to flopping. Again, give you that very mophead look.
And then we had a couple interesting ones that aren’t as widely available, but if you can find them, absolutely go get them. One of those was Hydrangea arborescens ‘Mary Nell’ [below]. It’s another great mix of horticultural and ecological value, very similar to ‘Haas Halo.’ It has more sterile flowers around the perimeter, so arguably it’s a more showy plant and it still has good pollinator value. And this was actually introduced by the same person who introduced ‘Annabelle’ back in the sixties. It’s just not out there as much. I know Hydrangeas Plus Nursery, it’s an online nursery, carries it.
Worthwhile checking there. And we’re trying to get this one out in the trade as well, but it’s a really great plant, particularly in some shade, it’s fabulous.
And then finally, Hydrangea arborescens ‘Total Eclipse’ was a wonderful plant as well. A great, beautiful plant for naturalistic gardens and very similar to the straight species Hydrangea arborescens from both a horticultural perspective and ecological perspective. I think the two of them attracted… I think there was a difference of three pollinator visits over two years between them. Very, very similar performance.
If you can’t find ‘Total Eclipse,’ Hydrangea arborescens is another just fantastic garden option. Not quite as showy as some of the mopheads, but tons of pollinator activity on these plants.
Margaret: Well, Sam Hoadley, at Mt. Cuba Center, thank you so much. And what are you trialing next?
Sam: Yeah, so we are currently in the trial garden we have a Carex trial, which is wrapping up this fall. So look forward to that. We’ll be releasing our Carex research report next January. We are also trialing goldenrods, ironweeds, and we are-
Margaret: Oh, great.
Sam: …doing a brand new oakleaf hydrangea trial.
Margaret: Right. Well, it’s always good to talk to you and I hope I’ll speak to you again soon. Thanks for making time today, Sam.
Sam: Thanks so much for having me.
more from mt. cuba center
(Photos from Mt. Cuba Center, used with permission.)
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MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 11th year in March 2020. 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 February 7, 2022 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes/Apple Podcasts or Spotify or Stitcher (and browse my archive of podcasts here).