FEW GARDENERS would dispute the fact that garden Phlox is a worthy addition to the summer landscape, and nectar-seeking butterflies (like the American painted lady, above) emphatically agree. But which varieties among the many offered at nurseries and catalogs do the best job of both adding beauty and supporting beneficial insects?

George Coombs managed the Trial Gardens at Mt. Cuba Center native plant garden and research facility in Delaware. In past conversations, George–who in 2019 was promoted to Mt. Cuba’s Director of Horticulture–has helped me make our way through the daunting selections of Heuchera, Monarda, and Baptisia. George and the trial garden team spent three years evaluating 94 different sun-loving selections of Phlox for eye and butterfly appeal and mildew resistance, plus 43 shade-garden choices, too.  

Read along as you listen to the February 26, 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).

phlox q&a with george coombs

Q. Whenever one of your reports arrives I feel very lucky to get it, because it’s such a good read all the time. Do you write them—do you write up the summary report? They’re great.

A. I do write them, yes. They’re a lot of work but they always turn out really well.

Q. It’s like a whole reference book. They’re wonderful (and people can read them online). I have to confess George, I didn’t even know that the genus Phlox had so many species in it. I only knew the ones we commonly use in gardens. Tell us just a little bit about the genus and how you decided what to focus on.

A. Yes. Well the main reason we wanted to trial Phlox was really to look at Phlox paniculata, the garden phlox, the common phlox that most people think of. It blooms in the summertime but is really prone to powdery mildew.

With all of our trials, we like to include as many different species as we can get our hands on and so we dove a little deeper into the genus to get some things that were a little more obscure, a little different. So some of the other species that we looked at were things like Phlox carolina, Phlox maculata, glaberrima, amplifolia, and so on.

Really a lot of these are very different from the garden phlox that most people are familiar with. They tend to bloom earlier in the summer time, late spring/early summer. A lot of times they’re much shorter. So they’re just a very different type of plant, but in many cases we found that a lot of those were really good performers. So they were some of the top performers in the trial that we highlighted as well as different selections of Phlox paniculata.

Q. Interesting, because I think there was even a discovery of a species that isn’t commonly grown in gardens, wasn’t there? I can’t remember in the report that I read which one it is.

A. Yes. That’s Phlox amplifolia [above], and that was actually a plant that I had never even heard of until we started researching the trial a bit more. What we found is that a lot of Europeans referring to Phlox ‘David’ [below] which is a popular Phlox paniculata cultivar, as Phlox amplifolia ‘David.’ And so it was like well what is Phlox amplifolia? I need to investigate that.

What we’ve found is that there is a species called Phlox amplifolia. It’s pretty different from paniculata. They both have the large panicles of flowers, but Phlox amplifolia actually is more of a rhizomatous grower, so it spreads like a Monarda. It’s about 2-1/2 or 3 feet tall and it blooms a lot earlier. It blooms in early June, whereas most garden phlox start blooming in July. The really interesting thing about it was that it doesn’t get any powdery mildew.

Q. Aha. [Laughter.]

A. It seems like something that maybe we should have been paying attention to all along but nobody really was.

Q. Yes. So let’s sort of spend some time on the garden phlox, the common Phlox paniculata. What did you want to look at? You said powdery mildew, but I think in collaboration with your neighbors at the University of Delaware you also looked into some more sort of scientific things, yes?

A. Yes. So we’re always interested in how these plants perform in the garden but being Mt. Cuba, focusing on native plants, we want to know well what do these plants do in the environment besides just being pretty.

Q. [Laughter.]

A. So the thing that we were looking at with the Phlox trial is we wanted to see what differences there were in their ability to attract pollinators. So we actually had a graduate student as well as some volunteers here at the garden to keep track of that.

What we found is that one cultivar in particular really attracted the most butterflies out of all of the sun-loving phlox types and that was Phlox paniculata ‘Jeana’ [above]. The really interesting thing is that that was also the top performer horticulturally, in terms of what blooms the most, is the most disease-resistant, etc., so that was a really nice coincidence. What they found is that even though this is the one that’s attracting the most butterflies, we’re not really quite sure what characteristics about ‘Jeana’ are bringing the butterflies in.

So the interesting thing about ‘Jeana’ is that its flowers are actually the smallest out of any of the flower sizes of garden phlox. They’re about the size of maybe, somewhere between the size of a pea and a dime. They’re quite smaller than normal, which you would think maybe an atypical flower size might deter butterfly visitation because they’re not necessarily used to seeing that. But for whatever reason that’s what they love. So the going theory is that well maybe that smaller flower size somehow makes it easier or faster for them to move between different flowers and visit and get more nectar quicker and easier.

Q. Jeana is very different looking. I only met it for the first time last year. Yes, it’s very different. When you say the flowers are small, I mean there are still lots of them massed together into these like heads, so to speak, but each individual one is small, is petite compared.

A. Yes. The flower display is still very impactful, very, very showy. It’s just the individual flower, they call it a pip in phlox. The individual flower, the little pip is quite small.

Q. P-I what?

A. P-I-P.

Q. Pip, oh a pip, of course. I’m sorry, a pip. Yes. My father used to call me and my sister pips as well. I don’t know where that comes from.

A. Yes, exactly.

Q. But yes, whatever. One thing that I read, I think in the report, was that one of the sort of wonderings of the scientific aspect of this looking at the appeal to butterflies is that in breeding, in sort of selecting Phlox over the years—horticultural people and plant breeders—we haven’t thought about what makes an ideal structure and nectar supply for the native insects that have long used and co-evolved with these plants. We may have made adjustments that weren’t to their liking, yes?

A. Exactly. It’s like when you’re breeding for certain characteristics, unfortunately, you’re always going to be ignoring a few others. I think a really good example is the breeding that they did in tomatoes, where they were breeding for a very solid red color but in doing so they also happened to breed out the taste, which you would think would be kind of important.

Q. [Laughter.] Oops. Oops, little problem.

A. It got left to the wayside, so yes.

Q. Yes.

A. It happens when you’re doing focused breeding like that.

Q. The range of colors, ‘Jeana’ is what? How would you describe the color? Is it a lavender-y pink or is it pink or what color is ‘Jeana’?

A. I would call it more of a pure pink, almost a bubblegum or cotton-candy pink.

Q. O.K.. And the range of colors in Phlox paniculata is not super wide but it’s not just … I mean we have white. We have sort of more hotter pink, coral pink kind of shades.

A. Yes. The natural color for Phlox is really like a bright garish pink, almost fuchsia-colored. Then you’ve got anywhere things from white to pale pink, there’s really coral orange-y colors now, there’s some that are almost a true red, as well as pale violets and purples.

Q. And some have eyes, like contrasting centers, so to speak.

A. Yes. And even there are some now that have a pinwheel pattern, stripes across the petals that are pretty interesting.

Q. Oh. So what were some of the other like winners besides ‘Jeana,’ who seems to have won on many counts, mildew resistance as well as appeal to butterflies?

A. Some of the other ones that are really good are Phlox paniculata ‘Glamour Girl’ [above]. That’s a relatively new cultivar but it’s also compact, gets to be about 3 feet tall, kind of a nice pinkish coral color. ‘Robert Poore’ is an old-fashioned cultivar which is very tall, it gets to be about 5 feet, in the more traditional Phlox paniculata color, that bright magenta pink color. That’s a really nice one. ‘Delta Snow’ is also one that was really mildew resistant. That’s been shown to be mildew-resistant in several other trials, too, so that’s a good one for a broad part of the country.

Q. Did you evaluate for duration of the bloom, like the length of the bloom season of the particular plant as well?

A. We did. We found some of them were a little big longer than others. I would say for the most part they’re all blooming for about three, maybe four weeks. Unfortunately, it’s really hard to gauge because so much of it just depends on the health of the plant, so the healthier plants are going to bloom longer.

Q. Right.

A. Our trial, I will say that we really put them through the test. It was a very difficult situation when you’re growing 90 different types of Phlox all in one confined space.

Q. [Laughter.] Woo, yes.

A. When these plants are prone to disease and you put that many in one spot you’re going to get incredible disease pressure. They were suffering. So I think in certain cases where plants were a little more inferior, a little less disease-resistant, they may have bloomed longer in a personal garden where there was just one Phlox or two. We really kind of tested them to their limits.

Q. With the mildew now, let’s see what happens? In the summer we get the warm days and cool nights and I think that promotes our powdery mildew moments, is that correct?

A. Yes, that’s usually when it starts to show up. It’s early to mid-June, it depends on the weather. As soon as you start getting those nice warm 80-, 85-degree days but the plants are still getting cool down to the 60s, that creates the perfect temperature fluctuation for mildew to first develop and then spread. It just really can take off in those kind of conditions.

Q. It used to be recommended, I mean I can sort of remember, being 100 years old, I can remember in vintage garden how-to books, one of the suggestions would be that as our Phlox clumps, our Phlox paniculata clumps, were emerging in spring we were to crawl around and knock off every other emerging little shoot, so to thin the clump before it emerged fully so that it would have more air circulation.

That was one of the kinds of crazy things that I was taught to do as a young gardener. What really is a good idea? Is there anything that helps? [Laughter.]

A. I think air circulation is always going to be important, reducing that humidity level right around the plant is helpful. I think really the easiest thing to do is to just try and pick a cultivar that’s been proven to be disease-resistant. We’ve definitely seen quite a range in terms of just natural resistance to powdery mildew.

But then also I think the thing that we forget about most often is that plants really get diseased when they’re stressed, so the important thing is to really put your plant, whatever it might be, to put it in a site where it’s going to most likely thrive.

So for Phlox paniculata, they really prefer a sunny spot that is somehow cool with consistently moist soil, which sounds like the perfect Goldilocks that doesn’t exist, but they naturally occur along stream banks and the edge of flood-plain zones so they’re used to having access to water but not necessarily being flooded themselves. So that’s why they do well in those kinds of situations. So I think having a richer soil, something with a lot of organic matter. Then if you are able to provide a little extra water during times when it does get dry, it’s going to make a world of difference in reducing the powdery mildew.

Q. I forgot to ask, what is their native range in the United States? Like how wide-ranging are they? Are they in eastern part of the country or what? Do you know? I haven’t looked at the range map anywhere.[Range map from GoBotany, using BONAP data.]

A. You know, I haven’t looked in awhile myself but I think they’re pretty broad spread, probably from Pennsylvania down to the deep South.

Q. O.K. So where in the garden do … I mean you tortured them in the trial garden [laughter], cheek-to-jowl in a monoculture of Phlox, Phlox, and more Phlox, but where in the garden do they go?  I know at Mt. Cuba some of your colleagues have been combining native perennials into landscape settings and so forth. Where do Phlox appear, these tall garden Phlox, what do they look good with or any ideas about using them? Or should they be in the back of the border to hide their messy legs if they get a little mildew or what?

A. Well a lot of it’s going to depend on the size of the plant that you want to use. I personally think that a lot of the taller ones, at least in our trial, were much more vigorous. So I’m more inclined to use those than some of the more compact varieties. I would recommend back of the border. They’re really great in meadow situations or kind of naturalistic plantings, where you can really crowd them around with other plants that, even though you’re creating the bad situation where mildew might develop, having a diversity of plants surrounding them, typically ones that don’t get mildew, it really doesn’t add to the disease pressure.

Q. O.K.

A. So those are, I think … other tall perennials, grasses, things like that are things that I would typically combine Phlox with.

Q. So any other stand outs that we should be looking for? And again, we should qualify it saying you’re only in one place, you’re in the mid-Atlantic, you’re in Delaware. You’re doing these trials there based on your conditions but there are probably extrapolations for a wider range of situations, about their mildew resistance and so forth. But any others that you loved?

A. One that I think is really good that’s not quite too popular is one that’s called Phlox carolina ‘Kim’ [above].

Q. Kim?

A. That’s another species. Yes, Kim, like the woman’s name.

Q. Oh.

A. So that’s just a really nice, 2-foot-tall, prolifically blooming phlox that doesn’t get any disease, doesn’t get any kind of powdery mildew or leaf spot. It’s just a really clean, clean cultivar. So that’s something that I think should be more popular than what it is.

Q. I wonder why not. I wonder why those haven’t been.

A. I think most people, when they think of Phlox they think of just the tall garden Phlox with the big giant flowerheads, and anything else is lesser by comparison.

Q. O.K.. So Phlox carolina ‘Kim.’

A. Yes.

Q. O.K., that’s one to be on the lookout for. Then, of course, there’s the other phlox. As I think I said in the introduction, you did trial, what is it, 43 shade-garden choices as well, is that right? They were in a couple of different species I think.

A. Yes, we figured we had some space in the shade garden, so we’re like well let’s just kind of go hog wild on Phlox altogether.

Q. [Laughter.] You’re just wild guys down there at Mt. Cuba.

A. Yes, we can’t control ourselves. [Laughter.]

Q. Yes. Party, party on.

A. So we had about 40 different kinds of shade-loving Phlox. They both represented Phlox divaricata, which is the woodland phlox, and Phlox stolonifera, which is the creeping phlox. Both of those are shade-loving types but they’re very different looking. [Above, P. divaricata ‘Blue Moon’.]

The creeping phlox, Phlox stolonifera, is more of a mat-forming groundcover. The woodland Phlox is more of a typical clump forming perennial about 1-1/2 feet tall. They do bloom at the same time so you can think of them in the garden as providing color at a similar point.

Q. What month is that for you that they bloom?

A. For us it’s at April.

Q. Yes, O.K..

A. But the timing in April can fluctuate depending on the weather and the season.

Q. Not that the weather fluctuates at all lately, at all.

A. Never.

Q. Never, no. George, I don’t know if I told you in our correspondence lately that it was almost 70 degrees here in Zone 5B in February and that was slightly, just a few days after an ice storm. So we’re doing real good over here. [Laughter.]

A. You know, I don’t mind the 70 degrees. The plants probably have a different opinion, but I don’t mind.

Q. Yes. It’s crazy. Anyway, that’s a digression. So these two sort of, are these like semi-shade, shade kind of denizens?

A. Yes, these are shade, semi-shade. Typically they want to grow in, again, in a nice moist, fertile soil. But they don’t want to be in something that’s really wet so they want to have well-drained otherwise they’ll typically rot during the summer months.

Q. And I saw, I think it was with the stolonifera, which I believe you said were easier to grow. Are they a little more robust?

A. Yes. We found that actually the divaricatas were challenging and I don’t know if it was because we just didn’t really get the healthiest plants in when we purchased them, or if they’re just naturally more difficult. I’ve heard from other gardeners that were seeing us struggle, they’re like, “Yes, we had the same problem. They’re great when they get established, but it’s getting them to establish that can be more challenging.”

Whereas the Phlox stolonifera, I think because they’re more of a groundcover. They’re a little more aggressive, a little more vigorous, that they were just much easier for us to grow in the garden. I think for a beginner person those are a lot easier.

Q. There was a great combination suggestion actually. A lot of listeners and readers ask me about this, about sort of a living groundcover so to speak. Making these, I always call them “mosaics” of plants, that play well together but give you sort of a long season of interest without tons of a complex care regimen. You suggested sort of using the stolonifera with a few other things I believe, yes?

A. Yes. So we’re actually, that’s kind of one of the things that we do best here at Mt. Cuba is just combining these plants in almost a groundcover where they all knit together and there’s seasonal interest throughout the year. The plant pairings for Phlox stolonifera that work really well [above] are foamflower, which is Tiarella cordifolia, Pennsylvania sedge, which is Carex pensylvanica, and then maidenhair fern, which is Adiantum. Together those are just a really nice, they all put on a great show in the spring time with a fresh flush of either foliage or flowers. Then the ferns and the Carex really carry you through the rest of the year with a textural interest too.

Q. And the Tiarella, yes, the Tiarella foliage I think is adorable, you know?

A. Yes, yes. Absolutely.

Q. Yes. So that kind of, in other words, not thinking about, “O.K., I’m buying 10 of this one plant and that’s it.” Then you’re going to be disappointed because then its moment is over and that was the whole thing, right?

A. Yes.

Q. These little like more combinations, the way nature might plant, actually, is a lot smarter I think besides beautiful.

A. And mixing those plants together you don’t really have to pay attention so much to, “This needs to go exactly here, this needs to go exactly there.”

Q. No.

A. They do their own thing and live with each other very well. You can just step back and enjoy.

Q. Right. That’s why I sort of think of it as mosaics as opposed to designs, do you know what I mean? The way that things sort of, it’s irregular, it does its own thing.

A. Yes. They self-regulate and self-sort themselves.

Q. Yes. So I have to ask you, what in the world are you trialing next? because you just keep going, George. I don’t know how you guys do it there. [Laughter.] What’s next?

A. They keep cracking the whip so I keep cracking out the trials.

Q. You’re not going to run out of native plants to trial are you?

A. I don’t think we will. At least not in my career, no.

Q. O.K.

A. So right now we’re halfway through a trial on sneezeweed or Helenium.

Q. Oh, that’s going to be a great one. Those are great plants.

A. That’s something that I think a lot of Americans aren’t really aware of. They’re taller, they tend to bloom in the fall, late summer and fall. They’re not very popular perennials here but Europeans love them.

Q. Exactly.

A. There’s cultivars available in Europe.

Q. The places I’ve seen them in gardens used prominently were, years ago, in English gardens. Then knowing they were native American I was like, “What’s the problem?” But they were not popular here.

A. Yes. So we’ve got about 40 different cultivars of those.

Q. Wow.

A. Half of which aren’t really available in the United States yet, so we’re looking at some of the ones that the Europeans love as well as the ones that we already have access to here. So we’re pretty excited about that. And we just started two new trials, one on Echinacea.

Q. Oh.

A. Yes. We looked at coneflowers about a decade ago actually, when the coneflower craze was just getting going with all the different colors coming out and what have you. Most of those cultivars that we trialed and reported on a decade ago are no longer available, so we don’t really have any guidance to give to people anymore. So we’re looking at those again to see what’s changed in the industry, if they’re performing any better than what they were a few years ago.

Q. Boy, they were sorry creatures. Those first ones it was all flash and dash.

A. Yes.

Q. The tag looked beautiful, because they were breakthrough colors that weren’t expected, and then a year later the plant was dead. I’m not the world’s best gardener, but boy, if I can’t grow a coneflower, if I can’t keep it for two years, it’s not much of a plant. Do you know what I mean?

A. Well and they are generally pretty expensive too so-

Q. They sure were.

A. When you’re spending that kind of money you want it live at least for a couple years. You know?

Q. [Laughter.] Yes. Yes, yes, yes.

A. Then the other trial that we just started is on sedges or Carex.

Q. Oh, great.

A. I think maybe most homeowners won’t really will appreciate but it’s certainly a plant that a lot of landscape architects and things are really getting keen on that these are very utilitarian plants that are great for groundcovers, just really versatile workhorses that we’re just scratching the surface on where these plants can grow and what kinds of conditions they can tolerate. So we’ve got over 80 different species of those that we’re going to be looking at for the next few years.

Q. Wow. Busy, busy, busy. Well George Coombs, like I said at the beginning, I’m always so happy to get another one of your reports and always really pleased to speak to you. Keep up the good work, because I think you’re really, the Mt. Cuba effort really is going to continue to change and evolve, influence great gardening and great plant selections.

A. Great, well thank you.

Q. You say the heleniums aren’t available yet, but guess what, if they’re in your report they’re going to be available.

A. We’re going to try, that’s our goal.

Q. Yes. You’ll move the market, as they say. Thank you for making the time today, George.

A. Thank you.

more on natives from mt. cuba

• All the Mt. Cuba Trial Garden reports
• Visitor information for Mt. Cuba
• Our past interview on Monarda
• Our past interview on Heuchera
• Our past interview on Baptisia
• Border design with natives, at Mt. Cuba

(All 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 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 February 26, 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).