ITS COMMON NAME–bee balm–tips us off that Monarda is a pollinator favorite, and though the name doesn’t also say so, hummingbirds consider it irresistible, too. But as important as various native plants in the genus are, many of us gardeners may hesitate to plant Monarda because of its reputation for getting powdery mildew on its foliage in summer, or its inclination to run sideways.

George Coombs, who manages the trial gardens at Mt. Cuba Center native plant garden and research facility in Delaware, tested 40 different selections of Monarda over a three-year period, including plants from seven Eastern U.S. native species, and he shared results of which fare best on both scores in the hopes that more of us will make room and make more pollinators happy in the process. (Like ‘Purple Rooster,’ above, a top pick.)

Read along as you listen to the June 26, 2107 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).

evaluating monarda with george coombs of mt. cuba

Q. We’ve talked before on the show about your past trials of other native plants like Baptisia and Heuchera—and native plants are the mission of Mt. Cuba, which is both a garden for visiting and a research center, right?

A. Mt. Cuba Center is actually a former du Pont family estate, the Copeland family estate, and they left their estate to become a public garden. What kind of sets us apart from others in the area is that we focus on native plants. We broadly define our nativity region as the Eastern United States.

We do a lot of work promoting plants in a display capacity in the gardens itself, and then we also do research like what I do, trying to help gardeners pick better plants and kind of evaluate the options that are out there.

Q. Every time I get another of your research reports—and they are also online—they’re almost like a chapter of a book on the particular genus. Really wonderful. Let’s start with a little history of Monarda, and its transition from wild native into a garden plant. It has a long history.

A. It does. They can put a date back to when the genus was first named, and it was named after a guy, Nicolas Monardes, back in 1571. So it’s been around a long time; people have known about this plant.

Historically it was kind of used as a flavoring—an herb almost. They used it as a tea substitute during Colonial times when the real expensive tea was hard to obtain. Indians used to use it for the same reason, and actually some of the common names, like wild bergamot, reflect that it has a kind of bergamot flavor—which is what makes Earl Grey tea have that flavor. It’s not the same plant, but it’s kind of a close second. That’s where its common name came from.

It’s been around a long time, but it’s maybe only the past 50 years that it has been used as a true garden plant.

Q. Initially most of the breeding just involved two species, and mostly was happening in Europe, wasn’t it?

A. The two main species for Monarda are Monarda fistulosa and Monarda didyma. Monarda didyma is more of a red-flowered species, and Monarda fistulosa is more lavender or pale pink kind of flower.

They’re both found in different kinds of habitats here in the Eastern United States, but being a native wildflower it wasn’t really appreciated by many people here. However, over in Europe, the grass is always greener, so they kind of took a liking to our plants, and started the development of the first round of cultivars over there.

Since then, I think people over here have made an effort to really appreciate them. particularly for their pollinator attraction, but actually interestingly enough, the Canadian government got into a breeding frenzy with them in the late 80s or early 90s, looking for more cold-tolerant cultivars and more disease resistance as well.

Q. Right, because when they were breeding them in Europe, they weren’t in “our environment,” with hot, humid summers or whatever—not the same conditions. So they wouldn’t have been selecting for things like getting powdery mildew in our summers.

A. Exactly, and they probably do have powdery mildew over there, but it’s going to be another organism. So it’s going to behave and affect different plants differently.

Q. Right. So when the Canadian government got involved, that started to be a little more consciousness as you said about cold tolerance.

A. Yes, we got better cultivars for our area I think out of that program. It gave us some good starting cultivars for us to get going with more locally.

Q. So we talked a little about its provenance, but botanically speaking it’s a Mint family member, which you can kind of tell by looking at it but if you touch the stems—the square stems—you can definitely tell. What else?  It has such an interesting flower structure, doesn’t it?

A. The flower structure is strange because whet you see is actually a collection of flowers. Each little thing that you might call a petal is actually a flower in itself. So it’s kind of this conglomerate of flowers that emerge from the top of the stem.

They are different colors depending on the hybrid or the species that’s involved. I mentioned the species are red and lavender, and they have pink and magenta and all the colors in between.

So they can be quite bright, but a lot of times you’ll also get color on the leaves that sits right below the flower—they call that the bracht. That can kind of be a colorful attribute as well.

Q. To me sometimes some of these Mint family things, and Monarda being one, they kind of look like spaceships. That’s a really distorted view—but I have a lot of mental-health problems.

A. [Laughter.]

Q. But they’re sitting on this bracht, like the little landing pad, but then if you are looking straight down at the flowerhead it would look like the center, like if it were a daisy—the calyx, like a knob in the middle—and then it has this crazy stuff coming out of the center, and as you say each of those little tubes attached to the calyx is complete flower in itself.

A. Yes.

Q. They’re pretty wonderful, and again: there are several sources of color, not just each individual little flower. The calyx and the bracht can each add color to it.

A. That was one of my interesting lessons that I learned. The colored calyx—a lot of times those can be a darker color, which I thought was pretty attractive, especially for a few weeks after the flowers had finished blooming.

Q. Yes.

A. It kind of extends the interest.

Q. I agree, and that’s why I wanted to mention it because you evaluated them and would have noticed that.

The first Monarda I grew when I was a young gardener, like many gardeners of a certain era, was ‘Cambridge Scarlet.’ I see it’s in the report. I looked back to check, and in my first garden book that I wrote in like 1989, I grouped it in what I call the “confidence booster” plants for new gardeners [laughter]–the ones that make you feel like you have a green thumb. Because, whoa, it’s everywhere. You get this little plant, and then you’ve got this big plant.

I love the chart in the report you compiled of how wide each variety was after the three-year trial (among other factors).

So ‘Cambridge Scarlet’ used to be one of the only ones out there, but now there are a lot, yes?

A. We were able to find 40 that were readily available in our area. But it was interesting, when we were doing the width measurements, there came a time—I think the second year—when we had to cut them back and then just add on the subsequent growth to the final measurement.

Q. [Laughter.] So everything you had lopped off, you had kept records of, and you said, “Let’s see, we’re at 42 inches but we had 16 inches we already took off.”

A. Exactly. That’s exactly what we did; they were growing into each other, so we had no choice.

Q. Oh, let me tell you how many years it took me to tell ‘Cambridge Scarlet’ to go away. [Laughter.]

A. We’re still pulling a few out of the garden.

Q. I bet you are. So the mildew thing: some years that’s bad, and some years not so bad. I assume it’s not just weather and cultural tips, but that the right variety can help also. That was the other thing you were really looking at them for?

A. That was really the main reason we wanted to trial this plant, because it’s a big complaint gardeners have; there are a lot of issues. There was no data for the mid-Atlantic region how these plants react to powdery mildew, so it certainly was the impetus for the trial.

Kind of the interesting thing that we found was that it didn’t end up being the deciding factor solely for what performed in the garden. Probably the most interesting lesson we learned was that the plants that seemed to be the most resistant to powdery mildew—i.e., the ones that are completely clean—generally have a poorer flower display than the ones that might get a little bit of mildew or maybe even are mediocre in the mildew department. So it’s a tradeoff.

Q. There was a cost to that defense.

A. Exactly. Really powdery mildew starts to show up when the plants are stressed, which is usually when the plants are flowering—not the best time to look at mildew. It’s a stress-related kind of reaction. A lot of times when they are done flowering, the mildew will go away, but by that point the damage is done and you might have lost a lot of the foliage already.

To my mind it just depends on what part of the garden you want to put it in. If you are looking for bright color for the back of your garden, and you are not going to walk past it every day, go for the ones that are most floriferous and most colorful, and worry less about the mildew. Whereas if it’s going to be on your front sidewalk or by the mailbox, someplace you’re going to see everyday where it is going to be frustrating, kind of go with the ones that are a little cleaner. You’re going to be looking at the foliage longer than the flowers.

Q. Do you want to tell us about some that made the grade? I don’t even know 40; there are some genera of plants that I could name 10 cultivars, but with Monarda, I think ‘Cambridge Scarlet’ might be the only one I can name off the top of my head because I just haven’t grown them.

A. I think going along with that mildew interesting observation, there are two worth pointing out. One is Monarda ‘Dark Ponticum’ [above]. This is one that does bloom really well, but it also gets mildew, so it’s something that you are going to have to live with for a few weeks. The interesting thing about ‘Dark Ponticum’ is that the mildew doesn’t actually seem to affect the foliage. You’ll see the kind of white-grayish mold on the leaves, but it doesn’t cause yellowing, and it doesn’t cause defoliation.

So what happens is once the plants are less stressed out by flowering, and the weather changes, the mildew goes away—but then you’re left with a relatively lush-looking plant through the rest of the summer, which is a pretty big deal in Monarda. They tend to look pretty ratty late in the summer. So having a plant that hasn’t been through that kind of leaf-destroying event is really impressive.

Q. So ‘Dark Ponticum.’ Funny name; I don’t even know what it means.

A. I don’t really know what ponticum means, either, but it is a dark flower—kind of a medium-purple flower. On the flip side of that is Monarda ‘Purple Rooster’ [photo, top of page], which was the cleanest cultivar that made it into our top 10. It does, as I mentioned before, have a slightly reduced floral display, but it was still good enough to be highly recommended. That’s a cultivar that has a really dark purple flower, but very, very clean foliage—we didn’t see a spot of mildew on that over our three-year trial.

Q. ‘Purple Rooster.’

A. And that’s a relatively new cultivar that you can probably find at a few mail-order places, but it’s not a blockbuster yet. [‘Purple Rooster’ at Digging Dog Nursery and at Bluestone Perennials.]

Q. And it’s not lavender, it’s purple.

A. It’s purple—it is the darkest purple cultivar we looked at, and it is a true purple.

Q. That’s interesting; that’s a nice thing. Most I think of being on the lavender-y end of things, probably because one of our natives in my area is that color.

A. It’s nice because it’s not necessarily a hot color, either. I feel like you either get the pale lavender or you get these very bright fuchsia and reddish—so I think it’s a little easier to design with sometimes.

One of my favorite cultivars is one called ‘Grand Marshall’ [above]. This is the only compact cultivar that did really well in our trial. And the reason why I like it so much is that it’s short—it only gets to be about 2 feet tall—but it still looks and grows like a Monarda. We found that a lot of the other compact cultivars were so dwarfed and so dense that they just really look like these kind of distorted, misshapen plants that have been plunked down in the landscape and left and forgotten.

But ‘Grand Marshall’ still spreads—it doesn’t spread quite as quickly as some of the larger cultivars—but it still spreads like a Monarda, and weaves in and out of itself. Which is nice, because one of the downfalls of compact cultivars is they tend to become misshapen over time. So each winter portions of the root system of the Monarda tend to die out, but these compact cultivars can’t really grow fast enough to fill in those holes. But ‘Grand Marshall’ is still kind of…I don’t want to call it aggressive enough, but vigorous enough…

Q. Robust. [Laughter.]

A. …to mend itself. We could say it’s self-healing.

Q. And that’s what color?

A. So that’s more of a fuchsia color, and it is a relatively large flower for the size of the plant.

Q. It’s a reddish-purple. So those are a few. Best red one?

A. Best red one for us was ‘Gardenview Scarlet,’ which is a cultivar that’s not very commonly grown. It was a very close call between ‘Gardenview Scarlet’ [above] and what probably is the most commonly grown red one, called ‘Jacob Cline.’

Q. That I guess is the other one I have heard of.

A. That’s a really famous selection of Monarda didyma, and a really great selection, too—I don’t want to downplay that at all. The reason that ‘Gardenview Scarlet’ did better in our trials is that it was slightly less prone to powdery mildew than ‘Jacob Cline.’

Now the interesting thing is that people say ‘Jacob Cline’ is completely mildew-free, but we didn’t find that; it got mildew moderately badly. It wasn’t a severe or as detrimental to the foliage as some of the other cultivars. But ‘Gardenview Scarlet’ was slightly better. However, they are very similar.

Q. Now in the three years these were in the ground, being observed under trial conditions, were there any really incredibly “conducive to mildew” years? What makes a good or bad mildew outbreak besides the cultivar? Like with lilacs, some years it’s just bad and some years they don’t get it.

A. I don’t have a really concrete explanation. Mildew typically starts when you have these really nice warm days and then cool nights. The temperature fluctuation is what gets the spores germinating and then spreading. So that’s really what gets it going.

But I think the years when it’s the worst are the years when it’s just stressful. So if it’s really dry—particularly for Monarda, because they like to be in a more evenly moist soil. So if you stress the plant out in other ways, the mildew is going to get worse.

Q. So even with the good ones, we want to try to avoid—like with any plant—the stressors that will make it do its worst thing, or bring out its weaknesses.

A. It’s worth mentioning that Monarda didyma, the red-flowered species, and its similar cultivars, want to be in a shadier, moister situation, whereas some of the others toward the purple range are much better in a sunnier, slightly drier spot. Though I wouldn’t advocate putting them in a dry situation, they can handle a little more drought stress than the red ones.

Q. That’s good to know; that’s a little bit of insight we can have depending on our site as well.

You’ve just mentioned some that came from those two primary species that the breeding has been done with. But the report, and your work, you looked into plants from seven Eastern United States species. Were there other good guys that you want to shout out?

A. That’s actually probably one of my favorite things about doing these trials, is that we’ve kind of looked at some of these species that we aren’t using in horticulture—and is there a reason why, or maybe they have just been looked over. So there are two that I really like:

The first is called Monarda bradburiana, or Eastern bee balm [above], a naturally compact species that only gets to be about 2 feet tall. It has a very pale pink flower color, which I think is a nice kind of subtle flower color compared to other Monarda. And it blooms much earlier, in late May or early June, so it can extend your Monarda season a little bit, because the others are more July bloomers.

But it’s also disease resistant; we didn’t see any mildew, or very little mildew really, on this species throughout the whole three-year period. It didn’t make it into the top 10 because it did prove to be kind of a floppy plant. But I think over all there is a lot of potential with this species to maybe make future selections or even breeding work into new cultivars to really give us some better disease resistance.

Q. So that’s bradburiana.

A. The other one I really like is dotted bee balm, or Monarda punctata. [above]. It looks very different from most other Monarda. The flowers are actually stacked one on top of another, so they create almost like this big popsicle of flowers. It belongs to a different branch of the Monarda family tree, so that’s why it looks so different, but the added benefit is that you get these multiple layers of brachts, which maintain their color long after the flowers are gone.

With this dotted bee balm the flowers are kind of yellow with little brown spots, and they aren’t the most showy. You don’t appreciate it as much as you do the white or pale pink brachts that are underneath each set of flowers. It almost seems like it’s in bloom for two months, even though it’s only really blooming for three weeks.

Q. And this is one that Xerces Society, the nonprofit in support of pollinators, and New England Wild Flower Society and places like that will say that this is one of the really powerhouse pollinator plants in the areas where it is native.

A. It was impressive. We didn’t see as many bees on this plant but the beneficial wasps we saw on this plant were just incredible. I the back of my mind was this is a great plant to have in your vegetable garden to help attract these beneficial predators that will do some pest control for you.

Q. Yes, absolutely. To wrap up, I guess have to ask this: You alluded to this, but how long did it take to dig all those three-year-old clumps of Monarda out of the trial beds? [Laughter.] George!

A. They were really easy to dig out, surprisingly, thankfully. They are so shallow-rooted.

Q. And they weren’t entwined—the thing is if you have a mixed perennial border and you put Monarda in a couple of places…

A. I know; that would be a nightmare.

Q. It’s going to entwine in the roots of your desired plants, so you can’t lift it out.

A. We could almost do a clear-cut through the whole thing, so it was probably easier for us than most people.

Q. And what are you trialing now?

A. We’re in the third year of our Phlox trial, so we’ve actually been trialing shade-loving Phlox as well as sun-loving Phlox. We’re pretty excited about that. Right now out there it’s mildew city, except for a couple of cultivars, which are looking pretty good. So I won’t give those away, but I will tease you that we do have some really disease-resistant cultivars in our trial.

Q. That’s great.

A. We’re really happy about that. And we just started a trial of Helenium, or sneezeweed, and I am pretty excited about those. I haven’t had personal experience growing them, so I am really looking forward to learning about them and what does well.

Q. The report on Monarda, besides giving the history and the botanical understanding of the plant, and looking at these cultivars—it makes me want to dare again, and give them some room in the garden. So good job. [Laughter.]

A. Thank you—that’s the goal. We want to inspire people to plant these plants.

Q. You convinced me.

more from mt. cuba

• The Mt. Cuba website and visiting information
• Get the report on Monarda
• The Top 10 Monarda
• Browse the other Mt. Cuba native-plant reports
• My conversation with George on Heuchera
• My conversation with George on Baptisia
• Using natives in formal perennial borders at Mt. Cuba

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MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its seventh year in March 2016. 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 June 26, 2017 show right here. You can subscribe to all future editions on iTunes or Stitcher (and browse my archive of podcasts here).

(Photos courtesy of Mt. Cuba Center, used with permission.)