LATE IN THE SEASON, when all else in the flower garden is losing its head, dahlias are coming on strong and having their moment. The 52d annual American Dahlia Society National Show just took place at Longwood Gardens in Pennsylvania. Longwood senior horticulturist and dahlia expert Roger Davis was heavily involved in bringing the national show there in late September, and he joined me to talk all things dahlia, from ribbon-winning varieties to cultural tips for best performance, and even timely ones for off-season storage of those tubers.

Read along as you listen to the Oct. 1, 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).

how to grow and store dahlias, with roger davis

Q. I’m so glad to talk dahlias with you. Everyone is excited about them right now.

A. Yes, it’s the time of the year for the dahlias, for sure.

Q. It is. Boy, they show off, don’t they?

A. Yes, definitely.

Q. Did you have a big turnout for the show? It seems like the dahlia shows really do well, even the local ones.

A. Yes. On Saturday, we had over 11,000 guests in the garden.

Q. Geez. That must be crazy; the parking lot must be a madhouse. [Laughter.]

A. Definitely.

Q. Longwood is really no newcomer to the growing of dahlias. It’s not like you just did this show and this is a first-time thing. This is a long tradition at Longwood, isn’t it?

A. Correct. Yes. Even Pierre–we have records that he planted dahlias on Flower Garden Walk when he first started planting. Each year in September, we have the local show. The Greater Philadelphia Dahlia Society always hosts a show here at Longwood each September.

This year was special because it was the national show.

Q. I love that you say “Pierre” as if he’s your friend. Pierre S. Du Pont, whose estate this was—that Longwood was, back in the day?

A. Yes, definitely.

Q. How long ago … was that in the first quarter or so of the 20th century, I think that he had dahlias there?

A. In the ’40s and early ’50s.

Q. Oh, O.K., a little later than I thought. [Update: The Longwood website says du Pont purchased around 500 batches of dahlia tubers between 1909 and 1934, so it seems like he started earlier and continued the tradition as he developed the Flower Garden Walk.]

A. Yes.

Q. They’ve been planted in various ways and places on the property there, both when it was a private place and as a public garden. Where do you use them now these days?

A. Right now, we’re using them predominantly on the Flower Garden Walk, mixed in with all the other summer annuals, as well they are highly used in the Idea Garden, as well.

For this year, we wanted to highlight the dahlias because of the national show, so we planted 12 large beds full of dahlias with a number of seedlings. Then we also mixed them in with some summer annuals, just to show people how you can mix them into a flower garden. You don’t have to have them all segregated out in your garden. [Part of the walk, above; Longwood photo by Fred Zwiebel.]

Q. What other kinds of things, just for sort of a visual in our mind’s eye, what other kinds of things worked well with them on the Walk? Summer annuals, I mean.

A. Right. Anything that kind of mimics those flowers, as well, like you can’t go wrong with a salvia during the summer. Right now, Salvia leucantha [above] is looking wonderful. It’s kind a late-summer-blooming salvia. Those work really well. We have everything from zinnias, as well, to geraniums. Grasses look really good with dahlias. Sometimes, dahlias can have ugly bottoms where the foliage isn’t as nice, so anything that’s kind of can dress down the front really works well.

Q. In this area, is this an area of the Flower Garden Walk that also has a spring moment, or is it something that’s from summer through freeze?

A. Yes. During the spring, that’s the location where there are over 100,000 tulips planted.

Q. Oh, my goodness. That’s a lot.

A. This is a changing, evolving garden all year. The goal is to have as much color there as possible.

Q. You also plant them in the trial gardens, the dahlias?

A. Correct. In the Idea Garden, we have an area called the Trial Garden. We like to experiment. Each year is a little bit different. This year was kind of using plant combinations with dahlias incorporated so that people could see how they, at home, could incorporate dahlias.

Q. Oh.

A. I think a lot of people have the idea that dahlias have to be segregated out for showing, exhibition, but they can be mixed in and done tastefully so that they kind of blend in with other things, because they don’t … They kind of really start showing off by September. Early in the season, they’re a little more quiet.

Q. Yes. We’re saying “dahlias”—so far, you and I have both used the word “dahlia” sort of generically. But this is a genus of plants that … I don’t know as well as you do all of the technical, the sort of categories and sizes and flower-shape descriptions, but, for me, they’re from little buttons to as big as a child’s head. Those would be my categories. [Laughter.] You know what I mean? They’re drastic, right?

A. Exactly, yes. Yes.

Q. Tell us a little bit about sort of the range of things, of dahlias.

A. Sure. Right now, there are 21 flower forms that are recognized by the American Dahlia Society.

Q. Oh, my goodness.

A. They range, like you were saying, from the little small pom-poms all the way up to the formal decorative, the cactus, semi-cactus. There’s also a form that’s called orchid, which is a star-shaped flower.

Then, there’s the anemone-type flower form that has a pincushion center. There’s a wide variety. I think that’s one of the things that our guests really take away from the exhibit, is just the diversity of flower shapes, because you don’t always see all the other interesting flower shapes around.

Q. Yes. The waterlily is another one I saw in some of the Instagram pictures and stuff like that. There were … [Above, ‘Koppertone,’ a waterlily dahlia; Longwood photo by Candie Ward.]

A. Yes, definitely.

Q. Yes, waterlily. It really is that … It really is distinctive. A dahlia isn’t a dahlia isn’t a dahlia. They’re not all the same, by any means.

A. Exactly. You just never really know. You have to do a little bit of research to kind of see what the plant is going to do, because sometimes the smallest blooms can be on the tallest plants and vice versa. Sometimes the large blooms can be on short. The flower size and shape doesn’t really dictate the size of the plant.

Q. O.K. What kind of winners? There’s the Court of Honor where you kind of line up the ribbon winners, I think? Is that right?

A. Yes, definitely.

Q. Yes. So all these people came and they saw these … I’m thinking, from having been to other shows, that competitors came and brought a cut dahlia in a regulation vase or jar of some kind and those are all put out on tables in classes or something, and then the judging happens? Tell us a little bit about what goes on before we get to the winners.

A. Yes, correct. Well, we had folks come from all over the country. We even had some folks come in from Canada. They brought small blooms in their suitcase in water vials. That is true dahlia dedication.

Q. [Laughter.] It is.

A. Yes, everyone brings their blooms. Then we spend pretty much an entire evening and morning staging the blooms, which is basically primping them and getting them prepared to go into the vases. Then everything is numbered and labeled with the varieties and the name of the person who brought it. Then they go out to the show floor, where they’re then judged. The different groups are judged together—so all of the AAs, the dinner plates, all of those are judged together. Then, the winner of that section then goes to the Court of Honor.

Q. And so on and so forth through all the different groups.

A. Exactly.

Q. Is there anything to infer, when you looked at sort of what got the ribbons, is there anything to infer about what’s hot in dahlias or is this about a well-grown specimen that’s true to type or something? Or does this ever have a trendiness to it, in terms of color or whatever.

A. A lot of the Dahlia Society growers, a lot of the gentlemen really like to see just how big they can get the AAs, whether they can get a 13-, 14-inch flower.

Q. Oh, my.

A. Some of the dahlia growers aren’t as interested in the smaller ones, but then the folks that came from Canada had some beautiful little small ones and they won awards, as well, with the small ones. Everyone has their different preferences and tastes.

Really, when they’re judging, it comes down to flower form. If you can get that perfect flower form that that cultivar is supposed to exhibit, that’s what the judges are looking for.

Q. I see. Now, on the other hand, in, say, your trial gardens, you’re trying different things. As you said, you’re sort of looking to provide inspiration for home gardeners, combining them with things, making suggestions, so to speak, by what you plant. Do you pick up sometimes on, “Ooh, there’s these new varieties or there’s new colors”? I’ve seen, for instance, some bi-colors like … Again, on Instagram and other social media, ‘A la Mode,’ is that a variety? Do you know what I mean by that?

A. Yes, that is. Yes.

Q. Yes. It’s got these white-tipped … are they petals?

A. Yes, that’s right.

Q. It’s dramatic. They’re dramatic or … Are there things like that, are there, that you say, “Ooh, we’ve got to try those,” or, “Everybody is into orange this year?” Are there any kind of trends? For instance, as a gardener and someone who likes to use plants in gardens more than … I don’t have a trial garden or even a cutting garden. What excited me most in the last 10 or 20 years about dahlias was when the ones with dark-colored foliage, like the ‘Bishop of Llandaff’ [left; Longwood photo]. I might have just mispronounced that, but the ones that came from the UK that had the dark foliage and kind of orangey-red flowers. They were good because they always look good in the bed even before they bloomed because of that dark-colored foliage. It was dramatic. Right?

A. Yes. Those are definitely very popular, yes.

Q. Oh, they are? They continue to be, sort of the offspring [of ‘Bishop of Llandaff’]?

A. Yes, definitely. I love the dark-foliage dahlias. Those are definitely … Like you’re saying, have a presence in the garden even before they start blooming.

Q. Yes.

A. I would say another thing that people are really interested in right now is cut flower dahlias for weddings.

Q. Oh.

A. There’s one called ‘Café au Lait’ [above] that’s kind of creamy-white with a little blush of pink to it. It’s not necessarily what you would consider an exhibition-style dahlia. You don’t technically see them at the shows a whole lot, but they’re very popular out in the industry. Every florist is looking for them. [Wikimedia photo by Mark Twyning.]

Q. Interesting. ‘Café au Lait,’ O.K., now I’m going to have to go look that up when I get home.

A. Sounds good.

Q. Wherever we’re growing them, how to grow them? You are an expert plants person and grower, and you’re growing in a very refined environment where it’s got to be right so you’re trying to do everything just right.

What’s the protocol, from the beginning of the season, to then we can talk about storage later. Where do we begin if we want to do well with dahlias?

A. Well, to be honest, with dahlias, I think the most challenging part is remembering that these plants have hollow stems and they definitely need support.

Q. Oh.

A. Staking is the major thing to take into consideration. One way you can get around that a little bit is if you choose more dwarf, shorter varieties that don’t require as much heavy-duty staking. Anything that’s over 3 to 4 feet, oftentimes some dahlias get up 5to 6 feet tall, you need some really strong either bamboo or metal rebar stakes to be able to tie up these beautiful plants, or those late season storms will come through and knock them over and damage them. [Staked dahlias with twine between stakes for support, at Longwood; photo by Becca Mathias.]

Q. Is it important to put those supports, even though there’s nothing showing, or very little showing above ground early on, is it important to put the stake in right away so we don’t pierce anything or what?

A. Exactly. You have it exactly right. You definitely want to go in first with your stake and then plant your tuber next to that, or your plant next to it, just so you don’t pierce that one tuber and damage the plant.

Q. How deep below the ground … Do you start them indoors in pots first or in trays first to get them a headstart and then transplant? What do you do at Longwood?

A. Yes. We are fortunate here to have greenhouse space to be able to start them ahead of time.

That also allows us to note … Every tuber, even at home, any time I plant tubers, I’ll plant 10, and there’s at least one or two that probably aren’t going to do well. Every year, that’s kind of the case. When you plant them ahead of time, you can see which ones are the strong ones and plant those, as opposed to having to find out a month later this one’s probably not going to turn into much.

Q. Right. Of course, it’s always the one you put in the most prominent spot. [Laughter.]

A. Exactly. Exactly. With the tubers, there could be rot starting on the inside of that tuber but it still feels firm and O.K. Then, as you see it start to grow, you’re like, “Something’s not right. It’s too stunted.”

Q. I might, then … I’m in a colder zone, even; ‘m in a 5B compared to you. Say I decided I had a space, maybe under lights or a bright spot in a shed or something with a skylight or whatever, where I was maybe propagating other things, seedlings and stuff early, and I decided to put some dahlia tubers in. Would I put them in a gallon pot? What would I do? With some potting soil? How deep? What would I do if I wanted to do that?

A. Yes, you could pot them up. It doesn’t need to be a large container because, by the time you’re going to plant them out, you don’t want them to be too far along. You want them to still be fairly small.

Q. O.K.

A. You also are going to be very careful not to over-water them. If the potting mix already has moisture to it when you plant them or pot them up, as most does, it’s probably going to have enough moisture.

Q. Oh.

A. If you do a lot of watering early on, you’ll rot the tuber.

Q. Oh. O.K.. Oh, that’s a really good point, because I think a lot of times, we think, “Oh, I’ve got to really douse that.” You know what I mean? Like we’re watering in seeds in the garden outside because we want to really … Seeds need that but, with these tubers, we don’t want to use that same idea.

A. Right. The general rule in the garden is, when you plant something, you water it, but that’s not true with dahlias. If you plant a tuber directly in the ground, there’s enough moisture already there for it to get started. You don’t need to water it.

Q. O.K. I forget if you said how deep. How far below the soil’s surface am I going with these?

A. You just want the crown, the eye of the tuber that’s at the end, to be an inch underneath the soil is fine.

Q. O.K., so not too deep.

A. If you have really good-draining soil, like sandier soil like on the West Coast, you can plant it deeper and it’ll stay cooler, which is good for them. Here in Pennsylvania, we have clay and, if you plant it deep in the clay, it will rot on you. A lot of times, I will even mound my soil up a little bit and make rows in my garden, and then plant them in the mound. I can always add more water, but I can’t take it away from the tuber once it starts to rot.

Q. Right. Are we either direct-planting the tubers outside around the frost-free date or just before? Is there a timeline here?

A. I always wait. My personal preference … Everyone does it a little bit different, but my personal preference is after the frost-free date. I’m not in any hurry because really, once the soil temperature warms up, the plants will start growing. If you put it into cold ground, it’s going to sit there until the temperature gets high enough. There’s no real rush. If you have your tuber in before the first of June, you’re fine.

Q. O.K. Then, the thing that I think that we gardeners, a lot of us, want to skip, and this is true with a lot of plants, is we don’t want to pinch, we don’t want to kind of do any cutting back, any …

A. Right.

Q. Tell us about that, because these are plants that could benefit from that because they branch more, I think, if you give them a nip in the bud occasionally.

A. Definitely. Once your plant comes up out of the ground and has started really about three or four leaves, once it puts on three or four sets of leaves, you’re going to want to top the plant, which is basically just pinching out the growing tip, out of the center. That’s going to encourage the plant to branch low to the ground.

You will get multiple branches that will then grow up, and you’ll be able to then tie to your stake to support for the blooms. If you just leave the plant to its own devices, it’s going to grow straight up and set a flower right at the top, and then it’s going to branch way up high.

Q. Right. [Laughter.]

A. It’s going to be very tall and the chance of that main stem breaking is very great. So it’s better to get that weight, to keep the weight, center of gravity, down low on the plant because they get so top-heavy.

Q. Right. O.K. I have to remember that. I have to be courageous and do it because, again, it’s growing so beautifully, you don’t want to head it back.

A. Exactly.

Q. Yes, but it’s good. It’s good, a little …

A. It’s definitely the right thing to do. Then, even when the plant is growing, a lot of people, for use in vases at home or for showing, will often disbud, which means you’re taking off some of the side buds. Typically on a stem, buds are set in sets of five, so the terminal flower bud is going to be the largest. Then, if you pinch the two sets out underneath that, more energy goes to that main flower. It also will make the flower present over the foliage, as opposed to the side shoots growing and covering up that bloom.

Q. Right.

A. Some varieties just have a tendency for the plant to outgrow the blooms, and so then all the blooms are tucked down in the foliage.

Q. They’re hidden, right. In the Northeast, where I am, before long it’s going to be time for aftercare if I want to store my tubers. When do I do that? What’s the signal? When do I dig? What are ideal storage conditions?

A. Yes. It’s fine to allow your dahlias to be killed back by the frost. It’s actually really good for them. After the first heavy frost, you can cut the tops off of the tubers, cut the plant back to the ground. Then, you want to make sure that your name label goes with that root mass, that you leave it on the stake or you leave it attached to the portion that you left, so that you don’t lose the name of that variety.

Q. Sure.

A. Then, you can use a digging fork. You want to make sure that you stay like 8 inches away from where the stem came out of the ground so that you don’t pierce through the tubers, and kind of lightly dig that piece up.

There’s a lot of different people who store them differently, but I store my tubers as one big clump. Then, I put them into a bulb crate, a slitted plastic crate, that has some ventilation. I don’t knock the soil off of mine, because a lot of them have those thin necks, and so if you … If that tuber then bends and breaks and is detached from the crown of the plant, then you lose that portion. I leave it kind of all intact.

Then, I pack it with mulch around to kind of keep it tucked in there so it doesn’t dry out.

Overwintering tubers, the catch is you don’t want it to be too wet, and you don’t want it to be too dry.

Q. Just right.

A. You want to kind of be right in the middle.

Q. Yes.

A. You want a location that’s going to be cool but not freezing. And you don’t want it to be too warm. Oftentimes, people’s basements are a little too warm, so the tubers typically dry out and shrivel up.

Q. We’re talking about what temperature? 40-something or 50?

A. Yes. 40 is perfect if you can do that.

Q. O.K.

A. A root cellar is perfect, but not everyone has that ability to have a root cellar.

Q. [Laughter.]

A. I keep mine in my garage that’s attached to the house. Then, if we do have cold night temperatures that are down into the single digits, then I’ll bring them inside into the basement for a couple of days until the temperature moderates, and then I’ll put them back out in the garage.

Q. And then we start over again with those expert tips. Roger Davis, I’m so happy to speak to you. Congratulations on what sounds like a wild success of a show.

more about dahlias and longwood

• How to grow dahlias at home, by Roger Davis
• Longwood website and visiting information

(Show photos from Longwood, by William Hill; used with permission. Salvia photo from Longwood website; others as indicated.)

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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 Oct. 1, 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).