Dahlias Made Simple: Growing, Feeding, Cutting and Storing with Kim O'Brien

Dahlias are bold, flamboyant, sometimes slightly outrageous - and one of the great stars of the late summer garden.

But they can also feel a little intimidating. When do you plant them? Which varieties are easiest to grow? Do you have to lift the tubers every winter? How do you support them, feed them, protect them from pests, and keep them flowering into autumn?

In this episode, Rod Whiting is joined by Dahlia expert Kim O’Brien - Head Gardener at an RHS-funded garden in Cambridgeshire, flower grower, floral designer, BBC local radio gardening expert, regular Gardeners’ World speaker, and columnist for Amateur Gardening magazine.

Her passion for Dahlias really shines through, and the tips keep on coming.

💬 Got a question for us? Or fancy sending us some fan mail? Drop us a line: rod@lovingyourgarden.org

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Transcript

00.00 - Welcome and why Dahlias intimidate some gardeners

SPEAKER_00 0:00

Hello and welcome to Lovin' Your Garden Better Gardening Podcast. I'm Rod Whiting. Late summer gardens are just full of stars, but few plants make quite the statement of a dahlia. Bold, flamboyant, elegant, sometimes slightly outrageous, and available now in almost every shape, size, and colour imaginable. And yet, for all their beauty, they still intimidate some gardeners. Just a little. Are they difficult? Do you have to live them every winter? Which varieties are worth growing, and how do you keep them flowering right the way through into autumn? Well, someone who can answer all those questions and much, much more is my guest, Kim O'Brien. Hello, Kim. Lovely to see you.

SPEAKER_01 0:53

Hello, Rod. Nice to see you.

SPEAKER_00 0:54

Kim is head gardener at an RHS funded garden in Cambridgeshire, a floral designer and flower grower, a regular speaker at Gardeners World Events, a BBC local radio gardening expert, and columnist for Amateur Gardening Magazine. Right, let's get into the garden then, because well, and let me start by asking you how it's going in in your garden at the moment. It's been an interesting start to the season.

SPEAKER_01 1:18

It has. How is it going in my garden? Well, I'm absolutely up to my eyeballs in dahlias of lots of different stages. So I have potted dailias which are giving me cuttings that are sending up all the shoots. That's still happening. I'm taking cuttings every day. I'll take uh typically a couple of thousand cuttings every year. Um, so that's still happening. I've got dahlias that are rooted cuttings that are waiting to be potted on. I've got dahlias that are um have been grown on and that are now ready to go out, either into my polytunnel, into my garden, or into my allotment or my field. So um, yeah, it's it's busy. There's lots of stages, lots to contend with at the moment. I've got lots of plates

Why Dahlias? From “Blousy” Blooms to Pollinator-Friendly Varieties

SPEAKER_01 2:04

spinning.

SPEAKER_00 2:04

Well, given your experience and your academic knowledge as well, you could have done any plant you like. Why did you choose dahlias as your sort of special?

SPEAKER_01 2:14

You know, I'm often asked this, and I don't think I chose dahlias, I think they chose me. I feel like I was just grabbed by the heart in a way that I feel I feel passionate about them in a way that no other flower has captivated me. Sometimes I'll talk to people and they say, What do you do? And I say, you know, florist and and um you know gardener, and I say, I particularly love dahlias, and they say, Oh, I don't, I don't like dahlias. And I just think, oh gosh, there's so many dahlias. How can you not love dahlias? I think they mean that they think they're a bit of a faff and they're a bit deverish and they can be hard. Uh, that might be what they mean, rather than not liking the actual flowers, because who could fail to love the flowers?

SPEAKER_00 3:00

Uh I don't know whether you've come across Richard Chuck, who's um a very, very good gardener who uh I I've done quite a bit with, and he he has a bit of a loathing of blousy flowers. Now, I don't know whether Dahlius comes into that uh category for him, but uh they can be a bit blousy to use his term.

SPEAKER_01 3:18

They can be, and actually, I mean, personally, my personal taste is I love the the blousiness and like you said in your introduction, the flamboyance. Um, I love that. I love that they're really attention-seeking. Um, but having said that, there are varieties that are more subtle, a little uh they'll they'll take a step back and you know, not stage centre. And uh, you know, you have like the singles and the collerettes that are a little bit, and some of the pompons as well, they're a little bit more dainty and less flamboyant. So there really is something for everyone. When you when you compare the different forms of dahlias, whether they are um, you know, collerettes, balls, pompons, uh water lilies, um, the dinner plate varieties, the cactus varieties. Sometimes it's hard to remember they're all part of the same genus. It's like, you know, they're just so there's so much variety within them. Um so I think there really is something for

Choosing the Right Dahlia for Your Garden

SPEAKER_01 4:13

everyone.

SPEAKER_00 4:13

Let's deal with the the choosing them first. Are there specific varieties that that grow easier than others? I mean, what if what are we looking for in a dahlia?

SPEAKER_01 4:22

Yeah, I mean, it really depends what you as a gardener want from them. So if you want a dahlia that is good for pollinators, for example, I would recommend the ones with the open middles, like the singles and the collarettes. Um, if you want something that's very a very statement piece, it's very flamboyant, then it would be things like you know, cafe au lait, which are the you know part of the dinner plate varieties. So it's really down to the individual what you're after. In terms of are there some that are easier to grow than others? Yes, absolutely. Uh we've got the patio dahlias, the mignons, uh, the happy single series. They've been developed especially to be easy, actually. Um, and they're often the ones that you find in flower in the garden centre because you know the nurseries can churn them out. It's not often you find rows and rows of cafe au lait. Uh, I'm not saying it can't be done, but you don't often see those in the garden centres. You tend to see the easier to grow varieties.

Growing Dahlias in Pots vs Borders

SPEAKER_00 5:20

And then when it comes to to growing days, uh is there a big difference in the way you would you would grow certain sp varieties, or uh is there a fairly general approach?

SPEAKER_01 5:32

Um I would put things that are the patio varieties and the easier to grow varieties. They do very well in pots, for example. Uh that's not to say that the larger varieties and the more flamboyant varieties can't be grown in pots. They can, but they would take another level of um care and dedication that you might not be able to commit that time to. So um, you know, in terms of feeding, they're very hungry plants. So the feeding watering schedule that you would have to have for some of the larger, hungrier dahlias um wouldn't be, it wouldn't be as easy to do that. So uh I would recommend, you know, growing the the minions and the the happy singles, for example, and the the bishop, the bishop of Landaff and the Bishop's Children, all of those grow very well in pots. Um, but they can get a little bit lost in a border, depending how big your borders are. You might want something that uh pops a bit more, so you'd want something that's a little bit meatier in size in the border.

Planting Dahlia Tubers the Right Way Up

SPEAKER_00 6:29

For people who've never grown them or or look at them and feel a bit intimidated by them. When you get them out of the the the the bag, um they they're not your typical uh corms or bul or bulbs, are they? They are quite uh quite unusual. And some people might struggle with which way up they are supposed to plant them.

SPEAKER_01 6:48

That's right. You're being very kind there, calling them very unusual. I mean you can say they're ugly, they are. They're really quite unattractive, aren't they, when they're tubers? But um and it's amazing to think something so beautiful comes from it. But yes, I mean they can be a little bit intimidating if you haven't grown them before, but they don't need to be. They they really are quite straightforward. So they will have last year's stem on them, which will be near the top, and then the dahlia tubers will actually be like a bunch of bananas, if you like. You usually have that like a clump of tubers. So a dahlia tuba is actually a single one of those, each one of those is a um a tuba, but what we what we are sold in uh the UK um and in Europe, we're sold the the bunches of tubers. So um you would plant them with the bunches, you know, the fingers of the bunch of bananas down and the stem up, and that's how you would you would put it. And you would bury the the tuba and the crown. So the crown is the area sort of around here, and then you've got the stem up there, so you can make sure that that crown is is buried.

Why Dahlias Need Strong Support

SPEAKER_00 7:56

And how far down would that be buried?

SPEAKER_01 8:00

Uh it doesn't have to be buried that deeply. I mean, just enough to cover the crown really is sufficient. You don't want them too that you don't want them too deep, really. They don't need to go, you don't get any better, there's no advantage from from burying them really deeply. Um it takes longer for the shoots to come through, obviously if they're buried deeper, and also you don't get better support. Dahlias really are the most terrible design um in terms of, you know, because you you get the attachment where the stalk attaches to the um the tuba, where those stems are, that it's so weak, and um, you know, you they they send up, you know, some of the stems are sort of five, six feet in height, and then you get sort of three or four heavy uh, you know, dinner plate blooms on them, and then you'll get a gust of wind and snap and it's broken off at the bottom. So you don't get any benefit from um burying them deeper. What you need to do is support them.

SPEAKER_00 8:59

Let's talk about supports then, because uh that that is so important with with dailies, isn't it? Um it's particularly those those taller growing varieties. So, what what how do you do that? What's your method?

SPEAKER_01 9:10

So again, it depends where you're going to do it. If I'm doing it in my field, I would have wooden stakes with nets between them. I'm guessing the average grower doesn't have that because most of us wouldn't be growing in a field. You could do that at your allotment, that's a good way of growing them, growing, you know, hammering some um uh some stakes, wooden stakes in wooden posts, and then or just fence posts and then stretching a mesh, a net um over them. That works well in an allotment. It's not as attractive in a garden. Uh, in a garden, I would use metal plant supports, or you can use garden canes. Um, ultimately, it doesn't matter what you use, but it does matter that you support them. You'll be so glad you did. That that wind will arrive mid-August. You'll have a couple of days where it's really, really gusty, and you'll be looking out the window thinking, oh, I need to go and rescue them. It's too late, you need to have done it before, so don't get caught

When Is It Safe to Plant Dahlias Outside?

SPEAKER_01 10:05

out.

SPEAKER_00 10:05

Let's talk about the right time to plant them because the the temptation is, of course, they sell them a bit like bedding plants. Garden centres sell it really early, far too early. When is it safe to to to plant them outside or in the pot?

SPEAKER_01 10:19

Yeah, it's only safe to plant them once the last frosts have passed, really. So um, and even then, keep checking the weather because obviously our weather is so it fluctuates so much, we we can't be certain. But usually it's the end of May, uh, certainly for my area, Cambridgeshire, it's the end of May, and then you can plant out. So even now I'm still holding back. I've got no dahlias in my garden. I have dahlias planted in my polytunnel. I have nothing at the field or the allotment yet. Um, and I I won't because there is still that risk. And as soon as the frost gets them, that's it. The shoots are blackened and they go, they go limp, they look a little bit like cooked cabbage, and you don't want to be in that just for the just for the in that situation, just for the sake of hanging on for an extra few days or an extra week or so, it's really worth hanging on to them, keeping them sheltered.

Can Dahlias Survive Winter in Pots?

SPEAKER_00 11:13

I put in now this is the thing, is it's a very hit and miss thing. I've I've I've tried taking uh the tubers out of the ground and and putting putting them away for winter with very little success, it must be said. Um so we'll come to that in a minute, but I'm just thinking in terms of the frost. Uh I've got a Bishop Landaff which um I put in a pot last year, and I just didn't I thought I I was gonna take it. I had good intentions, I thought, yes, I'll take that one out and I'll put it in the cardboard box and some straw uh in the garage and uh never got around to it, so I thought, oh well that's that's that's a goner. And here it comes, it's coming out.

SPEAKER_01 11:52

No, that's good. I mean it's made it through the winter. You probably found that all the compost that was around it and the pot has insulated it really well. If you are going to leave things in pots, it's a good idea to mulch as well. Um, but you've probably escaped this year because we had quite a mild winter.

SPEAKER_00 12:09

We did, yeah.

SPEAKER_01 12:10

Yeah, so that really went in your favour. But it's exciting to know that it's sending up shoots, but you do need to watch if we have um the temperature drops down to people say zero, but actually, if anything, if it drops to two degrees or less, I'm out with a piece of horticultural fleece.

Feeding Dahlias for Bigger, Better Flowers

SPEAKER_00 12:28

Right.

SPEAKER_01 12:28

Or even just an old piece of carpet and I just like throw things over. Yeah.

SPEAKER_00 12:32

Um, feeding. Once we've got it planted and it's starting to show itself in its all its glory, what about feeding?

SPEAKER_01 12:39

So I actually feed at the time of planting. Uh I will put a good handful of uh fish blood and bone into the planting hole, and that seems to stand it in good stead until it starts um showing buds. So I kind of treat my dahlias the same way as I would treat my tomatoes, in that I don't feed them. I put I put feed in when I plant, and then I don't feed them again until I see, you know, the the um flowers forming on the tomatoes, for example. When I see the buds forming on the dahlias, that's when I start feeding. And I would feed with either a liquid seaweed or a tomato fertiliser, um, those are the two feeds that I would give

Feeding Dahlias for Bigger, Better Flowers

SPEAKER_01 13:23

it.

SPEAKER_00 13:23

So staking we've talked about, uh feeding we've talked about, pests. Are there any particular pests we need to watch out for? Earwigs I I I know are an issue?

SPEAKER_01 13:32

Earwigs can be an issue once the blooms are there. Well, they actually they can be an issue before the blooms arrive. So um when the flower is in bud, the earwig will come along in the night, have a little nibble, and then when the flower opens, each of those petals has been affected, and you'll end up with a ragged flower, a ragged bloom. Um so yes, earwigs can be an issue. One way of dealing with um earwigs is if you just get a cane with an upturned um terracotta pot stuffed with straw, and then during the day what what happens is the earwigs will go in there to sleep, and you just empty, go and empty that, you know, into like a faraway place, your green bin or in the field nearby, you know, or the woods, wherever, just empty it out and then refill it with fresh straw the next day. It's a bit of a pain that you have to, you know, you you do have to keep on top of it every day, but it's really worth it for keeping the earwigs at bay. Another thing you can do is you can apply oil, uh, just olive oil. They did used to say petroleum jelly, but if you are an organic uh farmer, then obviously it uses flower or flower grower, it uses petrochemicals, which you might not want to have. So you can use something like olive oil, and just when the flower is in bud, just stroke the behind the bud with the olive oil, and the earwig will climb up, they come up the stems, and then it's slippery and they can't get up, so they give up and they go back down again. So it's a nice organic, you know, remedy.

SPEAKER_00 15:11

Fiendishly clever.

SPEAKER_01 15:12

Yes.

Deadheading, Pinching Out & More Blooms

SPEAKER_00 15:13

Okay, uh, so we're we've we're enjoying uh dahlias through the through the summer, and uh then we in order to keep if if you've got the the varieties where you've got lots of different heads, um presumably you do you do still deadhead, you know?

SPEAKER_01 15:29

I do deadhead, yes. And um this is kind of a a bit this is where it becomes a bit personal as to what you want in your garden. So for example, in my field where I'm growing commercially, I want long stems for floristry. So dahlia heads, dahlia uh blooms will come up in threes, and then you have the central bloom, which is going to be the largest and the strongest, and then you have the two lateral blooms as well. And when those are about P size, you would pinch those out to get a long as long a stem and as big a bloom as possible, because all the plant will send all of its energy into that one stem. And it's also if you're gonna exhibit or enter your garden show and you want better blooms, um, you'll get fewer but better blooms. In my garden, however, um, I'm not going to do that because I want more blooms. I want to enjoy more blooms. So what I'm going to do is I'm going to allow that first one to open and then I'm going to allow the centre to blow as well. Now I wouldn't do this as a florist because it's you wouldn't have a blown dahlia, which is when I say it's blown, the centre is blown open, exposing the reproductive parts, and the pollinators can come and enjoy it. Because we have those single varieties, the collarettes and the singles, where they always have their accessory organs available for the pollinators. But a lot of the fluff, very fluffy, frilly, flamboyant dahlias don't. But all dahlias will, if you just allow them to go just past that dead head stage, if you can bear to have them leave them for the pollinators, then deadhead, and then the two lateral buds will also develop into flowers, and you get more flowers. So you're looking after the pollinators as well. It's about getting the balance in your garden between nature and beauty,

Keeping Dahlias Flowering into Autumn

SPEAKER_01 17:19

really.

SPEAKER_00 17:19

So keeping the display going into water, um, because they they can go on for a long time, can't they?

SPEAKER_01 17:26

If you if you look after them, they can, they go on until the first frosts. So um at the first frosts, everything becomes blackened, all the foliage goes blackened, and the the flowers, you know, when they see a frost, that's it, they've they're done. Um but to keep them going, uh you've just got to keep cutting them, really. I mean, dahlias are cut and come again flowers, and uh once you and also if you cut deep into the plant as well, and I know it seems counterintuitive because you'll think you're taking the plant away, but really dahlias they just come back with such vigour. Um, you might delay flowering a bit, but if you cut into them deeply, it's almost like uh supersonic pinching out, really. They will they will come back so much stronger and with so many more blooms, and that just keeps them going longer. You will find that once the um the light, once the the light levels begin to fade, once the the days um become shorter, notably shorter, the stems also become shorter, and the the quality of the blooms starts to fade a little bit. Um they're not as as as vibrant and as as strong as they were maybe a two or three weeks beforehand. But um, yes, you can you can you can keep them going by looking after them, just keep

Lifting & Storing Dahlia Tubers Over Winter

SPEAKER_01 18:45

deadheading them.

SPEAKER_00 18:46

Okay, so then we get uh God forbid, we get into the the frosts, and another winter is approaching. Let's see if we can convert me into doing this religiously. Talk me through the process of wintering uh the tubers.

SPEAKER_01 18:59

Yeah, so ideally you want your dahlias to have seen two or three frosts, um, because it's the frosts that triggers dormancy in the tuber. So it tells the tuber, it signals to it, it's time to rest. And that's really important if you're going to lift your dahlias, because I have so many, I have to start lifting before the frosts come. And there's a noticeable difference in the quality of blooms from those tubers the following year, as there is from the ones that have seen the frosts, and it's because they don't necessarily realise they're supposed to be asleep, and sometimes when I check on them in the boxes, I'll see they're still trying to grow, and I've got shoots that I have to take off because they don't realise that it's uh uh time to sleep. So presumably, if uh presuming they've seen two or three good frosts, then you would cut them down, you would cut the uh the blackened foliage off, you'd leave about six inches of stem. And the reason that you do that is because if there's going to be any rot in the tuber, it tends to be in the stem initially, and it gives you a bit of grace. If you've cut really close to the tuber, the ro can get in. But if the rot is happening up here, you've got a few attempts of getting ahead of it before it reaches the tubers. So you've cut it down to about six inches, you're then going to invert it, and that's really important because I said we've got our bunch of bananas with the stem pointing up. You're going to turn it upside down, and all the water or any water that's in those is going to drain down and out of the stem. So that's the excess water because dailiers don't. I always say they don't like to have their feet wet. Um, and it's so true, if they're full of water, they're prone to rotting during storage. So you need to dry them out just the right amount, stand them upside down on a piece of um newspaper, and you'll notice the newspaper's damp. Move it, check it the next day. When that paper is no longer damp, then you know your tuba is ready to be stored.

SPEAKER_00 21:04

Wow. Okay. Well, there are some great

Watering Dahlias Properly

SPEAKER_00 21:07

tips there. Well, you talked about watering. Is there is there anything we need to be aware of when watering normally through the through the summer?

SPEAKER_01 21:14

Yes, don't over water. There's a huge temptation, particularly if you've got dahlias on your patio, they're in full sun all day and they're gonna look really limp, the leaves, the foliage will go limp. Um, but you'll find that when the sun's worked its way round, they'll pick back up again. That's a sign they don't need any more water. It's very tempting to water them. Um but if they pick up again, they don't need water. If they don't pick up again, then give them some water.

SPEAKER_00 21:42

Another great tip. Once you've dried them out, uh what do you store them in? And where do you store them?

SPEAKER_01 21:49

Yeah, so this is a really good question. So back in the days when I was like, you know, I had a normal amount of tubers, like sort of nine or ten, like most people will. Um I used to store them in in dried tubers. Compost in a cardboard box in the shed and they were perfectly happy there. Um, now because I have so many tubers, um, I can't do that. You know, it's a huge amount of compost, it's very expensive, very heavy, it's just not practical. So I have done things like wrap them in newspaper, I've used uh you can use things like straw or hay to store them in, you can use shredded paper, you can use sawdust, any of those things. Uh, there's no need to wash them. People get really hung up about I've got to wash them, I've got to wash all the dirt off. I actually think that if they've got a good chunk of dirt over the crown, brilliant, and that's frost protection. Why would you take that off? Um, and then I've rather seen other people say, Oh, you put your dailias in a box and they mustn't touch. I'm like, why mustn't they touch? They're not apples. I understand that apples mustn't touch, but dailias aren't like that. The rot doesn't spread from one to another. If there's a rotten tube in there, you just take it out, snip it off, and then put it back in the box. So after years of experimenting with all these different media, because I have so many, and because I am the lady laziest dahlia grower in the world, I just chuck them in a box. They all go in together in a cardboard box, touching, dirty, and then they the lid gets closed and they go into my conservatory. I have a conservatory on the back of the house which is unheated, which is perfect because during the summer months, uh, when we don't when we don't have need to store dahlias, my family can use it. And when it's too cold to use, my dahlias get stacked in boxes or in mushroom crates in there. If we're going to have a particularly cold night, even though it's frost protected because it's a conservatory, but if it's particularly cold night, I will just throw a duvet or a piece of old carpet over the top of the stack of all of them, and then they're extra cozy.

SPEAKER_00 23:46

Lovely.

Dahlias as Cut Flowers & Floristry Tips

SPEAKER_00 23:47

Okay, well, we've covered a lot of ground there, but now let's just touch on your cut flower, uh your floristry, because that's another uh string to your bow. Um, and I guess dahlias are just fabulous for cut flowers. Have you got any tips for for using them as cut flowers?

SPEAKER_01 24:04

Um, yes, so people always ask me as a florist, what did dahlias work well with? Which other flowers do they work well with? And actually, I think there is quite a limitation because dahlias are so big and so blousy and so overpowering. You know, if you put your dahlias with, say, some little cosmos or you know, you've got little cornflowers or something in there, um, it's they get, you know, the other flowers, bless their hearts, they're just so lost. Even the filler flowers, they're they're they're totally lost. So I actually said when people say to me, What do dahlias go well with? My answer is dahlias go well with dahlias. And because there are so many different varieties, and maybe I'm just lucky because I grow so many varieties. I've got, I think at last count I had 236 varieties, but I actually stopped counting a couple of years ago. So goodness knows how many there are now. But because there are so many different forms, you can get, you know, texturally in a vase if you put, you know, a pom-pom next to a cactus, and then you've got a lovely floofy um dinner plate dahlia, the different textures they may as well be different flowers anyway. So dahlias work really well with dahlias.

SPEAKER_00 25:17

Well goodness. Uh I knew we got a full episode just about talking, just talking about dahlias. I mean, they're they're they're amazing uh flowers, and uh so got some wonderful tips there. So thank you so much, uh Kim, for for for sharing your passion with us. Uh it it's uh it really shines through.

SPEAKER_01 25:35

That's my pleasure, Rod. Thank you for having me on the show.

SPEAKER_00 25:38

Oh, you're very welcome. So and and

Where to Find Kim O’Brien Online

SPEAKER_00 25:40

if people want to follow you and and learn more about what you're doing and pick up some more tips, what's the best way?

SPEAKER_01 25:46

Yeah, so um, like you said, I do talk a lot of the garden shows. So if you're at any of the the BBC shows or um other local shows local to you, I may be talking there. But the main way to find me is on Instagram, and my handle is at garden.with.kim. And um, yeah, I will I can tell you anything you need to know about growing dailias. I've been doing it. This is my 26th year of growing dahlias, so I do know a thing or two about them now.

SPEAKER_00 26:14

I think we gathered that. Listen, it's been I absolutely love this. It's been really, really useful. Kim O'Brien, thank you very much indeed for your time. And thanks for listening uh to our Loving Your Garden podcast. Don't forget, you can contact us through Bud Sprouts fan mail. You can uh do take a look at our Facebook group, Loving Your Garden. Uh, and we have a YouTube channel as well. Thanks for listening. See you soon.