By: Bonnie L. Grant, Certified Urban Agriculturist
There is absolutely nothing like a big bed of dahlia flowers. The blooms come in many colors and sizes, giving any gardener’s taste an outlet. When planning your bed, it’s a good idea to consider what to plant with dahlias. Dahlia companion plants can be the perfect accents to set off the flowers but may also aid in deterring pests or even act as a decoy and sacrifice themselves to insect predators. There are many flowers that complement dahlia and do double duty to prevent pests.
Dahlias grow from tubers and produce flowers that range in size from dinner plate giants to diminutive discs. Dahlias are perennials and work well in a mixed perennial bed. Consider the huge water needs of these plants when selecting companions for dahlia. Plants can also grow several feet tall and nearly as wide. Lower plants should be able to tolerate some shade and taller plants can be used at the rear of the bed as an accent.
Choose plants that assist dahlias by repelling pests so the garden bed is free of insects and dahlia health is protected. Herbs are often good choices, as many of them have pungent scents and oils which seem to deter pests.
Annuals and perennials can coexist peacefully in an ornamental dahlia bed.
As you choose other perennial companions for dahlia, remember to check the size of your plants. Since most dahlia form large bushes, taller plants will set them off nicely if installed as a backdrop. Towering joe pye weed, butterfly bush, and agapanthusare excellent vertical choices to brighten up the background of dahlias.
Sunflowersseek sun’s warmth as much as dahlias and will thrive in the same garden bed, adding their cheery faces as part of the area’s charm. Helenium, marguerite daisies, and goldenrodare other taller plants to add height.
Tumble a scattering of annuals in the front of the tubers such as petunia, marigold, ageratumand bacopato add a rainbow of color while pulling the whole effect together.
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Read more about Dahlia Flowers
Show off dahlias to their best effect by pairing them with other border favourites.
Published: Tuesday, 2 July, 2019 at 7:26 am
Few border plants can match dahlias for sheer exuberance.
The incredible range of available colours and forms can be overwhelming – you could easily fill a border with dahlias alone. However, they really come into their own when paired with contrasting and complimentary border plants.
Remember, to keep your dahlias flowering until the first frosts, you’ll need to keep on top of deadheading and feed them regularly with a potash-rich feed to encourage further blooms.
Discover some of the best dahlia planting combinations to try, below.
Cosmos and dahlias are a classic pairing, and if you opt for single-flowered varieties of both, you’ll attract an abundance of bees and other pollinators. Pictured here is orange Dahlia ‘Happy Single Kiss’ and pink Cosmos ‘Dazzler’.
While eye-catching, dahlia flowers aren’t scented, so combining them with nicotianas is an ideal solution for a fragrant display. Not all nicotianas are scented, though, so check this before buying – varieties to consider include ‘Perfume Purple’, ‘Only The Lonely’ and ‘Grandiflora’.
Airy Verbena bonariensis is perfect for blending with dahlias in borders. The loose stems blend well without competing with the dahlias too much and help to break up areas of dense planting. It’s also popular with pollinators, so makes a great choice for wildlife-friendly planting schemes.
Achilleas come in lots of colours, but this fiery pairing is ideal for a hot-toned border. Achilleas, like dahlias, tolerate a wide range of conditions as long as they’re not waterlogged.
Many ornamental grasses like this Pennisetum ‘Red Head’ are reaching their peak just as dahlias burst into flower, so the two make natural partners. They look lovely in a vase, too. Other grasses, such as Stipa gigantea, would also work well.
Grow dahlias for gorgeous, colorful flowers that bloom from midsummer through autumn, when many plants are past their best! The tubers are planted in the ground in late spring. In colder zones, you do need to dig up and store the tubers in the fall if you wish to grow them as perennials (or, treat as annuals). Here’s how to plant, grow, and store dahlias.
Dahlia is a genus of tuberous plants that are members of the Asteraceae family related species include the sunflower, daisy, chrysanthemum, and zinnia. They grow from small tubers planted in the spring. Picking a favorite dahlia is like going through a button box. As well as coming in a rainbow of colors, dahlia flowers can range in size from petite 2-inch lollipop-style pompoms to giant 15-inch “dinner plate” blooms. Most varieties grow 4 to 5 feet tall.
They are considered a tender perennial in cold regions of North America. They are only winter hardy in planting zones 8 to 11. Gardeners in zones 2 to 7 can simply plant dahlia tubers in the spring and either treat them as annuals or dig them up and store for winter. Dahlias love moist, moderate climates. Though not well suited to extremely hot climates (southern Florida or Texas), dahlias brighten up any sunny garden with a growing season that’s at least 120 days long.
Check out our video to learn more about growing dahlias in your garden.
Pinching, Disbudding, and Staking
Dahlias are beautiful in a vase. Plus, the more you cut them the more they will bloom. To gather flowers for a bouquet, cut the stems in the morning before the heat of the day and put them into a bucket with cool water. Remove bottom leaves from the stems and place the dahlias in a vase. Put the vase in a cool spot and check the water daily. The bouquet should last about a week.
Unless you live in a warmer region, you have to dig up dahlias in late fall before there is a hard frost in your area. Native to Mexico, Dahlias won’t survive freezing temperatures. Digging and storing dahlias is extremely easy and simple, and will save you the money that would otherwise go into buying new ones each year.
If you live in an area where your ground doesn’t freeze, you don’t need to dig up your tubers. The general rule is: If you live in USDA Hardiness Zone 8 or warmer, you can leave dahlias in the ground. In Zone 6 or colder, dig them up. In Zone 7, you may be able to get away with just covering the plants with a thick layer of leaf or straw mulch, but if a freeze hits, you may lose them.
When to Dig Up Tubers
Dig up dahlias before the first hard freeze. A light freeze (32°F / 0°C) will kill the foliage, but a hard freeze (28°F / -4°C) will kill the tubers, too. See your fall frost dates.
A good indication of when to dig your tubers up is when the plant starts to turn brown and die back.
How to Dig Up Tubers
Digging up tubers is easy:
That’s it! Cut rotten tubers off the clump and leave the clumps outside in the sun upside down to dry naturally.
How to Store Dahlia Tubers
Re-planting Tubers in Spring
If this all seems like too much bother or you do not have the right storage place, skip digging and storing, and just start over by buying new tubers in the spring.
There are about 60,000 named varieties and 18 official flower forms including cactus, peony, anemone, stellar, collarette, and waterlily. The American Dahlia Society recognizes 15 different colors and color combinations. Here are some popular choices:
Image: Kidd’s Climax. Longwood Gardens
The Dahlia you brought to our isle
Your praises forever shall speak
‘Mid gardens as sweet as your smile
And colour as bright as your cheek.
–Lord Holland (1773–1840)
I'm HOOKED on dahlias. I keep perusing the on line catalogs and I've picked up quite a "starter" sampler.
My big worry: dahlias are relatively late-season. What does everyone do for late spring/early summer color? I don't want to crowd the dahlias as they are growing. What's my zone 6 best bet for companion plantings??
My husband does pretty much the gardening some things we have to hire help that he can no longer do. But we grow a lot of what could be Cottage Garden Beds ! We enjoyed growing Sunflowers, Marigolds and Zinnia with our Dahlias . We also had vines growing behind them up and over Morning Glories, Moon Flower , Red Runner, The Purple bean one forget the name. . In front we had a small circle garden with a yukka plant and low ground blooming annuals. Off not far we had another large round garden bed they all complemented each other ! We had lots of flowers year round here in Florida and it was a lot of fun :))
Violet, do you have a picture? It sounds beautiful. The purple one is the Hyacinth Bean, I believe.
I am basically what you'd call a cottage gardener myself, however, my problem is TIMING. I find that my sunflowers bloom at the same time the dahlias do. And I ONCE planted a Morning Glory it took over. It re-seeds every year and attacks my rose arbor, forsythia, and lilacs. I think it's a different kettle of fish here in New England. We get this flush of early color (peonies, lilacs, iris, allium) then there is a lull, then sunflowers, cosmos, dahlias, etc. SO MUCH depends on how much sun versus how much rain. It is really frustrating! Roses can bloom at any point, but if it's been hot and dry, fuggetaboutit!
I've been searching for a picture as it was a couple years ago and we sold that home and moved down the street. When you cam out our front there was small porch ramp and roof over hang what ever this is called. Along the front going to the side L shape was vines . They went up the poles and my husband hung fish wire where ever to bring them up and across. We just let everything grow ! I did fine this year with the more rare Morning Glories we had less foliage and more flowers and many or the flowers were so large and bright ! I think they were the ones from Japan. Having different vines together gave different flowers all the times. Any type of vines can be used for a back ground of flowerings . Ours were going up and across with the fish line .
Our Zinnias and marigolds bloomed non-stop. We had different sizes , colors and types. As the blooms died we would ead head let dry and plant the seeds right back again. I did this all around a few of our garden beds as they can help keep pest away and always had flower blooming !
Then any lower or ground covering annual or premarital can work. Lot's of time we would go to lowes and just pick out all kinds of flowers plants ! Choose plants with colors you enjoy , ones that will bloom from Early Spring threw summer maybe even to frost.
We planted multi head sunflowers that were out of this world. I did start some inside, We had several different colors and heights.
We had a long garden bed this one was long about 25 feet.3 ft out At the end we planted a Mexican Sunflower stick my sister bought us for a dollar!. It grew taller than our home and wide and it bloomed all the time too. People would stop and comment all the time. It was fun !
early Glads will be nice too ! We like to try a few different plants into gether cant go wrong this way !
We both lives up North all our life. I grew up Mostly in RI and MA . The growing seasons are different. We grow everything here in Florida ever plants people say can't be grown here !
Hope you can find a nice combination !
I'll look for pictures most we have on CDs are single flowers shots. But I know there are some larger ones around here some where !
Personally I don't combine them with anything else. I can see your point though. I have many other beds that have stuff blooming in them at other times to help. I guess a few of the points would be, the beds that the dahlias grow in I like to fully work up before and after planting for the year, and to be able to plant other annual or liftable times at the same time as the dahlias but have them bloom before is tricky. I have also started a lot of my dahlias early inside and had them blooming in July.
So I wasn't much hlep. Maybe to work out your cottage style you could have smaller groupings mixed in with perennial or shrubs or something instead of having just big dahlia beds(like I like).
this is a bed at my neighbors I have which has dahlias,perennials and a few shrubs
I like that garden ! and the home back ground colors it so pretty too !
I like gardens like that more than one plant ! Makes nice for people like us with little room and many likes !
That bed turned out pretty good in it's first year. The dahlias are alone in the middle so my theory works fine there with the seperation. There are some plants like the bleeding hearts and lupine that flowered before them and the hydrangea and knautia continues afterwards.
Cottage gardens get so weedy, it's tough. Al, that's a great idea, starting some early. I am a bit cramped for indoor space. Would a one-gallon plastic bag w/ damp medium be OK? I could put them in a big plastic Rubbermaid tub and mist them, so they won't get soggy. I've started some seeds/bulbs this way I clamp flourescent spotlights on the sides of the container.
I have 2 acres of land, which includes a 1/2 of a pond and plenty of trees. I have LOTS of shade, so I am a big-time hosta collector. After my experience last season, it seems Dahlias aren't nearly as fussy about having full sun all day, every day. I have LOTS of room, so storage may become a big problem some day. I get addicted to collecting, and I already have about 55 named dahlia tubers in the basement. That's 55 varieties, at least 100 tubers.
I am going to have the fattest slugs in the country.
I am all gung-ho for planting now, but I am going to hate myself when I have to dig 'em all up next fall.
I was going to offer the suggestion of putting Trumpet lilies behind the dahlias -- no matter how tall the dahlias gets, the lilies are still taller! BUT they need fuill sun, so as I read down the thread I'l have to recant the suggestion, at least in your case (part shade)
I'm growing my dahlias next year behind, and in front, of some roses, depending on height. These are sort of landscape roses withthe heaviest flush in June.
Sounds real pretty Suzy ! UT will be fun seeing everyones gardens !
Illoquin, I had to give up on lilies. That horrid little red lily beetle has set up World Headquarters in my garden. I moved to this house 15 years ago and grew Asiatics, Trumpets, and Tiger lilies. After about 5 years I saw one red beetle.. the next year, about 6, and had damage the next year I only got 3 or 4 blooms before they were eaten, and now I can't grow anything in the lily family. Between friendly deer, Japanese beetles, woodchuck, moles, and an overly enthusiastic Helper Kitty, I'm lucky if I can grow mold at this point.
One of the aformentioned critters loves my Hollyhocks. They mow them down like a weed-whacker. I wonder who it is. I went to a great deal of trouble to buy seeds from especially nice varieties and start them indoors. They grow beautiful foliage, but they never get higher that 2 feet due to someone eating them. I should post this on the pest board, but maybe I should just use hot pepper spray.
Lilies can take part sun. I have a large group of trumpets in front of the house which faces west. It gets afternoon sun for only a few hours now since the street trees have grown over the years. These trumpets are several years old so they grow to at least 5 - 6 ft tall. In sun or shade they always list sideway since the flower head is so large. I have a lot of stuffs growing in front of them, so it's not a problem.
I think I've solved my own problem. My full-sun bed is 40 feet long and 20 feet deep. It is a raised bed with a fieldstone retaining wall. In the front row is a hedge of "The Fairy" rose. I was going to start the dahlias after the roses, but I think I'll move the low-growing perennials up behind the roses, and put the dahlias behind them. I can also squeak in coleus, which has showy foliage all year.
P. S. I also ordered some glads to sort of "punctuate" the dahlias they have such a radically different habit that I think it will make a nice display, but clearly I have to devise some sort of elaborate support system.
Jax, I just have to say " You've got it bad!" I mean Dahlia Fever. You may as well rip up your roses now and just get on with dahlias. How can you say no to one? They each offer their own superb highlights: sheer size- humongous or petite, radical or delicate coloration, fascinating petal formation, determination to grow and bloom despite our efforts to thwart them! Why, I can get lost in one bloom for minutes at a time. I'm a goner.
But, to go back to your question: I have interspersed glads, mums, zinnias, liatris, asiatic lilies, trees LOL, catmint, rudbeckia, peonies, and low-lyers like lamium among the dahlias as long as they all get a fair shot at the sunshine they like, they do well. I like the verticals of liatris and glads- especially pretty in a vase with dahlias. I gave my roses away several years ago: way too fussy out here where Blackspot abounds.
40x20 sounds like dahlia heaven to me- not too big, not too small. You have a lot of deciding to do for your collection!
My dahlias go in back of daylilies and in back of some of the dahlias are Orientals or other tall lilies. I'm hoping the foliage from the dahlias will hide the lily stalks. I have a new bed with Phormium (has to be brought in for the winter) in front of where the one dahlia will go.
In another spot there's a short trellis "fence" and that's backed up by tall white phlox, David, and the dahlia will go on the other side of that fence so the background will be the phlox.
Last year I tried three dahlias in obviously more shade than what they'd like. They bloomed but not a lot.
I enjoyed planting them around the vegetable garden as the rails were used to hold the dahlias as support. If I could eliminate 40 or 50 daylilies I'd be able to surround the vegetable garden!
I've read so much about various support systems, it's scary. I've heard "rebar" thrown around on this board- yikes. It sounds like dahlias are the Brooklyn bridge or something. I bought 5 wooden birdhouses this year, and 5 six-foot dowels to mount them on, plus spikes. I'm hoping to stagger the 5 poles through the midst of my dahlia patch, and then run wire from pole to pole. Then I can plant around the supports. My brother gave me 3 steel spikes with finials I can add those, too. My problem is how to attach wire to each pole so it doesn't slip down.
But I love my rebar!
My only knock on it is the price, but it's an investment. Wood dowels will work if they are 3/4-1" thick, you can't underestimate the weight -especially when wet and windy.
Yep, I love my rebars too. I don't mind the price tho. I get a 20' long one, have the lumber store cut into either 4' or 5' long for like $5. Isn't too bad if you buy a few at a time.
But, you have to paint the rebar. How do they cut it? I have lots of tools.
What about the green steel garden fencing posts? They are designed with fins on the bottom to hold steady, and little "spines" on the side to hold up the green "Yard Guard" fencing.
Nope, I don't paint mine. Within time, they are covered up with the plant. I know other people paint them like Al here.
Yep, those fence post work but are much stronger than you really need.
My main problem is New England "soil". Although I've added truckloads of compost to the garden, the rocks keep percolating up. Fun trying to drive stakes. I think I'll drive 5-7 stakes and run the wire fencing in 4 foot lengths between them. Then I can plant the tubers along the fencing! Woo-hoo! I'm rarin' to go!
Carol - what dahlia people buy a few of anything?LOL
I'm sold on rebar too. Sturdy, reliable. They may rust, but won't rot like the 150 wood stakes I have :((((((( LOL I spray painted mine green, but not like Al who painted his like Picasso! Not one plant that was staked with rebar fell over last year.
You can cut it with a hacksaw very easily. A very worthwhile investment and if you don't mind the gardenful of stakes for a couple months, they disappear in the foliage for the rest of the season as Carol said.
The green metal fence posts with flanges/wings are also very sturdy and have a little hole on top for a tag- how convenient. I have any number of those too. Two years ago I did have a couple instances where the tuber clump grew around the flanges so there was some damage to new tubers when pulling the stakes out, but this year I did not run into that problem. I tried to plant the tubers a little bit further away from the stake and that worked. Someone also said they always dig the tubers first, then pull the stake.
If you make a wired support system as you said, that should work just fine. A mature 4 ft dahlia can get to weigh a lot, so I bet several heights of wire would offer you good support against wet and wind.
Dahlias and vegetable gardens have gone hand-in-hand for many years and the bright, bold blooms of these tuberous rooted perennials still have much to add to our gardens today. Their flowers are a late summer and autumn beacon for honey bees and beneficial predatory insects. In addition, their roots exude chemicals that are said to suppress and deter microscopic worms that attack vegetable plants. The sky is the limit when it comes to colour – dahlias come in yellows, oranges, pinks, reds, purples, lilac, white, bronze and multi-coloured. There are 10 recognised groups that describe the differing dahlia flower shapes. You can choose from flowers that are simple and open faced, double flowers – with layers of petals, tight and compact rounded flowers and bizarre cactus-like flowers that are spiny-looking and complex – almost other-worldly. The size of flowers ranges from the diminutive ‘lolipop’ to the huge ‘dinner plate’. Dahlias are so varied and at times dramatic they can become a bit of an obsession – you have been warned!
Dahlias are planted as tubers. Tubers are effectively a bunch of fat finger-like roots with an upward-pointing growing stem in the middle. Tubers can be ordered from suppliers in the autumn or purchased from garden centres in the spring.
Companions Plant with cleome, echinacea. sun flowers.
Plant tubers in early spring and summer countrywide when all risk of frosts has passed.
Dahlias grow best in full sun but if all you have is partial shade then they are still worth a go. They can be planted around the fringes of your vegetable garden as well as sporadically amongst beds where space permits. Smaller growing varieties can also be grown in pots. Plants can tend to have brittle stems so shelter from wind where possible.
Dahlias prefer a reasonably rich, well-drained soil. They do grow on most soils, however their tubers may rot in persistently damp and poorly-drained soils. If your soil is sticky then dig in coarse sand or fine pumice and organic compost to help improve drainage and soil structure.
Before planting dig a hole the size of a bucket and fill with well-rotted compost, rotted manure and some coarse sand or fine pumice to help with drainage. Mix together with garden soil. This can be done in the autumn to allow for ingredients to settle, combine and integrate with surrounding soil.
If you are in a cooler part of the country you can plant your tubers in pots of compost several weeks before you expect conditions to become warm and favourable for planting. This encourages tubers to start to shoot early – enabling you to get a head start on the growing season when they will be planted out in the garden.
When the weather has settled and is reliably warm and sunny, plant tubers at an average spacing of an arm’s length apart – this will vary from variety to variety so check growing dimensions for the varieties you choose.
First dig a hand deep hole in your pre-prepared planting spot. Make a shallow mound in the centre of your hole and sit the tuber on it with the upward-pointing stem in the middle and the fingers (if there are more than one) of the tuber radiating around it.
Cover the tuber with a thumb-deep layer of soil. Don’t water tubers at time of planting – this can cause them to rot. Now is the time to drive in a strong stake just outside the area of the tuber if your dahlia grows to more than knee height. (if you do this later when the plant has grown you might damage the tuber by accidentally driving a stake through it).
When shoots start to appear through the covering layer of soil you can start watering and then cover again with another thumb-deep layer and repeat until soil meets ground level.
Pinching out the growing tip when centre stem has three pairs of leaves will encourage branching and therefore more flowers.
Water once or twice per week during dry periods. If plants grow well they will need tying in to stakes for support in windy areas. Tie your string tightly to the stake and loosely around the dahlia stems – to prevent constriction and damage. Mulch around the base of your plants to keep soil cool and moist and to suppress weeds.
Flowers can be cut for use indoors. Use secateurs or a sharp knife to cut them off. Plunge straight into warm water before adding to your flower arrangement.
Text and Photos © 1995-2000 by Barbara Jenke (Do not use for any commercial use without the permission of the author) Hot Springs, South Dakota
When I first started to grow dahlias in containers, I was frustrated because I could not find anything written in the general gardening magazines or books on this subject. The first years I grew only those dahlias listed as “dwarf” or “low growing.” One year I realized that if I could grow a dwarf tree or bush in a tub, I could cultivate any height of dahlia in a container. A dwarf variety may be a bit easier to handle, but raising 4 feet or taller dahlia bushes is just as easy, and could make a nice privacy hedge on a patio or balcony. Different heights of dahlias also add variations of elevations to a deck garden as well as adding different colors and textures from the flowers’ different forms and sizes.
Over the years I’ve had to deal with a short summer growing season with gentle-or monsoon- type rains summers of heat and drought with or without muggy humidity or the coolest, wettest foggiest conditions still air to refreshing breezes to wind gusts of 50-60+ MPH cool days with cumulus clouded skies or bright sunny hot days thunderstorms electrical storms or hailstorms. Through these schizophrenic weather conditions I’ve experimented with different potting soils, fertilizers, staking, how to start the tubers without having them rot, etc. It’s been a challenge, but I’ve developed a method of planting dahlias in containers which has been successful for me.
By starting the tubers indoors during the first week in April, I have had some early varieties start to flower by the last week in June. Because of weather conditions, most dahlia suppliers don’t send the tubers until at least the first week in April. I probably could start my over wintered tubers earlier than April, but I am usually too busy sowing seeds and transplanting seedlings of other annuals and perennials.
When I get the tubers out of storage or when they arrive in the mail, I inspect them for rot and look for good eyes. As I check each one against its invoice and write down any bonus tuber I may have been sent, the tuber is laid horizontally in trays with the eyes (or any growth which has already begun) facing up. I make sure that each tuber is identified either by having its name written on it or making a plant label for it and laying the tuber on the label. Tubers which look like they have no eyes or damaged eyes(sometimes stems break off in shipment) are placed in individual small trays on moist potting soil. This way I can watch them closely for any signs of sprouting and don’t waste time and energy planting a tuber which won’t sprout a new stem.
The tubers should be planted before the roots start to grow and get entangled in the flats. Separating the roots damages them. While they are forming new root hairs, it delays the plants’ upward growth and first bloom date.
I use heavy-weight, sturdy, plastic pots in which I can drill holes for the later described stake inserting procedure. The diameters of pots I usually use are 8 1/2-inches and 10 1/2-inches. I have a few 11-inch and 12-inch pots for planting really long tubers. For base stability, I prefer the azalea style of pot, meaning that a pot has a short height in relation to its diameter which makes it look short and squat, as opposed to a tall, skinny looking container. I do not match pot size to the final bloom size, because miniature flowers could be on a tall bush and giant blooms could be on a short plant. I match the pot to how tall the plant could be or the length of the tuber.
Use a clean container! A dirty pot could spread a disease to the plant or have unseen insect eggs in it. Before placing the potting mix in the container, I remove the saucer from the base of the pot. This will allow the excess water to drain and keep the soil from becoming water-logged from the daily drenching rains we can receive. If there are not enough holes or the holes are too small for fast drainage in the base of the container, drill another hole or two in the bottom to enhance drainage.
In the container, if the tuber is placed in the bottom of the pot and then the potting soil is immediately filled in up to 1-inch below the rim and then is thoroughly watered, the tuber could very likely rot. Planted higher up to prevent rotting, the tuber would become exposed to the surface and the base of the stalk would be sitting on the surface of the soil. Then the stalk could the be easily broken off from the tuber. When planted in the ground, directions usually state to dig a hole about five to six inches deep and fill in the hole with soil as the plant grows. I learned the best way to plant the container grown dahlia is to plant the tuber by following the same method: plant the tuber deeply and slowly add more potting soil as the plant grows.
The potting mix I use is a coarse soilless mix, Ball’s Growing Mix #2. It is a nice loose blend of fine bark, vermiculite, peat, and perlite. I use it for all of my container plants. Grace Sierra has a soilless mix which is like the Ball’s Mix #2 called Metro-Mix 700. I have noticed that garden centers are selling other brands of coarse soilless mixes which dahlias should like. Whatever potting mix you want to use, make sure it is very loose whether wet or dry. Make sure the potting soil does not have clay in it. Dahlias do not like to grow in clay soil. The clay turns the soil into mud when wet or into a block of cement if allowed to dry out.
An important “secret” ingredient I stir into the potting soil when I plant the tuber is a product made of acrylic copolymer crystals. The copolymer crystals absorb water and release it to the roots of the plant when the soil dries out. They protect the plant from being overwatered or dying in dry soil. The products I have used are TerraSorb™ and Soil Moist™. They provide the added benefit of keeping the roots cool during a hot day, and from keeping the soil (and tubers) from freezing when the frost kills the top of the plants in the fall. I use the copolymer crystals in all my containers of non-cactus plants. What’s nice is that if I don’t have time to water on a hot day, I don’t have to worry about the plant dying.
The copolymer crystals are easier to work with in their reconstituted form (slurry). Use the amounts for each pot diameter as directed on the label. To turn Soil Moist™ crystals into slurry, add 1 cup of warm water to 1 teaspoon of the crystals and wait about 5 minutes. The slurry looks like little gelatin-looking globs. I like to make a batch at time in a 3 quart container. Left over slurry can be covered and stored for later use or allowed to dry out and be stored for reconstituting at a later time. Please watch out when used around children or pets! It is also very slippery when wet!
Planting the tuber: Place an inexpensive, biodegradable drip-coffee filter, or two if necessary, over the holes on the bottom of the flower container in order to keep the soil in and the sow(or other) bugs out. The roots will appreciate the extra space that pot shards or rocks would have used.
Fill the flower pot 1/3 full with pre-moistened potting soil. In that bottom layer, mix in an amount of copolymer slurry as recommended by the package directions for the pot’s diameter. Try not to pull the coffee filters off the bottom pot holes.(The soil and slurry can be combined outside of the pot and then put back in) Lay the tuber horizontally on top of that layer of mix and slurry. If at all possible, place the tuber so that the eye end will be in the center of the pot. It is okay if this can’t be done. Many times I’ve had to place a 6″inch tuber in an 8 1/2-inch pot. Just be sure to leave about 1/4-inch space between the root (non-eye) end of the tuber and the side of the pot to allow for roots to grow. If a sprout is already growing out of an eye, place the tuber so that the sprout is pointing upwards. Write the variety of plant and what other information you want on a plant label, and insert it in the soil next to the eye end of the tuber. This marks the spot where the stake will go. It also keeps roots from growing in that spot.
Cover the tuber with more pre-moistened soil, but just enough soil to hide it. The eyes may be exposed, if desired, to watch for growth. Using a spray bottle filled with warm water, mist the tuber until the surface is damp. Do NOT fill the container to the top with potting soil at this time. By just covering the tuber, the plant’s growth can be easily watched and prevents overwatering of the awakening tuber.
As the stalk grows, carefully add more potting soil to the container, so as to not break the stalk from the eye of the tuber. Do not cover the upper set of leaves. Do NOT add any more of the copolymer slurry. The gel rises. If it’s put in at higher soil levels, lots of little globs of gel will be sitting on the top of the soil after a heavy rain. Believe me, I know!
If started inside, place the containers under plant lights. The dahlias should have light from above to keep them from bending towards the light of a window and to grow compactly. Set the lights about six inches from the tops of the pots of newly planted tubers. Raise the lights as the plants grow. Suspending the lamps from chains on hooks makes them easier to raise and lower. I use plant gro lights or a combination of two fluorescent lamps: one cool white and one warm white fluorescent tube in a 48-inch two-lamp shop fixture. This provides the proper light spectrum to raise plants without having to pay for the expensive grow lights. I learned this from the Floralight Company when I bought some of their stands. The timers are set at sunrise to sunset times (12-14 hour days) The containers can be set near a south window. Be sure to turn the pots so that stalks will grow straight.
The stake should be inserted in the pot before the plant label is covered by soil additions. Stakes help to prevent the stalks and stems from breaking off in the wind or if/when the pot falls over. Also, when uprighting or moving the pot, the stake can be grabbed onto instead of the plant. I prefer to use steel stakes which are covered in green plastic. Besides being strong and easy to work with, they can be cleaned and disinfected at the end of the growing season for use the next year. (If you can only find bamboo stakes, then try doing what I used to do: for added strength, tie three stakes together with tape, tie wire or string.)
If the average height of the variety is known use that length of stake. If not known, judge what length to use by the height of the trunk when the stake is inserted. Otherwise, use a 4 ft stake, because most dahlia plants seem to have an average height of 4 feet. To keep the stake upright, tie the stake to the container using plant tie-wire (or string, if preferred). Drill 4 holes in an “X” or “+” position (depending on the plant’s growth) in the sides of the pot near the rim. Cut a piece of plant tie-wire at least four inches longer than the diameter of the pot. Fold the wire in half and wrap the middle of the wire around the stake at the same level with the height of the pot’s rim and twist the tie wire to the stake. Remove the plant label which is near the eye end of the tuber and replace it with the stake. Next, thread one wire end through one of the pot’s holes and twist to secure it, then thread the other wire through the opposite hole and pull the wire taught until the stake stands upright next to the stalk of the plant. The base of the stake should touch the bottom of the container. Repeat with the other set of holes. Occasionally, two stakes may be needed if there are two main stalks growing from the tuber. Tie the stalk(s) to the stake(s). Return the plant label to its pot. (To keep the labels from becoming separated from their pots, mainly due to curious baby raccoons, I am going to experiment and use those aluminum name tags which have tie-wires thread through holes in them and attach them to the drilled holes in the pots.)
After being staked, when the plant has grown taller than the top of the container, add the rest of the potting mix to within one-inch of the top rim of the container. It is okay to cover the leaves below the soil line.
When the plants get at least three or four sets of leaves, pinch out the growth tip of the stalk. Pinching helps to make a bushier, sturdier plant. It does not delay the blooming time of the plant, but the plant does make more flowers. When I don’t pinch, I usually get tall, skinny plants. My husband and I like larger as opposed to more flowers so, the side buds get pinched when they start to develop. The miniature flowered buds don’t get pinched.
Watering: While the plant is developing roots, let the soil almost dry out before watering again. The copolymer crystals will prevent the tubers from drying out. If the soil is kept too wet before the roots and top growth get a good start, the tuber may rot. Water the plants after adding more soil to the pot. The city’s water here is hard with a pH about 7.4 and contains lots of calcium sulfate, magnesium, and other minerals. I understand that dahlias like a soil towards the alkaline side, so this water’s pH does not seem to bother them (they grow and bloom well!).
The indoor water goes through a water softener which replaces the calcium with a sodium salt. While they are inside, all my plants get watered with softened tap water. I use tepid water because the cold water that comes from our faucets is really cold. The softened water does not seem to harm any of my plants. While inside the pots will need to be placed on a saucer to protect the floor. But do not attach the saucer to the pot.
Outside, all the container plants get watered daily either from rain showers or the hard unsoftened city water directly from the garden hose. If the the soil is moist one inch down from the surface, do not give the plant any water. Because of the copolymer crystals, on cool days plants may not need to be watered. In August, the roots have usually filled the pots, so in the evening after a day in the 90’s or more, I check the soil in the containers to see if they need more water.
On hot, dry, sunny days, I will take the garden hose and mist the plants and the deck, so that the evaporation of the water will help cool the plants. I will do this in the hot mid-day sun if they look like they need it. Contrary to some people’s beliefs, the drops of water on the plants will not leave little magnification burn marks on them. Don’t you feel cooler after getting sprayed by a sprinkler? My fertilizer of choice is Ra·pid·Gro Bloom Builder (19-24-18 with micro nutrients). I’ve tried other formulas with the soilless mix, but so far, prefer this one. I like the foliage, blooms and good tuber production I get with this formula. The plants get fed every 7-10 days. I mix the fertilizer and water in a watering can and pour it into the individual containers until the liquid runs out of the bottoms. I stop feeding the plants at the end of August, because we usually get our first killing frost in the first weeks of September.
I spray the combination fungicide and pesticide, Orthene III, in the early evening (if the air is still and cool) at the first sign of powdery mildew, spider mites, or thrips. The first time I used it, I thought that I would wake up the next morning to find that all the plants’ leaves were burned or the plants dead. Instead, the plants looked happy and healthy!
The plants get hardened off to the outside weather conditions when the nighttime temperatures rise into the high 40’s F, usually in the first weeks of May. The containers are set on the southeast facing deck under the front porch stairs. At night I cover the plants with a reemay fabric blanket and/or a giant piece of bubble wrap which are both made for covering vegetable gardens. All the pots get taken back inside if there is a frost warning. The first week of June, the sun loving plants get hauled upstairs to the top deck.
Until the end of August, I add more potting soil to the container when the soil level looks like it has dropped, exposing the roots. When the lower leaves start dying, I cut them off. In August, outer green leaves get thinned out to allow the inner branches to receive light so they can grow and make more flowers.
Some dahlias are affected by sudden drops in temperature(within 20 minutes) from the 80’s into the 50’s(F) due to a thunderstorm (with rain or hail)or by an electrical storm (lightning and thunder only). These situations seem to trigger a winter season shut-down even though it’s the middle of the summer. The leaves droop as if the plant doesn’t have enough water, even though a check of the soil proves that it is moist. Let the soil in the pot dry out before watering again. Many times this will get the plant to regenerate and start growing again and flower nicely. Sometimes only the buds will continue to develop and bloom. Sometimes the plant will die.
Keeping a growth record of each dahlia and taking a photograph of the plant and flower is a good idea for future year’s reference. Besides the usual classification data such as bloom type, color and size, I include pot size, height, first bloom date, supplier, and how well it did. I also write a summary on what the season’s growing conditions were like, so that I can see why a plant may not have done very well that summer or did exceptionally well.
The best thing about container gardening is that the plants can be moved! The ones in flower can be turned around or moved to the front for the best flower show, the ones growing too tall go to the back, or the sun stressed ones can be moved to a shadier location. When there’s a hailstorm warning or dark thunderstorm clouds are seen moving in our direction, I move the plants next to the house, under the eaves, until the storm passes or until the next morning. In early September, when there is an early frost or snowfall warning, and we are home, my husband and I make a mad dash out and bring inside the dahlias which are in full bloom or have a lot of promising buds on them. After that first frigid spell, the temperatures usually warm up again the plants go outside and give us another month of beautiful dahlia flowers.
This way of growing container dahlias is not written in stone. This is a starting point to help develop your own system of growing contained dahlias. I saw some very nicely grown container plants when I was at the ADS National Show in Kalispell, Montana, so if you have found a method which works for your dahlias, by all means continue to use it!
This article was originally published in the Bulletin of the American Dahlia Society
Author’s update:If you are growing potted dahlias on a cement patio, put the container in the shade during the hottest part of the day or the plant could get too hot and burn from the reflected heat of the cement. Morning sun is the best, especially, if you live in a hot summer area.
Check the label of ingredients on the potting mix. Some of today’s mixes already have the copolymer crystals added to the mix. If so, do not add any of the crystals to the pot.
Some mixes already have fertilizer added to the mix. While the roots are filling the pot, you do not need to add any more fertilizer, but once the plant starts really growing, it consumes fertilizer rapidly, so you will need to give it additional plant food.
Schultz brand potting mix with fertilizer for Roses is a nice potting mix for dahlias. If you have an Eagle Hardware Store, they have a Cole’s brand planting mix which grows dahlias well, too. I do not recommend Peter’s brand potting mix for dahlias, because it has too much water retaining peat moss and a wetting agent which keeps the soil too wet for the tubers.
PHOTO NO. 1: The potting mix just covered the dahlia tuber when it was placed on the bottom 1/3 mixture of soil and copolymer slurry. The eye of the root was left exposed. The plant has grown beyond the top of the pot and has been staked, so it is ready to be completely filled in with potting soil. The lower leaves will be covered with soil.
PHOTO NO 2: The staked dahlia’s pot has been filled in with potting mix. The top growing tip has been pinched off. The leaves below the soil line were covered over with the mix. The plant is ready to be tied to the stake and then watered.
Hot Springs is located in the southern Black Hills of South Dakota in the southwestern part of the state, and has an altitude of 3800 ft above sea level. Please refer to Barbara’s Letter to the Editor in the June, 1993, issue of the ADS The Bulletin. Photos of the deck gardens appear in that issue and the September, 1994, bulletin.