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How to pot up dahlias for a jump on summer

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It should come as no surprise to anyone who reads this blog or follows me on Facebook or Instagram that I have a dahlia addiction. This has been an ongoing problem and my collection seems to keep growing (although I continue to play with different methods of saving tubers from year to year and have yet to hit on one I am happy with).

I garden in zone 5b, but there’s a lot more to it than than. Because we are about 500 feet from Lake Michigan, it is very slow to warm up here. In fact, it was a gorgeous weekend, but each morning at my house was foggy and overcast and it wasn’t until later in the day that the sun was shining. On mornings like that, when I drive up the hill near our house, less than a quarter-mile away, every window in the car fogs up and I have to pull over to open up the windows and turn on the defrost until the car acclimates to the 20-degree (or more) temperature change). The air temperature at the top of the hill is about 70 degrees. The air temperature at the house struggles to reach 50 degrees, thanks to the influence of the enormous 40-degree lake we love to look at from the window.

That means that zones that are much cooler than ours are actually much warmer in spring (although we are warmer in fall and winter). It’s a funny little microclimate, and a good reason why you really have to know your own garden and not rely on just the USDA hardiness zone map. But the practical application is that I can’t plant things as early as many of my neighbors to the west and even north can.

‘Cafe au Lait’ dahlias are some of my favorites. They change color throughout the year. By autumn they were all lovely, buff and cream.

And dahlias fall into that category. Dahlia tubers absolutely will not stand cold soil. Cold, wet soil is likely to make them rot in the hole, and spring in my garden adds up to a whole bunch of cold, wet, soil. That leaves me with two options: Wait until the soil is sufficiently warm to plant tubers in the garden, or get a head start on growing dahlias by potting them up for a bit.

There are pros and cons to each method.

The benefits of just waiting until the soil is warm enough to plant tubers directly into the ground probably start with it being easy. You plant them once and forget about it. But there are a lot of cons: You get a later start on the season so blooms will come later; it can be difficult to store tubers properly that long (especially if some that you buy have already sprouted); and it’s easy to forget where you planted them in the garden and you may accidentally dig them up or plant something else too close.

Planting in pots has obvious benefits that counteract most of those cons. You can get tubers out of storage sooner, get plants going so they have a healthy root system and good amount of top growth by the time they can be planted out, and you’ll never forget where you planted one because you can see it. But that all comes at a price. You will need a lot of gallon-sized nursery pots (I save all my nursery pots for planting up dahlias and seedlings but did have to buy some to supplement my collection a few years ago), a lot of potting mix (I probably went through 3 cubic feet potting up my dahlias), and there’s a lot more time in the planting and ongoing tending of the dahlias.

‘Art Deco’ gallery dahlia.

When you’re as in love with dahlias as I am, the choice is pretty clear, but just planting them straight in the ground may work better for people with warmer springs.

You can see some of the dahlias I’m growing this year here.

Dahlia tubers all potted up. And no, I didn’t count them. Some things are better left unknown.

Planting them in pots couldn’t really be simpler. I just put a few inches of potting mix—I never use potting mixes with fertilizer added. It’s like baking with unsalted butter to me; I like to control my ingredients—in the bottom of a gallon-size container, put in the tuber either on its side or pointed upward (you want the “neck” of the tuber where the eyes are pointed up) and then cover it up with more potting mix so the top of the tuber is buried at least a couple inches. I don’t worry about filling the container all the way to the top with potting mix. Some tubers are smaller and don’t need as much.

Then—and this is a step I learned from NOT doing it—label every pot. I know you think you’ll remember that a grouping of pots all has the same kind of dahlia so you’ll just label one and keep those together. That will not happen and you’ll need to plant these well before you see any blooms. I finally broke down and bought a case of cheap plant labels so I will stop scrimping and forgetting what plant is what. Affiliate links at the bottom of the post will point you to those and some of the other products I like for potting dahlias.

Assuming that your potting mix is nicely moist, as it is when you open a fresh bag, there’s no need to water tubers in pots right away. In fact all they need to get growing is warmth, so I just put them in bins or laundry baskets and bring them in the house. When shoots start appearing I’ll water them and gradually move them outside to the temporary greenhouse. If you’re potting up tubers that already have shoots, don’t worry about them too much because they’ll probably be heading off in odd directions. Just plant the tuber as above and cover the shoot, unless it’s heading in the right direction and ends up above the level of the soil. In that case, I water the container and put it in a bright spot, usually in the greenhouse.

Then it’s just a matter of giving your dahlias water and light as they grow until it’s time to plant them out, which is usually the first or second week of June in my area, but depends entirely on the weather. Happy dahlia growing!

50 Responses

  1. Good Afternoon Erin,
    I just found your web site and have been an avid gardener of dahlias and everything else here in the mountains of New Jersey up in the North West Corner of the state. Yes we are part of the Appalachian Trail! If you could pass along to your followers that most potting soils contain peat. In the UK peat will be outlawed for use sometime in 2023. The reason for this is that peat boggs are being decimated and they hold and catch more carbon from the air than a rainforest. It take a year to make one centimeter of peat. The alternative is Coir which is coconut husks. This is not a new growing medium as it has been used in hydroponic tomato farming for years. You can purchase Coir blocks as well as coir seed medalions from any of the big box stores as well as on Prime or other internet garden supply sites. The blocks of Coir are compacted and all you do is add water to expand and use. For seed starting this is a game changer. Seeds sprout in under a week.

    Potting up Dahlias is another matter as I raised 85 pots of dahlias for my daughter’s wedding and then cut and transported them in solution to Maine for the bouquets . I did my very best to work with an Organic Soil, perlite and Coir mix (My own recipe) and fertilized every Friday once the plants were about a foot tall with a concentrated liquid seaweed feed. A tomato feed works just as well. I had massive dinner plates, tight balls and everything inbetween. It is a truly perfect mix.

    I overwintered them in their pots in the garage and have just put them out. We will have 2 nights of 30 degree weather and it just rained buckets but it shouldn’t hurt them as they haven’t sprouted yet and we hit the 50’s in about two days. I keep them on the black tarmack of the driveway to generate heat.

    Hope this all helps anyone!

    1. Susan, I love hearing that you grew dahlias for your daughter’s wedding. I want to do the same for mine, and since it will be a June wedding, I want to grow my dahlias in pots in the house. Do you have any advice for me?

  2. HI Erin, I have a question about putting my dahlias outside. I am in zone 3b -4a (actually ON the line) I have a few Dahlias I potted up a couple weeks ago, then set them down in my cool dry basement. I noticed today that some are peaking out from the soil, my intention was to put them into my little unheated greenhouse (similar to your temp one) when they did this, but wasn’t expecting it to happen so soon. Right now and for the next week, our daytime temps are in the mid to high 50’s and night time in the high 20’s to mid 30’s. Is it to cold to put my Dahlia’s out in the greenhouse? (which gets up to 80+ during the day)

    1. The easiest compromise is to drape some gardeners’ fleece over the pots in the greenhouse on nights where you expect a frost, indeed you can just leave the fleece on if you wish, as the plants will just “think” they are growing in light shade. Its quick and easy to do, doesnt matter if you forget to remove it some days, and will protect your precious shoots until the night temperatures in the greenhouse are safe. Then move the plants outside after the last frost date – but keep the fleece handy in case you get an unseasonal cold night.

  3. Hello Erin/ Hello everyone,

    I am a huge fan of Erin and watch all her videos. Anyway, I am just getting my dahlias out of storage and just received a shipment of new tubers. I plan on potting them up and was glad when my online search brought me here!
    I’m in Maryland, zone 7a and we receive a fair amount of moisture this time of year (March/April 2021), so I will use a fair amount of perlite in my homemade potting mix. Also used is worm castings, peat moss and sand.

    Take care and stay safe,
    AJ’s Green Thumb, LLC

  4. Oh dear. Oh dear. Oh dear. This is the year that I finally took a nose dive into the world of growing dahlias. I ordered thirteen different varieties from Longfield Gardens (with 3 tubers each) and they arrived the other day. Since we’re about four weeks out from our last frost date (zone 7B/8A) I think I’ll get these guys started in pots to give them a head start. Crossing my fingers that I’ll finally have a bounty of beautiful blossoms that I can enjoy. :-). Any other tips I should know about before I plant them outside? I assume I need to amend my clay soil, right?

  5. I live in Michigan, Zone 5, and this was my second year for over-wintering Dahlias and I was finally successful. No loss! I stored them in the large paper ‘yard-waste’ bags filled with perlite to absorb any moisture. They were kept in the garage over winter where the lowest recorded temperature was 40 degrees but average was 50 degrees. All the tubers remained dry and fungus free (even though they were not pre-treated). Some have now started to sprout, but I am going to hold off planting outdoors for a couple more weeks due to unseasonably cold/wet weather. My first attempt to over-winter was almost a complete failure. I filled an extra large cooler with perlite, and several cardboard boxes to store all my tubers. I though the ones in the cooler would have survived over the ones in the boxes due to being insulated. All that did was cause moisture to be trapped and all the tubers rotted. The ones in the cardboard boxes survived. The moisture is more of a culprit than the cold can be. I never replaced the lost tubers last year as I had so many and became so overwhelmed during planting time and digging up before winter. A good lesson learned!

  6. I planted my tubers from last year in pots and gave them inside for now by the window . I read that some people say water them and some , don’t water until it sprouts . Which one is it ?

  7. I’m in Chicago and I started my dahlias indoors at the start of April. They exploded! They’re all over a foot tall. What do I do? I pinched the top but they’re just on steroids, it seems. I think it’s too cold to bring them outside, right? Soil temp is about 50 in my garden. I don’t know what to do… everything I’ve read says that at their size they really should go in the ground already. Help!

    1. Check out Laura from Garden Answer she is amazing. She says to cut the main stem of the dahlia to make it bushier. She also says to wait until they are 12 inches tall. She is on Facebook and Utube.

  8. Hello! Wondering if I can use 8 inch or 6 inch pots if the tubers fit and still have couple inches on each side or if they will just outgrow those sizes too quickly. Thank you!

    1. I was wondering about this too. My dahlias don’t seem to be growing as fast as they should and wonder if the pot was to tight – there was an inch either side of the tuber and I’m wondering about repotting. Can anyone advise?

  9. So I am new at growing dahlias. I got really excited and started them early inside…probable too early (beginning of March). It is now the middle of April and my plants are pretty big… about 2 feet tall not including the pot they are in. I live in Ohio and I can’t plant them outside until about the middle of May. Should I be cutting the plants back? I saw that some people pinch the top growth off to encourage a bushier plant. What should I do? If I need to pinch them back where do I do that at on the plant? They are getting pretty tall and aren’t very bushy… more leggy.
    Thanks!

    1. I watch Laura from Garden Answer she says to cut back the main stem on Dahlias to make them bushier. She says to wait until the plant is 12 inches tall and then cut back the main stem. Check her out on Facebook and utube. She is amazing.

  10. This my first time growing dahlias. I am in 5A, Upstate NY. I have them in pots under a grow light. Today it has been 2 weeks. I have 4 and only “Evangeline” has sprouted. Do other varieties take longer? Anything I can to “motivate” the others?

  11. I think I may have planted tubers outside too soon. Can I replant into containers? You think I have a chance after two weeks in the ground? I live in CT. Thanks.

  12. I have a Dahlia addiction as well! I’ve done it both ways – just putting in the ground – this is usually when I was not organized enough to plan ahead with potting them up first. This year, I’m potting them up. I just received a batch of new tubers this week and one of them is the “Café au Lait. First time I’m growing them. One of my favorite Dahlias is Edge of Gold. It photographs beautifully. I’m having trouble finding it this year though. Looking forward to our flowers! I’ll follow you on Instagram to see the progress. 🙂

  13. Thanks for the great tips for starting dahlias early in pots.
    My question is how do you plant the dahlias in the pots afterwards? Do you bury the tuber deeper? Do you cover any new leaves when burying?

  14. It’s my first year with dahlias and I will be starting them in pots. My problem is I don’t have a greenhouse to move them into. Will setting them out in the sun and bringing them in each night until soil is warm enough work. I am in zone 6.

    1. Yes that will work, but you could also build some kind of temporary structure. All you’d really need is thick plastic supported over something. You could throw a sheet over the top at night if it was getting cold. My temporary greenhouse isn’t really much more than thick plastic over a frame. Good luck!

      1. I started pots inside in early/mid April – taking them out into sun when warm, and bringing them in at night. I think I should have waited till late April/early May, because some of them are nearly 2′ tall already – my question is, are they “sturdy” enough to risk a few cold nights in the ground? Is it just the baby tubers/shoots that hate the cold wet ground? (I don’t usually plant in ground up here in NY till late late May…) I don’t think they will actually die with a few cold nights, but I am wondering if I might “damage” their ability actually bloom? Thanks!

        1. I think they’ll be fine. The foliage is fine right up to a frost (so assuming you’re past that you’re ok on that). It’s the tubers that don’t love cold, wet soil because they can rot. If it’s just a couple days, I think you’re safe.

  15. After the dahlias start sprouting, do you add more dirt to the pots? Also, when you transplant them into your garden, how deep do you plant the corms? If I plant the corms 6 inches deep, the dirt will cover up some of the stalk and leaves – is that okay? THANKS for your advice!

    1. Hi Rachel!
      If you plant them 5-6 inches deep in the pot (or really whatever you can manage depending on the pot/shape of the tuber), I don’t add any more soil to the pots. When I plant them out, I typically plant them at roughly the same depth they were in the pot, but if I have one that is really shallow for some reason, I may plant it a touch lower. It’s OK to bury a small amount of stem, but not leaves.

  16. Thanks for the article. I started 75 Mignon Dahlias, ( from seed ) and was wondering how much potting soil each 1 gallon pot needs. I was thinking half a gallon, and putting leaf mold and soil at the bottom. I imagine the tubers will only be a couple of inches at the end of the year. They are little Dahlias only growing 12 – 18 inches. Any thoughts would be appreciated. ( So very busy at this time of the year, :))

    1. Fyi, I read that chopped leaves and garden debris get too dense and absorbs the moisture, so I used the small size yogurt containers upside down and potting soil. 🙂 I have saved so many over the years! I could have just used half gallon pots but I wanted the height. Just thought id let you know. Happy gardening!

      1. Hi Linda,
        I’ve never grown dahlias from seed, believe it or not, so I’m not well-versed on that process. But I would imagine I would treat them like any other seed, starting them in cells. soil blocks or tray until they are growing and then pot the on. Are you planning on keeping these in the pots or planting them in the garden?

    2. I’m very curious how long it takes to grow dahlias from seed! I’ve never even seen dahlia seed, does it eventually grow into a tuber? And do they bloom for you in a couple of months? Thank u!

      1. I grew one dahlia from seed last spring, planted it out in May and it bloomed in August or September right along with the dahlia tubers I had purchased and planted at the same time. I dug it up in November and it had tubers!

  17. This post was really helpful as it will be my first year growing dahlias and I’m going to have 6 cafe au lait tubers, and 4 watermelon. I live in Niagara Falls, Canada so a similar season as yours, so I think I will try potting them up. I am curious as to how much sun they need indoors in pots as I’m not sure how much room I have by my window for 10 dahlia pots. Thank you!

  18. This post was so helpful. My dahlia tubers are due to arrive mid April. This year, I want to start them in pots because when I start them in the ground here in Zone 6a in NJ, I don’t get blooms until September which is ridiculous. I read that dahlias should be started in a “soilless potting mix”. I’ve asked for this at two garden shops already and in both places, they looked like I had three heads and told me they had never heard of “soilless potting mix”. I also had no luck with googling. I will now start them in regular potting mix (I’m now wondering- is all potting mix soilless?). I was planning to start them in terra-cotta planters. Do you think the plastic garden pots would be better or just different? I could really do either one. Some day when I am not restricted to a tiny plot outside a condo, I will have a dahlia collection to rival yours. They are just so beautiful and I love having vases full of them in every room.

    1. Yep … that’s exactly why I started growing them in pots early. Now I have blooms nearly all summer so it’s well worth the extra work to me.

      Almost anything called “potting mix” will be soilless, so you should be good on that front. As for the pots, you can start them in anything you like. I just like plastic pots because they are light and there’s a fair amount of moving around as the tubers sprout and I have to start moving things out to my temporary greenhouse.

      Good luck! I bet you’ll be pleasantly surprised at how much more you get out of your dahlias by starting them ahead of time in pots.

      1. Thanks, Erin! Yes, even if I had more than just six dahlias, I feel the effort would be well worth it. Thanks so much for sharing your experience. Looking forward to a bumper crop of dahlias this year.

    2. Soilless mix is usually Peat moss w beads of white Perlite or flaky Vermiculite in it and no compost …labelled ‘seed starting mix’ because it’s considered sterile without any soil bacteria. And seeds get all their nutrition at the beginning from within the seed till they have a first set of leaves…but! You can certainly add some good compost to that above combination, just not too much because The advantage of using the Peat with Perlite is that it’s very light and doesn’t get heavy and compacted… so it’s easy for baby rooots to spread out and seeds to poke through the surface… plus it holds water well. I make a giant garbage can of my own every yr. for a fraction of the price and add only about 1/4 of compost after potting up to next size pot after germination…. my Dalia tubers do fine with compost in the seed starting mix because they’re big and retain a lot of their own water unlike a little tiny seed.

  19. I live in southeastern Michigan and fell in love with dahlias last year!!! I ordered a cafe au lait in November and just potted it yesterday. Fingers crossed. I watched several videos. We will see. Looking forward to it. Thanks for your post. Very helpful. I just couldn't stand to wait until our ground is warm enough! It's Michigan, for cripe's sake. It takes forever. LOL!

  20. Your obsession seems to be contagious as I seem to have started a small obsession too. Largely due to seeing your beautiful collection. Last year, I lovingly dug out the tubers, stored them in a straw filled box in the unheated garage and waited. Last week, uncovered a dozen mushy tubers. So I've started again with new ones. Do you start fresh every year with new tubers?

    1. I wrap all of my tubers in Saran wrap after I have washed them off. I remove any soft and broken tubers. I leave them sit for a couple days in a cool, garage. I spray a small amount of Lysol spray on them and let them dry for an hour or two. I then wrap each one individually with a description tag and put in a box. I put 20 or 30 in aq box. I then store them in a consistent 50 degree garage in the dark. So far this has worked the best for me . Done this for 3 years. I also then plant in pots in house with a very soft and fluffy substrate. Doesn’t need to be in a bright room yet. Once greenery is up around an inch or two, I move the ports to a bright room. If it is nice out, I put them out every day and keep watered as necessary. never too wet. Gotta love those Dahlias.!!

  21. This is wonderful! I love dahlias and have never grown them but have recently been contemplating it. I'm a baby beginner gardener in general and think I need to find some sort of dahlias-for-beginners book, but I love hearing about (and seeing) yours!

    1. I have a Dahlia that I planted late last winter, it was dirt cheap. To my surprise, this summer it has blessed me with deep red blooms. I live in SC, so our soil is red, hard clay. The tubers on the Dahlia are long and langley. Is this how they normally grow? Do I cut the tuners back after it finishes blooming?

      1. Do you mean the tubers are long and gangly, or the shoots? Generally, you do not cut back tubers. You divide them in the dormant season. The shoots, on the other hand, are fine to cut. I usually do it once the plant is about a foot high. Also, once the dahlia is blooming, the more you cut the blooms off, the more blooms you’ll have. I never let the flowers start to die off on the plant. I always cut them when they’ve first fully opened. I just moved from CA, where I could keep my dahlias in the ground, to New England, and my yard can be pretty soggy, so I’m going to have to learn some new techniques.
        If your shoots are long and gangly, not strong, then definitely cut them back.

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