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.
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| ‘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.
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| ‘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.
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| 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!
Oh my goodness, you do have a dahlia problem, ahem, I mean collection. Not a ad one to have I might add.
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!
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?
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!
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.
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.
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.
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!
Thank you so much for your detailed response! I will definitely try this.
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, :))
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!
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?
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!
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.
Thanks for your advice!
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.
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!