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How to pot up dahlias for a jump on summer


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.

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.

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.

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!

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