Dahlias are the queen of the cut flower growing world, but they can be tricky to grow and care for if you garden in a cold climate and short growing season. By the time they’ve grown and started blooming, the growing season is almost over and you might not even get to enjoy your blooms before the first frost.
I’ve been growing dahlias for cut flowers in Zone 3, which means I only have between 90 and 110 days to work with every year. In this post, I’ll share how to start your dahlias so you actually get to enjoy the blooms and how to care for them, and then I’ll answer some of the most common dahlia growing questions.
P.S. I’m assuming that you’re wanting to grow dahlias for cut flowers and not to put in pots. While you can grow cut flower dahlia varieties in pots, it’s better to grow them in the ground or in a raised bed.
Are dahlias easy to grow?
Yes and no. The actual growing once you put them in the ground is easy. The hard part comes when the season is over and it’s time to dig them up, divide them, and store them over the winter.
In a short growing season, the biggest challenge is to get enough blooms before the first frost comes. To combat this, I take all of my tubers out of storage in mid-March or early April and set them in front of a window or under a grow light. They don’t need to be watered or anything, but exposing them to light will help them to start growing.
You don’t HAVE to do this step (especially if you have a longer growing season), but it really does help. I’ve planted many a dahlia on the late side, only to be stuck with a cool summer and have only one or two blooms. Not fun!
Don’t you need soil to start dahlias?
Surprisingly… no. However, once the dahlias start to develop buds I will transfer them to soil. I start my dahlias this way to save both space and soil. If I feel that the dahlia tubers are starting to dry out, I will mist them with water or transfer them to soil.
Once all risk of frost has passed (usually a week or two after your last frost date), it’s time to plant the dahlia tubers in the ground. Dahlias prefer full sun, so plant them in an area that gets at least 8 hours of sun a day. I have had success planting them in an area that only got 5-6 hours of direct sun a day, but I didn’t get as many blooms.
Plant them in well-drained soil. The tubers can rot if they don’t get enough drainage. Water them regularly and fertilize them with a water-soluble fertilizer every two weeks (or sprinkle on a slow-release fertilizer at the beginning of the season and forget about it). I like to spray them with fish emulsion every week.
Once you start to get blooms, keep cutting them to encourage even more growth. Cut the stems as long as possible (even if you don’t need them that long), so that you don’t end up with a bunch of weird, short stems.
Dahlias are very sensitive to frost and will die if they freeze. Once your first frost hits, your dahlia season will be over, unless you cover them to try and extend the season. Once they freeze, cut off the flower and dig out the tubers. Take off as much soil as possible, label them, and store them over the winter.
Where do you buy dahlia tubers?
There are many great places to get dahlia tubers from, it just depends on what kinds of tubers you want. If you want the prettiest kinds that all the flower farmers grow, you’ll want to head to this post to get a list.
However, I will warn you that the price of dahlia tubers has gone up significantly over the years. While they were never cheap, some varieties are now going for $20 a tuber… *cough* Peaches and Cream *cough*
An average tuber now costs around $10-$15, with some of the cheaper varieties around $7.
What Should I Know Before Buying Dahlia Tubers?
When buying dahlia tubers, please remember that you are buying a living product and that while sellers do their best to ensure the quality of the tubers they sell, they can not guarantee that the tuber will live. I find that it’s normal to lose around 10% of my new tubers–sometimes more, sometimes less.
If tubers are mushy when they arrive, then please contact your seller so they can make it right. If tubers are small, or they don’t have eyes immediately, that’s not cause for concern. Dahlia tubers come in all shapes and sizes and can take up to two months out of storage to develop eyes.
Avoiding Crown Gall and Leafy Gall
Crown gall and leafy gall have become very problematic in recent times, and once they enter your soil, your soil becomes infected and the infection will not leave for YEARS. The best way to keep gall out of your garden is to buy Canadian (or American, depending where you live) grown tubers instead of Dutch grown tubers.
However, buying from North American sources doesn’t guarantee that you won’t get gall. Many dahlia sellers are desperately trying to build up their stock to meet increased demand, and sell Dutch-grown tubers in the meantime. If you know a tuber is Dutch, it is best to grow it in a pot for two years, and keep it separate in storage. After that, if no gall appears, you can assume the tuber is disease-free.
As far as I know, if you buy a tuber from a big-box store, it is reasonable to assume that it is a Dutch tuber.
It’s also best practice to clean your tools often when cutting dahlias, and especially between tubers when dividing tubers. If you discover gall in your dahlias, immediately throw the diseased plant in the garbage–not the compost–and disinfect any tools that touched it.
Can you leave dahlias in the ground over winter?
Unfortunately, no, unless you live in Zone 8 or higher. In that case, cut them off, top them with a thick, dry mulch, and forget about them.
As for the rest of us, if you do not dig them out in the fall they will not come back in the spring. They will simply turn to mush and become compost for the soil. Frankly, dahlias are too pretty and too expensive to succumb to this fate!
If you desperately want to try and tempt fate, maybe put a whole bale worth of straw over them? However, I’m really skeptical that that will work in a -40 winter. If you’ve tried this and it actually worked, let me know in the comments!
Broken, Shrivelled, and Dead Dahlia Tubers
No matter how well you try and store your dahlias over the winter, you will likely end up with some broken, shrivelled, mouldy, or dead dahlia tubers. Some can be salvaged, and others can not.
Broken dahlia tubers are the best candidates for actually surviving. As long as they have an eye on them, they should bloom that year. If a tuber breaks, let it dry until it has a bit of a scab over the broken part, then try planting it in the spring.
Mouldy dahlias tubers are generally not worth planting, and will just eventually rot. Put all mouldy tubers into the compost.
Shrivelled dahlia tubers happen when the area you store them in is too warm. If the tuber feels papery and hollow, it has shrivelled too much and is dead. If it still has something to it, it can still possibly grow and thrive.
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