Dahlias are the most gorgeous flowers that really hit their stride in late summer, making them the queen of early autumn, when most other flowers are beginning to pitter out.
If you haven’t done so already, there’s still time to purchase tubers (the thing dahlias are most commonly grown from)! And if you ordered them over the winter, you should start receiving them in the mail right around now. However….
don’t be tempted to plant them outdoors just yet!
Here in central Virginia, the rule of thumb for planting dahlia tubers outdoors is to wait until the last frost - or, if you want to go old school, until the tops of the mountains are green. So that typically means the last week of April. Or, the first week of May if you want to be extra careful.
That’s because dahlias are fairly quick to sprout once planted out, and they’re also very sensitive to cold weather. So, if you were to get the early spring bug and put them outdoors too soon, it’s very possible that a random cold night could kill the baby sprout and leave you without a dahlia.
That being said, just because you can’t put them outdoors just yet doesn’t mean you can’t start them indoors! Tubers are no different than seeds in that way: giving them warmth and sunlight (window light or artificial grow lights) indoors is a great way to give them a head start, so you’ll have a baby plant to put outdoors when all risk of frost has passed, instead of putting the tuber directly into the ground and waiting for it grow there.
All you’ll need is a 1-2 gallon pot, potting mix, a little bit of grit to help with drainage (which dahlias appreciate), and the tuber!
In case you’ve never grown a dahlia before, below is a picture of a tuber. Sometimes you’ll receive only have one little potato looking thing (dahlia speciality shops such as Swan Dahlias and other small farms will most likely send them this way), and other times you’ll have many small potato like things all connected together (most likely found in garden shops). The tuber pictured below is one I received that had even more pieces than shown here, but many were dangling by a thread. So, I cut those off and only left the strongest ones.
If you look extra closely on the left side of the tuber, you’ll see two tiny white dots. Those are called eyes, and it’s what is proof that this tuber is viable and will grow into a baby plant (or two in this case, as there are two eyes). If I wanted to take the time to carefully cut those two tubers apart, I could separate them and plant in different pots (or holes outdoors). For the sake of time, I’m okay with just putting this size of tuber into the ground and receiving two plants in one spot.
Now, to start this tuber early indoors, you’ll simply fill the pot up with compost about 3/4 of the way up, place the tuber on top, and put only about 1-2 inches of compost on top. Dahlias like to be planted shallowly! If your tuber has the leftover stem from the previous plant still on it (like that one does above : it’s the upright piece), then it should be sticking out of the compost a bit, as pictured below.
If you’re putting your pots in an unheated greenhouse outdoors like I do, and slugs have the potential to show up (which they always seem to do when they sense dahlias are nearby), adding crushed seashells or grit at the top can deter them from getting to the baby sprout once it arrives.
One more important thing to note: though a little misting won’t hurt, it’s best to not fully water the tuber until you see a sprout appear. This helps avoid potential tuber rot, and I promise that the sun and warmth will trigger the growth! However, once your baby plant appears, keep the soil moist (but not too wet).
Then, when the last frost has come and gone, find a sunny, weed free spot in your garden to plant it out! Or, an extra large pot on a sunny patio can suffice for its home too. All you’ll need to do is carefully lift the tuber with the baby plant on it and shallowly plant it into its new home.
Hope that’s helpful! I’ll be sure to share more on caring for your dahlia in future posts this summer.
