Want to know how to plant dahlias and grow these beauties in your garden this year?  Chances are you’ve come across a gorgeous picture of dahlias on Instagram and inadvertently fallen down the dahlia rabbit hole.  We totally get it.  We did that, bought a boatload of tubers, and then hit repeat.  We’re currently twenty-seven varieties in and still looking for new ones to add.  We don’t have a problem, do we?  🙂

Today I’m sharing our best tips for how to plant dahlias.  We aren’t the experts by any means, but we’re happy to share what we’ve learned about growing these beauties in the Midwest.  We’ll talk about the easiest way to plant dahlias, two ways we’ve used to get a jump start on the season, and how to start them from seed.  All of the tips I give today will be from our dahlia growing experience as we learned it in garden zone 5b/6a.

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How to Plant Dahlias from Tubers

We’ve used several methods over the years when it comes to planting dahlia tubers. Choosing a method mostly boils down to how much effort and time you want to put into them and if you want to get earlier blooms.  With the last two methods, while we do get earlier blooms, the dahlias still bloom most prolifically in August and September.

The easy method

I’ll start with the easiest method.  If all you want is low fuss, wait to plant your dahlia tubers until your last frost and the ground has warmed in the spring.  Then dig a hole in the ground and plant your tuber.  Done and dusted.  The downside to waiting until the last frost is that your tubers won’t really start flowering until August and September.

If your tubers have sprouted by the time you’re ready to plant them, simply plant with the stalk facing up.  It’s perfectly fine to lay the tuber horizontally in the hole as long as the eye or stalk is facing up.

Cold soil that doesn’t drain freely is a recipe for rotten tubers and all your dahlia dreams dying an early death.  For success with this planting method, wait until the ground is at least sixty degrees Fahrenheit before planting.  Then wait to water them until they begin to show green leaves.

Pot them up for earlier blooms

If you’re impatient for dahlia blooms, you can pot them up three to four weeks before your last frost.  We’ve gotten blooms as early as the end of June by using this method.  Not sure when your last frost is?  The National Gardening Association can help you find that.  Pots can dry out quickly so make sure to water them consistently if they need it, but don’t overwater since the tubers can rot.  You’re aiming for lightly moist soil. Again, too much water before they’re fully leafed out will most likely result in the tubers rotting.

Dahlias can’t take frost or cold weather.  If you don’t have a greenhouse this method will take a bit more work. You’ll need to move your dahlias out into the sunshine in the morning and then indoors every evening until the weather warms.  Cold, rainy spring weather can be a challenge for this method since dahlias like soil temps around sixty degrees Fahrenheit.

After the soil has warmed and the danger of frost is past, plant them like you would another potted flower.  Be as careful as you can while taking the dahlias from the pots to plant in the ground.  The stem’s connection to the tuber can be fragile and dahlias really don’t like their roots disturbed.

Micro tunnels for earlier blooms

Another method we’ve used to get earlier blooms is to plant them out under micro tunnels about 3-4 weeks before the last frost.  We used 18″ hoops and clear greenhouse plastic to make the tunnels.  The concept is similar to our low tunnel for ranunculus, just a lot smaller.  Several weeks before we planted our dahlias, we covered the beds with our micro tunnels to warm the soil.  This also kept the the beds from getting too waterlogged.  Once the soil warmed, we planted all our tubers under those tunnels.  The tunnels did get snowed on that spring but the dahlias were unharmed.

This method took a fair amount of work since we had about nine of these micro tunnels that were 25′ long.  Whenever the sun shone brightly that April, we had to pull the plastic off all those beds so as not to scorch the dahlias.  That usually meant uncovering them in the morning and closing them back up again at night.  But the work paid off because we had early blooms and the plants were able to get established before the summer got too hot.

You’ll want to have drip irrigation in place for this method.  Since the rows were covered in plastic for most of those wonderful spring rains, the ground will be a bit drier than typical when the dahlias start growing.  Once the leaves start showing make sure they get adequate water.  Insufficient water stresses the plants and will encourage bugs and disease.

In summary…

Each of these methods has it’s own merits.  The first method of simply planting the dahlia tubers is the least work of all.  Potting them up is a bit more work, but it’s also easier to keep them from overheating than if they’re planted in a micro tunnel.  With the micro tunnels we had the benefit of getting them planted early before that crazy planting time in May.  Plus the roots weren’t disturbed by potting them and then planting them.

How to Plant Dahlias from Seed

First up, growing dahlias from seed means they will most likely not come true to the parent plant.  If you want to grow a certain variety, stick to growing tubers.  But if you want to experiment and possibly find new varieties, seeds are a good way to do that.  To get dahlia seeds, you can either save seed heads from your own dahlias or order them.  Johnny’s seeds has some dahlia seeds for sale if you don’t have any saved seeds and you’d like to try them this year.

Tools and supplies

When starting dahlias from seed, I use a good soilless potting mix and plant the seeds into 1020 trays with inserts.  You’ll also want a humidity dome of some sort.  You can probably start them on a windowsill.  Dahlias do like a bit of warmth, though, so I’ve had good success with growing them under grow lights.

Sowing the seeds

Sow your dahlias seeds and cover to 1/4″ with soil or vermiculite.  Cover the tray with a humidity dome and place under lights until at least fifty percent have germinated.  Then remove the dome and grow on until you’re ready to harden them off and plant in the garden.  If any seedlings lack vigor or look stunted discard them.  Not every seed will produce a healthy vigorous plant, and culling the duds is necessary.  Tubers will start forming pretty soon so you may need to move them up to larger pots to keep them from getting root bound.

How To Plant Dahlias in the Garden

You’ve chosen your planting method and now you want to get them planted in the garden.  Here are a few tips on spacing and general good practices.

Bed spacing

We plant our dahlias in a single row in 24″ wide beds.  The pathways between the beds are 3′ wide and get covered with landscape fabric to suppress weeds.  This helps with several things.  The wide paths allow us to still get through when the dahlias are big and spilling everywhere.  Planting them in single rows also helps with air flow around the dahlias and can help with preventing disease.

Dahlia spacing

When digging the hole to plant, we try to plant the tubers at least two to four inches below the ground.  Always plant with the eye facing up.  We plant most of our varieties 18″ apart.  Larger plants like dinnerplate varieties will need close to 24″ spacing.  Plant them in rich, well draining soil that’s been amended with compost.  We also top our dahlia beds with compost or grass mulch for water retention and to keep the soil cool and weed free.

Check out this blog post we wrote a few years ago for more tips on how to grow dahlias.

Final notes

2022 was an excellent year for dahlias here in Ohio, and I’ve concluded it was mostly due to all the rain we got.  We had consistent rainfall and the ground never got hard and dry like it typically does at least once a summer.  That fact and a recent podcast I listened to on the No Till podcast about pulse watering has me reconsidering how I water dahlias.  This year LaRonda and I are both planning to use a shorter, more frequent watering method.

Wishing you all armloads of dahlias in 2023!

Rosita