When and How to Plant Your Dahlias (And Strategies To Get Dahlias to Bloom Earlier)
When can you plant out dahlias?
One of the frequent questions we get is “When can I plant out dahlias?” and it’s kind of a difficult question to answer because everyone has different growing conditions and different situations that they’re growing dahlias in.
The short answer? Around your last frost date just to be on the safe side.
Of course this has some caveats, but for most people that’s going to be the correct date to plant out.
The biggest dangers to dahlias actually are going to be two things - freezing and flooding.
Freezing is the obvious danger to dahlias, which is why we wait until the last frost date to plant out. Because their tubers are full of water, freezing temperatures will cause the moisture inside dahlias to freeze - which will cause ice crystals to form, tearing through the cells and turning your firm healthy dahlia tubers into rotting mush.
Warm soil will actually protect your tubers slightly, acting as a form of thermal insulation for your tubers. Although that won’t protect them from extreme cold that causes the soil to freeze, it can protect them from a short dip in temperatures in the spring.
Even if you do get a cold snap and freeze, guess what? So long as your dahlias are buried into the ground and the tubers are safe from freezing, they will be just fine. Even if the top growth gets a bit nipped by the frost (or frozen - that’s happened to us before too) they will send up more shoots from the tubers.
Of course this all depends on the level of moisture present as well. We live in a drier environment and can get away with just digging our dahlias into the soil because the soil stays much drier here and without moisture, dahlia tubers won’t suffer freeze damage.
Conversely, dahlias don’t like too much moisture. Especially if you live in an area where you get a lot of spring rains that cause massive flooding (we’re talking biblical proportions of precipitation) your dahlias will suffer as a result. Clay soil especially with heavy flooding means rotten dahlia tubers even if your temperatures are well above freezing.
That’s why it’s important to ensure that you plant your dahlias in a well drained soil. You can accomplish that by living in a dry environment like us - we only get a few inches of precipitation over the winter. You can mimic our dry environment by covering your dahlias in the ground in clear plastic to keep the soil dry and free from precipitation.
You can also plant your dahlias in a higher spot that allows for water and precipitation to drain away from them. I’ve seen this done by digging drainage trenches on either side of a bed they are planted in (so that water and moisture drains away and into the trenches around the bed) as well as digging in french drains. You can also of course plant dahlias in raised planters, but you’ll want to ensure that you don’t do this until later in the season - raised planters and beds are less insulated than the ground and the soil can get far colder than the ground, meaning that your dahlias run a higher risk of freezing.
If you can keep your dahlias from freezing and getting flooded in early spring, you can then push the envelope a little bit as far as planting out and getting your dahlias going. We’ve had dahlias blooming as soon as mid-June with some of the strategies I’m going to share with you (whereas when planted out in the garden or the field they only just start blooming in early July at the earliest).
When do dahlias bloom?
Although the actual bloom date of dahlias is variable - it depends on the species, the amount of light, the fluctuation in temperatures and precipitation - we generally say it takes around 90 days for a dahlia to start blooming.
Since we plant ours in April, ours will start blooming around 90 days after planting, which is around mid-July. Some, such as ‘Penhill Watermelon’ will take longer to bloom, while a lot of the pompon or ball-shaped dahlias such as ‘Burlesca’ will bloom earlier.
July is generally the time for most people when dahlias start to bloom, and by August all the dahlias should be blooming. September is usually one of the heaviest blooming periods for most growers, and then October the production starts to die down a bit as the light levels and hours of daylight start decreasing.
So how can you get more blooms, and how can you get dahlias to bloom earlier?
It’s important to understand that dahlias are technically a perennial flower that will continue to bloom so long as you provide it the necessary conditions. It’s only the killing frosts that stop our dahlias from continuing onward. Thus with enough heat, enough light, enough water and enough nutrients, dahlias will theoretically bloom indefinitely (which is one of the reasons why they are such a popular cut flower for the garden and a workhorse of the flower farmer or professional grower).
So if we can provide those conditions earlier you will be able to have the dahlias start blooming earlier. This does not detract from their ability to bloom later on in the season, since the only thing that stops them from blooming is a hard freeze. And if you could have an entire month or two of additional dahlia season, wouldn’t you love it?
And as we’ve learned, dahlias are some of the most productive, spectacular plants that you can grow. There’s a reason that they’re so popular with flower farmers and gardeners and dahlia enthusiasts - they offer an abundance and volume of flowers that is staggering. And who wouldn’t want to extend the dahlia season longer, even if its just for a few weeks earlier?
Early Dahlia Strategy #1: Pot up your dahlias
To get your dahlias to bloom earlier, the simplest solution is to give your dahlias a head start on the growing season.
To do this, you’ll want to just get your dahlias started earlier by potting them up. You see, part of the reason why it takes so long for dahlias to bloom is that it takes a while for the tubers to get going and grow out into big enough plants to start flowering. By starting your tubers earlier, you’ll get to blooms earlier - it’s as simple as that.
I’ll usually give an early start to dahlia tubers about a month or two prior to our last frost date (this year I started some on February 15th) so that they have adequate time to grow and get a good head start before we plant out in the middle of April.
This works of course only if you have a heated space large enough to start your dahlias in. If you have just a few dahlias to start, a sunny window or space in your home may be enough to get them started. We have a few dahlias that are actually growing well on a southern facing window (one even bloomed during the winter).
On the other hand, if you’re growing thousands of dahlias as a flower farmer or professional grower, you’ll need a (slightly!) bigger space to start them off. They will need light as well to start growing properly, so a heated greenhouse is of course ideal, but a minimally heated tunnel or a minimally heated garage or barn with natural or artificial lighting would also work OK.
To start the dahlias, we’ll fill a gallon pot with good potting mix and then plant the tuber as we would out in the ground - just below the surface of the soil - and give it a good water. If we want them to get off to a good start, we’ll also pop them onto a heating mat to really get them going.
It takes a couple weeks for dahlia tubers to “wake up” and start growing. You’ll see it usually as a shoot coming from the pot - same as you would in the ground - and that will eventually continue to grow upwards. Continue to water the tubers as needed, checking to ensure that your soil is moist (but not sopping wet so as not to rot the dahlia tubers). Dahlias need a little less water when they’re growing in these cooler conditions than if they were growing at the height of summer in the heat.
At this point, I’m mostly looking to get some good vegetative growth going - not necessarily flowering or bud formation. And if the growth gets to be too tall, I just will cut the stems back to the base of the plant to encourage a stockier, bushier plant and keep them from getting too leggy.
When it comes time to planting them out (after your last frost date), you’ll just tip them out of their pot and get them planted. You’ll see that they will have taken up almost all the space in the pot with their roots - they’re quite vigorous plants! - and will be ready for planting as normal. We like to hill the dirt up around the stems of the shoots - this not only helps to ensure that they are firmly into place but also encourages tuber formation around the stem as well.
Early Dahlia Strategy #2: Take cuttings
One of the reasons why we actually start our dahlias earlier indoors is to take cuttings.
A lesser-known method of propagating your dahlias (most people are familiar with dividing the tuber clumps) taking cuttings of dahlias is actually a very efficient and powerful way to increase your dahlia stock - especially of varieties that are super expensive or rare or that don’t produce a lot of tubers.
The interesting thing is that dahlias grown from cuttings are very vigorous - and many times will bloom far earlier than dahlias grown from tubers. I think it’s partially because we start the cuttings so far ahead of time that they in essence get an early start like the tubers we’ve potted up early, but also because they seem to grow very quickly in comparison to tuber dahlias (especially in the beginning stages).
For this reason, we’ll start a lot of our dahlias from cuttings (at least 50% of our dahlias that we plant out) and we get flowers a lot earlier. It’s also a great way of expanding your dahlia collection or tuber stock very quickly (although it does take some work).
There are two types of cuttings - shoot cuttings and leaf cuttings (otherwise known as nodal). Although leaf cuttings are fun, they are considerably more difficult if you don’t have experience in rooting cuttings so I would recommend you stick with shoot cuttings as they come straight off the dahlia tuber.
The shoots coming off the dahlia tuber are vigorous and strong, pushing out of the tubers with a force that shoves aside soil and other impediments. The shoots are also readily primed to do other activities at this point, including rooting - and they will root very quickly and easily.
To take a shoot cutting, you’ll want to start your dahlias early - like December or January if you can - and give them a couple weeks to start growing their shoots. You’ll also want to plant them so that their eyes are preferably above the level of the soil so you can easily access where the shoot attaches to the tuber.
Once your shoot starts growing, you’ll want to wait until it’s a few inches tall - anywhere from 3-6” - and then you’ll cut it off the tuber. We use a very sharp scalpel, but you can use a knife or anything that has a sharp blade (don’t use scissors, since scissors will crush the stem) and you’ll remove the shoot as close towards the tuber as you can.
Some dahlia growers will actually take off a chunk of the tuber when they’re taking the shoot, but I prefer not to do that - it can damage the tuber and also slows down the dahlia from producing more shoots (which you can then additionally root). Technically so long as your dahlia tuber receives good conditions and plenty of moisture and fertilizer they will keep growing shoots for you to take cuttings of - that’s how the professionals do it. We’ve actually kept “mother” tubers to take cuttings off all summer long when building up our tuber stock.
You’ll strip the lower leaves off the cutting - all of them except for the top two (this reduces the risk of the leaves rotting in the soil) and pop the shoot into potting mix - preferably a soil-less mix to reduce the risk of the shoots rotting. We’ll also dip the cut end of the shoot into rooting hormone to encourage rooting.
We’ll pop a humidity dome over the cuttings and leave them in a relatively warm (70 F) area for a few weeks to let them root. We’ll check and see if they’ve rooted by tugging on the leaves - if there is resistance, then the cuttings have successfully rooted. If they slip on out, simply slide them back into the potting mix.
I particularly like this video our friends at Pepperharrow Farm made on dahlia cuttings. Adam and Jennifer grow thousands of dahlias every year and explain the process really well
Early Dahlia Strategy #3: Plant your tubers out earlier
This is actually a really cool strategy that works well for us here because we are very sunny and very dry in the springtime (if a bit windy). As a result, the soil warms up pretty quickly for us and we are actually able to plant our dahlia tubers out relatively early.
We can actually overwinter our dahlia tubers so long as we bury them deep (6” at least) since our soil stays very dry, which means that our soil is still dry in the spring and so we can plant our dahlias tubers even earlier since the soil is workable and we don’t have to worry about our dahlia tubers freezing.
We’ll plant our tubers deep and then we’ll cover them with clear plastic on hoops. The clear plastic is important because it helps to 1) heat up the soil and 2) keeps any excess moisture off the tubers that may come in the form of precipitation (including snow).
By heating up the soil, we’ll in essence create a nice little greenhouse over the dahlia tubers and encourage them to sprout earlier. And in this case, I’m not really worried about a late frost or freeze because if the growth above the ground gets frost damage, it won’t be an issue - the dahlia tuber will just sprout more shoots and continue to grow since it is nice and snug down below the ground. And because the plastic will keep the area dry, I’m not worried about rain or snow causing the dahlias to get flooded.
Once the shoots are up and the dahlias are growing, we’ll pull off the plastic and then place floating row cover on top of the hoops until the dahlias are fully grown under there. The floating row cover works well because it gets less hot under there, but builds up the moisture and heat especially in the earlier parts of spring and summer when the nights are still cool - in essence, creating a mini greenhouse again.
It’s a very simple system, but one that can allow you to really get a great head start on the season with your dahlias with just a bit of cover. You do have to know your local conditions because if you get a ton of precipitation (i.e. 12”+ of snow) or freezing conditions that will freeze your dahlia tuber or drown them, this won’t work the same way for you in the early spring months.
However, if you live in a growing zone like a 3b or 4a and you’re wanting to get your dahlias blooming earlier, you can use this technique even after your last frost date to encourage them to grow more quickly and earlier so that you can get more blooms prior to frost. We have some friends living in those conditions, and they have to use plastic and row cover in order to get some of their dahlias to bloom at all.
I hope this helps you to get some dahlias blooming earlier
They’re truly one of our favorite flowers and are so productive and actually fairly easy to take care of once you understand them and have some experience in growing them.
If you’re interested in learning more about growing dahlias, check out more posts here:
How to Grow Cafe au Lait Dahlias
Happy growing!