We deliver Dahlia Tubers from late February, which is much too early to plant them outside.
So, you can start them off in pots in a frost free place while you wait for the weather to warm up.
Dahlia tubers are tender, and planting them out into cold soil at best sets them back (makes them grow slowly) and at worst kills them.
If they sprout outdoors and there is a frost, the shoots will die, wasting a lot of energy.
Dahlia tubers want to be planted out into warm soil, which means around 10C.
That soil temperature is typically reached by late-April in the South, May for most of the UK, and as late as early-June for colder inland Northern and Scottish regions.
Therefore, you can give your Dahlias a running start by potting them up early, keeping them in a sunny frost-free place, then planting them out later into warm soil.
- This video is about starting Dahlias in Spring, by potting them up early into small pots (2 or 3 litre is perfect for most tubers, unless you are dividing them, in which case you can use smaller pots) for planting out in your flowers beds later.
- If you are growing your Dahlias in pots anyway, rather than your beds, and have space for those pots in your greenhouse, then of course you could put the Dahlias into their big pots directly and move them outside later, but it’s still better to move up pot sizes if you have time, see the second video below.
Note that you cover the tops of the Dahlias with only 1cm of potting compost, so that the new shoots can reach the light immediately: if you can see the “eyes” at the top of the tubers bulging ready to sprout, try to get them at soil level.
Starting Dahlias in pots that you plan to grow in beds
We forgot to show adding Bulb Starter Rootgrow, which is optional but highly recommended.
TRANSCRIPT
Right, these are the Dahlia tubers that you’ll receive from Ashridge. So just, when they turn up, just have a quick look, see if they’ll all right. What you will have to find out is that’s the top of the tuber. So this piece here is where the old stems were cut off last year. So that’s your top, that’s the sunny side up.
When you receive them, if you want to pot them straight up, put your compost in. Just make a little mound in the middle, so the tuber could sit on top. Give it a quick wiggle, push them down a bit and then kind of leave, try and leave this piece, the cut piece, at the top of the pot.
Put your compost in, give it a quick tap, pushing down a bit.
That’s it. Leave them in a frost free greenhouse, conservatory, or porch until the shoots start to come up, and then we’ll keep on taking the picture of this one. So keep checking on the blog or the website, and you can compare ours to yours. (editor’s note – those videos are coming later in 2025)
Starting Dahlias in pots that you plan to grow in pots
There are several benefits to starting Dahlias in the smallest suitable size and potting them up into progressively bigger pots until they go into their final pot, which should be at least 10L.
- Saves space and water
- Helps prevent problems from overwatering
- Uses the same amount of potting compost in the end, but you get the best out of it because you aren’t washing nutrients out of compost that has no roots growing it yet
- Lets you select the best specimens for your display
Note: It’s not a big deal, and watering should make the compost settle, but in our opinion, Zoe planted her tuber slightly too deep, because you can’t see the top at all.
We prefer ours to be peeking out the top. You want the eyes to shoot and immediately reach the light. This both saves energy for growth, and encourages sturdy stems.