How to plant dahlia tubers for non-stop blooms all midsummer and autumn long

Every bit as flouncy as they are fabulous, learning how to plant dahlia tubers is a surefire way to transform your garden into a riot of colour come midsummer – and, if you do things just right, your homegrown dahlias should bloom all through autumn, too.

It doesn't matter whether you prefer to grow dahlias in pots or weave them into your garden border ideas, knowing how to plant your tubers is every bit as vital to them thriving as, say, learning how to overwinter your dahlias like Monty Don.

With that in mind, then, we've pulled together an expert how-to guide for your convenience, so that you can look forward to a beautiful display of dramatic flowers later this year.

How to plant dahlia tubers

Oh sure: you could buy potted plants in summer from your local garden centre (in full growth and full bloom), but, while this is key to instant dahlia-induced gratification, it's a far more expensive way to approach things.

Instead, we recommend getting to work in the early spring and learning how to plant dahlia tubers like a pro.

Whether you're buying a new stash of tubers, or you've been overwintering yours in a frost-free shed or greenhouse from last season, there is a key month to add to your calendars.

That's right: April is the time to check on the condition of dahlia tubers that you dug up and stored away last winter, just as it is the best time to get planting old tubers and new – especially if you want them to bloom this summer.

What you will need

Before you set to work planting your dahlia tubers, it's a good idea to first make sure you have everything you need to hand:

• Dahlia tubers: if you're planting in pots, try a dwarf variety such as the Dahlia 'G. F. Hemerik' from Crocus. If not, though, go grand with the Dahlia 'Dinnerplate Collection' from Thompson & Morgan or the pom-pom flowers of the Dahlia 'Wizard of Oz' from Crocus
• A good all-purpose compost: the Westland John Innes No.2 Peat-free Compost from B&Q is ideal if you're potting your tubers up, or try something like the Peat-Free Compost from Sarah Raven if you're popping them straight into the soil
• A slow-release fertiliser: try something like this Miracle-Gro Continuous Release All Purpose Plant Food from Amazon

You will, if your aim is to plant your tubers in a container, need a pot of at least 30cm  – try this Greenville Recycled Plastic Pot from B&Q if you're on the hunt for options.

Step-by-step

As dahlias are tender, they won’t survive heavy frost, so must be kept indoors until temperatures are consistently above freezing. That's why it's best not to begin our How To Plant Dahlia Tubers 101 course until at least April.

When the time is right, though, here's what you need to do...

1. Inspect your tubers

If you've bought your tubers new, then you can skip this step. However, if you're using tubers that you've overwintered, it's a good idea to check that they're in good nick.

To do this, just dust away the compost, make sure they're still firm and plump, and ditch any that look shrivelled or rotten (a little bit of mould is fine). 

2. Plant them up

Dahlia tubers are best planted in containers and kept indoors or in a greenhouse, so they start into growth early. 

'If you have a greenhouse, warm windowsill or anywhere undercover and sheltered from frosts, dahlia tubers can be potted from mid April onwards,' says Morris Hankinson, director of Hopes Grove Nurseries. 

Morris Hankinson

Morris Hankinson is the founder and managing director of Hopes Grove Nurseries Ltd, the UK’s only specialist grower-retailer of hedging plants, which he established after graduating with a Commercial Horticulture Degree from Writtle College, Essex in 1992.

'Half fill a pot with a good quality peat free compost and place the tubers on the compost with the main stem upwards, then cover with more compost and lightly water,' says Morris.

Then, pop them in a warm spot that gets 6-8 hours of direct sunlight each day.

Alternatively, 'if you'd prefer to plant directly in the ground, make sure all frosts have well and truly passed and plant the same way with about 75cm between tubers,' he adds.

3. Get them outside

Starting your dahlias off indoors means that they will need to be gradually acclimatised to outdoor conditions before you pop them out in the garden. 

'They can be planted outside when all frosts have passed, usually from May, depending on where you are based, so keep an eye on the weather forecast,' says Morris.

Aim to plant them in warm spring soil of around 15°C. And the Royal Horticultural Society (RHS) advises that you dig in in lots of organic matter, like well-rotted manure or compost, so they thrive.

Water them well, apply a layer of mulch, and let them do their thing.

4. Give them some TLC

Dahlias are greedy feeders, so be sure to treat them to a good quality fertiliser once they've started to flower – a potassium-rich liquid fertiliser, such as tomato feed, should do the trick.

They are also thirst plants, so be sure to water them well (but don't let them sit in boggy conditions). And don't forget to learn how to deadhead dahlias if you want them to keep blooming well into autumn, too.

FAQs

Do you soak dahlia tubers before planting?

'Soaking tubers isn't necessary before potting but if you notice the tubers are particularly dry, you could leave them in some slightly tepid water for an hour or so before planting,' says Morris.

What month do you plant dahlia tubers?

Morris and the RHS are in full agreement: it is best to plant dahlia tubers up indoors in late March or early April, the planted them out in the garden once all threat of frost has dissipated (usually May).

Which way up do you plant dahlia tubers?

A lot of people talk about using the dahlia tuber's eye to help navigate which way up it goes, but it's far easier to identify the joint where the roots meet the stem.

Make sure this is facing upwards, and voila!

Overall, dahlias are a great plant to grow, as they suit most garden styles – and, while they need protection from pests and winter frosts, they more than make up for it with their gorgeous long-lasting blooms.

Time to start, then, researching which dahlias you'd like to see flourish in your outdoor space, so you have plenty of time to order your tubers and set to work planting them ahead of the growing season.