With their long and generous flowering period, dahlias are invaluable for bringing life to the garden from early summer until the first frosts. There are countless varieties, with single or double flowers, in classic or extravagant styles, and in a wide range of colours. They are usually grown from tubercles, but they can also be sown, which allows you to obtain a large number of new young plants at a reduced cost. Discover all our tips on when and how to sow dahlias!

Also check out our complete guide to growing dahlias.

Double pink-flowered dahlia
Dahlia 'Café Au Lait Royal'

Why sow dahlias?

Dahlias are usually grown from tubercles, but sowing is a great technique for multiplying the varieties you already have in your garden or for obtaining a larger number of young plants at a reduced cost. Seeds are indeed much cheaper than tubercles! This will allow you to easily create more abundant flower beds.

When to sow dahlias?

Sow dahlias indoors between February and April, depending on your climate. This will give them enough time to develop before being transplanted into the garden.

Why not sow them directly outdoors? Dahlias are sensitive to frost. By sowing them under cover (in a greenhouse, conservatory, or indoors), you protect the young plants from cold temperatures. Once the risk of frost has passed (usually after mid-May), you can move them outside.

What you'll need

Before you start, make sure you have all the necessary equipment to hand. Here's what you'll need:

Dahlia seeds
You can collect seeds from your dahlias once they've finished flowering, or buy them directly

How to sow dahlias?

Follow these steps to successfully sow dahlias:

  • Fill seed trays, small pots, or alveolate trays with a light substrate, such as seed compost or fine, well-draining soil.
  • Gently firm the compost to avoid air pockets.
  • Lightly moisten the substrate before sowing to stabilise the seeds during watering.
  • Sow the dahlia seeds on the surface of the substrate, spacing them sufficiently to allow room for growth.
  • Cover the seeds with a thin layer of sieved compost (about 2 mm thick) to retain moisture while allowing light to pass through.
  • Mist gently with water using a spray bottle to moisten without displacing the seeds.
  • Place the seedlings in a warm (20–22°C) and bright spot, such as a sunny windowsill. You can also place them in a heated mini greenhouse.
  • Keep the substrate slightly moist by watering regularly, but avoid overwatering to prevent rot.
  • Wait for germination, which usually occurs within 7 to 14 days.
Dahlia seedlings
Young dahlia plants from sowing
  • When the young plants have developed 2 to 4 true leaves, transplant them into individual pots to give them more space.
  • Handle the young plants carefully during transplanting to avoid damaging their fragile roots.
  • Water lightly after transplanting to encourage rooting.
Transplanting dahlias
Once the young plants are large enough to handle, transplant them into individual pots

How to plant them in the garden?

Once your dahlia plants are sturdy and the risk of frost has passed (around mid-May), you can plant them in the garden:

  • Choose a sunny spot, as dahlias love light. The soil should be well-drained and rich in organic matter. Amend it with compost if necessary.
  • Dig a planting hole slightly larger than the dahlia's root ball.
  • Place the plant, ensuring the base of the stem is just at soil level.
  • Backfill the hole and firm gently.
  • Water generously.

Remember to space your dahlias 30 to 50 cm apart, depending on the variety, as they can become quite large.

Planting dahlias outdoors
Plant your dahlias in the garden once the risk of frost has passed

How to care for them?

To ensure your dahlias flourish all summer, regular care is needed.

Dahlias like moist soil but dislike waterlogging. Water regularly, especially in dry weather, focusing on the base to avoid diseases. Mulching around the plants helps retain moisture and reduces the frequency of watering.

Dahlias with tall stems or heavy flowers may bend under their own weight. Install a stake at planting time to prevent stems from breaking in wind or rain. Tie them gently with raffia or a soft tie.

To encourage bushier growth and more abundant flowering, pinch out the main stems when they reach about 30 cm in height. Simply cut the tip above a pair of leaves: this will promote the growth of new stems and increase the number of flowers.

Remove faded flowers regularly by cutting just above a node. This encourages the plant to produce new buds and prolongs flowering until the first frosts.

In autumn, when temperatures start to drop and the foliage turns yellow, it's time to prepare your dahlias for winter. Carefully dig up the tubercles before the first frosts, let them dry for a few days, and then store them in a cool, dry, dark place (in sand or turf, for example). You can replant them the following spring for another season of flowering.

If you live in a mild climate with little risk of frost, such as the Mediterranean basin or the Atlantic coast, you can leave dahlias in the ground by protecting the base with a thick mulch.