Dahlias: how to force them to prolong flowering?

Dahlias amaze us in summer with their bright, colourful flowering. There are thousands of varieties, available in a wide range of shapes and colours. Sometimes extravagant and sophisticated, sometimes natural and simple, their flowers offer endless choice! Dahlias are usually planted in the garden after the last frosts, to flower from June–July and until November depending on variety... but did you know that it is possible to get them started earlier to hasten flowering and enjoy them for longer? We explain everything in this tutorial!

Our full guide: “Dahlia: planting, maintenance and growing tips”

Why pre-cultivate dahlias?

This technique mainly allows them to flower earlier and for longer: you can gain up to 6 weeks! By starting your dahlias under cover, you give them a head start compared with dahlias planted directly outdoors. It also provides protection against weather hazards. Young dahlias are sensitive to cold and late frosts that can occur in early spring. By growing them first indoors, you protect them until frost risk has passed, ensuring their survival and good growth. Forced dahlias are also more robust, less prone to disease and pests, because they have had time to strengthen before being put in the garden.

When to plant dahlias in pots?

You can plant dahlias in pots in late winter, around February–March.

How to do it?

Choose firm, healthy dahlia tubercles. Avoid those that are too soft or show signs of rot.
Before planting, it can help to rehydrate them by soaking in lukewarm water for an hour or two. This awakens the tubercles and encourages root growth.
Fill pots with a light, well-draining seed compost. You can use seed compost for sowing and propagation by cuttings.
Plant the tubercles then cover them with compost so they are buried under about 5 cm of substrate.
Water lightly to moisten the compost.
Place pots under cover in a bright spot, in a greenhouse, a veranda, or under a cold frame. Ideal temperature is around 16–20 °C.
Dahlias will begin to root and shoot. Keep the substrate moist but not waterlogged, as excess moisture can cause the tubercles to rot.
In April or May, as soon as frost risk has passed, you can plant them outdoors in a sunny spot, in rich, well-draining soil.