Planting Dahlias

Before planting: important reminder about tuber size and quality

Before planting, assess the quality of your tubers!

Many gardeners are surprised when they receive their dahlia orders by the size of certain varieties. They often worry about not receiving an enormous tuber when it comes to a giant or large-flowered dahlia. However, this is perfectly normal because, with dahlias, the size of the tuber is not a reliable indicator of quality, as it varies greatly between varieties. Keep in mind that a large tuber does not necessarily produce large flowers, and vice versa!

The quality of a dahlia tuber is mainly assessed by:

• its firmness: the root should not be shrivelled or mouldy.
• The condition of the area separating the tubers and the crown, where the buds are located: it should not be broken.

Finally, it sometimes happens that parts of the dahlia’s tuberous root (the “potatoes”) are damaged or partially detached. This isn’t too serious, but it’s important to trim them cleanly before planting, as they could rot and contaminate the entire root system.

Where to plant Dahlias?

Planting Dahlias is straightforward, provided you respect their nature: they are sun-loving plants that thrive in warmth, as long as they receive adequate moisture during their growing season. Choose a well-drained soil where water doesn’t stagnate, particularly in spring and autumn. Dahlias prefer light soils that remain cool and sufficiently moist, maintained with a good mulch during the growing period. However, waterlogged conditions may lead to tuber rot.

While they can grow in almost any soil that isn’t too acidic or chalky, they flourish best in fertile garden soil, rich in humus, fresh and well-drained, preferably neutral. Dahlias are greedy feeders: to flower well and grow vigorously, they need a good base fertiliser. If you’re unsure about the soil’s nutrient quality, add a generous amount of organic matter (compost or manure) in autumn. Tall Dahlias struggle in strong winds or heavy rain. An overly exposed position may dry out the foliage or cause stems to bend. It’s best to plant them in a spot sheltered from prevailing winds. For taller varieties, staking may be essential to support flower heads and prevent stems from snapping in the wind, especially if planted alone. To keep Dahlia flower stems from drooping in mixed borders, plant them alongside other perennials and shrubs for natural support.

When planting, allow enough space for their growth in your flower beds. Ensure each plant remains easily accessible for fertilising, staking, deadheading, and winter storage.

While they suit any garden setting, including vegetable plots, Dahlias truly shine as mid-border or background plants, adding vibrant colour. Dwarf varieties are better suited to sunny flower beds, pathway edging, or container growing on sunny patios and balconies.

When to plant Dahlia tubers?

In cold regions, plant them in late April or early May after the last frosts. Packaged in pouches, your Dahlias can wait in their packaging until the right planting time. Simply store them frost-free.

Pre-sprouting

To gain several weeks of flowering, if you have a greenhouse, conservatory or cold frame, start your dahlias into growth: plant them in trays of moist compost, covered with soil, and they will begin to root and produce shoots. You can transplant the already sprouted clumps in May.

Pre-sprouting to gain several weeks of flowering

How to Plant Dahlias?

• Work the soil to a width and depth of about 20 cm, mixing in a handful of roasted horn meal or dried blood with your finely crumbled soil
• Pour 1 litre of water into the bottom of the planting hole, position your dahlia and cover with 8 cm of soil
• Place your stakes at the same time to avoid damaging the tubers
• Space tall dahlias 80 to 100 cm apart, and shorter varieties 30 to 50 cm apart
• Mulch the soil once the plant is established
• Protect tender shoots from slugs
• Remove faded flowers. You can pinch the main stem to encourage branching and remove side buds to obtain larger flowers

Note: Shorter varieties make excellent plants for growing in large pots on terraces and balconies, just provide them with good potting compost and water very regularly with a good fertiliser.

Planting Dahlias is simple, as long as they’re given the right conditions

Should dahlias be lifted for winter?

Even in winter, Dahlias need attention, and this is when they require a bit more work. Extremely sensitive to cold, they cannot tolerate frost. In regions with harsh winters where temperatures regularly drop below -5°C, lifting the tubers is essential to ensure their survival. In areas with mild winters where frosts are light and infrequent, and in well-drained sandy soil, the tubers can remain in the ground over winter, simply protected by a thick mulch of dry leaves or straw. Lifting the tubers allows for division, pre-growth in pots for earlier flowering, and also protects them from slugs in spring.

• Before the first frosts, as soon as the foliage has blackened, cut the stems back to 15 cm above ground level with secateurs
• Carefully dig up the tubers – you’ll notice they’ve doubled in size!
• Remove as much soil as possible from between the tubers
• Label each clump at the base of the stem to easily identify them in spring
• Wipe the tubers thoroughly to remove moisture
• Place the tubers on a layer of peat, dry sand, or dry leaves in a cardboard box or deep crate
• Cover the tubers with another layer of peat or dry leaves, at least 30 cm thick
• Store them away from light, draughts, rodents, and most importantly, frost, in a dry, cool, well-ventilated, and dark place where temperatures do not exceed 8°-10°C (a dry cellar, frost-free garage, or attic). Under these ideal conditions, dahlias store very well with almost no risk of loss
• Check the tubers regularly for signs of disease
• In early spring, you can either pre-grow them in pots or plant them directly in the ground later