What is Dahlia imperialis?

Dahlia imperialis is a tuberous perennial belonging to the family Asteraceae — the same as ox-eye daisies, asters and sunflowers. Native to the mountain regions of Mexico and Central America, it belongs to the genus Dahlia which includes 25 wild species endemic to this region. Its European history begins in the eighteenth century: the Aztecs had cultivated it for centuries for its edible tubers before Spanish conquistadors brought it to Europe, where its ornamental qualities quickly eclipsed its food uses.

D. imperialis is the botanical type species of the genus, before all horticultural hybridisation — a wild dahlia in all its natural majesty. Among its many nicknames, "bamboo dahlia" is particularly well-deserved: its hollow stems, marked by regular raised rings, recall with startling precision the canes of giant bamboo. The species includes several forms — single lavender-pink flowers (most common), double flowers, and white flowers.

What are the unique characteristics of Dahlia imperialis?

• Absolutely spectacular height — 2 to 4 metres in a single season: this is the most striking characteristic of this plant. Starting from a tuber planted in spring, D. imperialis rises at an extraordinary rate of growth — sometimes several centimetres per day at peak season — to reach 2 to 4 metres in height (occasionally 5 metres in the most favourable conditions) for 1 to 1.5 metres in spread. In a few months, an ordinary garden is transformed into an exotic, tropical landscape.
• Hollow bamboo-like stems: the stems of the imperial dahlia are robust, hollow, green to purplish in colour, marked by regular raised rings that exactly recall the internodes of giant bamboo. This unique vegetative architecture is decorative long before the flowers appear.
• Large simple lavender-pink flowers of 12 to 20 cm: at the tops of the stems, large inflorescences reuniting 30 to 35 flower heads open in autumn. Each flower is simple — one to two rows of slender lavender-pink to pink-violet petals, lightly veined with white, surrounding a luminous orange-brown centre. The delicacy of the petals and the size of the flowers create a striking contrast with the gigantic vegetative scale of the plant.
• Late autumn flowering — September to December: the imperial dahlia flowers precisely when other perennials and annuals finish. This late-season flowering in October-November-December is one of the most precious and rarest in the ornamental plant world. In very mild-winter regions, flowering can extend into January.
• Large, very decorative compound foliage: the leaves are composed of 7 oval, toothed leaflets of a deep green, reaching up to 80 cm in length. This exotic, luxuriant foliage is decorative from spring onwards, long before the flower buds appear.
• Extraordinarily rapid growth: few ornamental plants offer such a speed of development. In a single season, D. imperialis is capable of structuring an entire garden space, dressing a façade, masking a wall or creating a spectacular living screen.

How to grow Dahlia imperialis?

• Light: full sun or light partial shade. D. imperialis produces its finest growth and most generous flowering in full sun. It must be sheltered from prevailing winds — its 3–4 metre stems, laden with inflorescences, are vulnerable to strong gusts.
• Soil: rich, light and free-draining. Organic matter richness is essential to sustain its explosive growth. Incorporate a generous mix of compost and quality potting mix before planting. Heavy, waterlogged soils cause tuber rot and must be avoided.
• Planting: in April-May, after the last frosts, once the soil is sufficiently warm. Plant tubers 10–15 cm deep, spacing plants at least 80 cm to 1 metre apart. Stake immediately and solidly — with robust posts or by training against a wall, fence or pergola.
• Watering: regular and sustained during active growth and flowering. In hot weather (above 25°C), water in the evening without wetting the foliage. Reduce after flowering.
• Feeding: essential given its phenomenal growth. Apply a balanced fertiliser in spring and summer, then switch to a potash-rich fertiliser in August-September to encourage flowering and tuber development.
• Frost hardiness: limited to -5°C. D. imperialis is not frost-hardy and does not tolerate prolonged hard frosts. In regions with mild winters (Atlantic coast, Mediterranean, Brittany) where temperatures do not durably fall below -5°C, it can overwinter in the ground under a thick mulch. Elsewhere, tubers must be lifted and stored.
• Staking is essential: from 1 metre onwards. Use robust stakes, giant bamboo canes, or train the stems against a fixed support (wall, fence, pergola). Several successive staking operations during the growing season are necessary.

What are the care requirements?

• Pinching young shoots: when young stems reach 4 to 6 leaves, remove the growing tip. This stimulates lateral bud development and produces a more branched, bushy and floriferous plant.
• Deadheading: remove spent flower heads progressively to encourage new bud production and extend flowering until the frosts.
• Overwintering the tubers: once the foliage blackens at the first frost, cut all stems to 10–15 cm above the ground. In cool regions, carefully lift the tubers, allow them to dry for a few days and store in a cool, dry, dark space (cellar, garage, loft) until the following spring. In mild regions, leave in the ground under a thick mulch of dead leaves (minimum 20 cm).
• Disbudding: for larger individual flowers, retain only the terminal bud on each stem and remove lateral buds below it.
• Pest and disease watch: watch for powdery mildew, grey mould, aphids and slugs on young shoots.

Where to plant Dahlia imperialis?

• As a solo specimen or back-of-border statement plant: its imposing stature naturally makes it a star plant — grown alone in the centre of a lawn or at the back of a border where its arborescent silhouette can be fully appreciated.
• Against a wall, façade or fence: it is ideal for dressing bare vertical surfaces — it efficiently masks any unattractive support while creating a spectacular tropical living screen.
• As a temporary seasonal screen hedge: several plants spaced 80 cm apart form an original and impressively tall summer and autumn hedge.
• Near a pool or water feature: its exotic habit and lavender-pink flowering create a tropical atmosphere alongside water.
• In an exotic or jungle composition: pair it with cannas, ornamental bananas, giant miscanthus or phormiums for a spectacular "jungle" or "exotic" planting. Explore our other exotic botanical curiosities to enrich your compositions.

What are the advantages for collectors?

• An unequalled vegetative scale among perennials: no other tuberous perennial grown in Europe reaches this height in a single season — D. imperialis is in a category of its own.
• Absolutely unique late-season flowering: flowering in October-December with blooms of 12 to 20 cm in diameter is an ornamental achievement that very few plants can match at this time of year.
• A spectacular growth rate to witness: the speed of development of D. imperialis is in itself a spectacle — watching stems reach 3–4 metres in a few months is an unforgettable gardening experience.
• A fascinating historical lineage: growing D. imperialis means growing the Aztecs' plant in its original botanical form — before all horticultural hybridisation.

How to propagate Dahlia imperialis?

Two main methods:

• By tuber division: in spring, at replanting time, divide the tubers by hand or with a sharp, disinfected knife, ensuring each section has at least one eye (bud). Allow cut sections to dry for 1 to 2 hours before replanting.
• By stem cuttings: in spring, when the first stems are 10–15 cm long, take cuttings just below a node. Place in moist substrate at 18–22°C in indirect light. Rooting in 3 to 4 weeks.