Dahlia imperialis - Giant Dahlia
Dahlia imperialis, known as giant dahlia, tree dahlia or - in English-speaking countries - bell tree dahlia, is the tallest species of the Dahlia genus and one of the most spectacular perennials that can be introduced into an exotic-style garden. While popular garden dahlias rarely exceed 1.5 m, their mountain relative shoots up several metres in a single season, forming a column of bamboo-like stems topped with a canopy of large, feathery leaves. The species was described in 1863 by Benedikt Roezl and Karl Eduard Ortgies, and the epithet imperialis - imperial - perfectly reflects its majesty.
Despite its impressive size, this is not a plant exclusively for collectors. In mild regions of Europe, it grows in the ground without major intervention, while in cooler temperate climates it can be grown like an ordinary dahlia: digging up the tubers for winter and planting them again in spring. It only requires an understanding of its rhythm - the plant grows throughout summer and blooms only when the days become short, at the onset of winter.
Botanical description and plant habit
Dahlia imperialis belongs to the daisy family (Asteraceae) and is a tuberous perennial that takes the form of a climbing shrub or small tree in warm climates. Upright, hollow, four-angled stems emerge from the tuberous roots, with distinctly swollen nodes spaced approximately 30 cm apart. They are usually 3-5 cm in diameter, reaching approximately 10 cm in older, partially woody specimens. The plant most commonly reaches 2-6 m in cultivation; specimens 8-10 m tall have been recorded in natural mountain habitats, and some sources report even greater heights.
The leaves are opposite, large, and twice or three times pinnate, with broad, dark green, toothed leaflets resembling elder leaves. As the plant grows, the lower leaves fall, making the bare, segmented stems resemble bamboo poles even more closely. The entire plant is succulent and soft - the stems consist mostly of water, which explains both its rapid growth and its sensitivity to frost and wind.
Growth rate and development
Giant dahlia begins growing in spring from tubers overwintered in the ground or in a cellar. It initially grows relatively slowly, but accelerates in midsummer and can grow by as much as 30 cm per week, reaching its full seasonal height within 4-5 months. Observations by gardeners in regions with mild climates show that by August the plant is approximately 2-3 m tall, while in autumn, as the days shorten, it adds another 2 m or more just before flowering.
In frost-free climates, the stems survive for several seasons, become woody at the base, and grow thicker each year. Where frost occurs in winter, the above-ground part dies back, and the plant regrows in spring from the tubers and root crown. Well-rooted plants are long-lived and produce increasingly numerous stems each year, eventually forming a dense clump with many stems. Young tubers, however, are susceptible to rot, so good drainage is particularly important in the first few years.
Flowering and propagation
The flower heads, typical of the Asteraceae family, are 7.5–15 cm in diameter and borne in loose, branched panicles at the tops of the stems. They hang or droop downwards like bells, which is a distinguishing feature of the species. The ray florets are lavender or lilac-pink, while the central disc florets are yellow. White forms and cultivars with double, white and pink flowers are also known in cultivation. Each stem can produce from several to twenty flower heads.
Dahlia imperialis is a short-day plant. It only sets buds after a series of long nights, so flowering takes place in late autumn – from the end of October to December in mild regions. This is why full flowering is rare in cooler parts of Europe: the first frosts often arrive before the buds open. The flowers are frequently visited by bees and other pollinators. In cultivation, the plant is propagated from seeds, by dividing the tubers, and from stem cuttings – sections of stem about 30 cm long with at least two nodes, laid horizontally in moist growing medium.
Environmental requirements and tolerance
The species’ natural range includes the mountains of Mexico, Guatemala, Honduras, El Salvador, Nicaragua, Costa Rica and Panama, extending as far as Colombia and Ecuador. The plant inhabits moist thickets, steep slopes, wet meadows and the edges of pine-oak forests at elevations of 1200–3800 m above sea level, most often between 1500 and 1700 m. The climate of these highlands is characterized by cool nights, wet summers and dry, mild winters, while the soils are porous, sandy-loamy and often contain an admixture of gravel.
These conditions determine its cultivation requirements: full sun or light partial shade, fertile and consistently moist but well-drained soil, and protection from wind. The plant tolerates prolonged rainfall as long as water does not accumulate around the roots. However, it does not tolerate drought, heat combined with dry air, or frost. Temperatures below 0 °C destroy the water-filled stems, while tubers in the ground survive the winter only in USDA zone 8 and warmer (the Royal Horticultural Society RHS assigns the species a hardiness rating of H3). Soil pH is of secondary importance – the dahlia grows in slightly acidic, neutral and alkaline soils.
Cultivation in a temperate climate
In colder regions of Europe, the giant dahlia is grown like garden dahlias. Plant the tubers in the ground after the last frosts, in the warmest and most sheltered position, preferably on the south or west side of a wall or hedge of evergreen shrubs that block the wind. It is best to install sturdy stakes, bamboo canes or metal posts when planting, and tie the stems every 50–60 cm with soft string, leaving enough slack for them to thicken. The soil should be enriched with compost and covered with mulch to reduce evaporation.
Throughout the season, the plant needs plenty of water - it is better to water less frequently and deeply than a little every day. In spring and early summer, you can use a balanced fertiliser, while from late summer it is worth reducing nitrogen in favour of potassium and phosphorus, so as not to encourage further growth at the expense of the buds. After the first frost, cut the stems back low, dig up the tubers, dry them for a day or two and store them in slightly moist peat or vermiculite at around 5–10 °C. In mild regions where the soil does not freeze, it is enough to cut back the stems and cover the crown with a thick layer of mulch, several centimetres deep.
Growing it in a container is possible, although experienced gardeners generally do not recommend it - the plant requires a very large pot and reaches noticeably smaller dimensions. A compromise is to grow it in a tall greenhouse or conservatory, where the tree dahlia is protected from wind and early frosts and therefore has the best chance of flowering fully in autumn. Pests and diseases to watch out for include aphids, leafhoppers, spider mites, slugs, powdery mildew and grey mould - the same problems that affect garden dahlias.
Uses in ornamental gardening
Dahlia imperialis is a plant with an exceptionally architectural character. It is grown primarily for its form: within a single summer, it creates a green wall with a tropical appearance, which can be used as a backdrop for a perennial border, a living screen to hide a fence or a specimen plant beside a house wall. It combines perfectly with banana plants, cannas, miscanthus, castor bean and other plants with large, exotic leaves, forming the backbone of so-called jungle-style gardens.
In regions with mild autumns, its November flowering is one of the last colourful accents of the season - its drooping, lavender flower heads several metres above the ground are best admired from below, against a bright sky. The plant is also valuable to insects because it provides nectar when other food sources have already dwindled.
Cultural significance, history and etymology of the name
The name of the genus Dahlia commemorates the Swedish botanist Anders Dahl, a student of Linnaeus, while the epithet imperialis means “imperial” in Latin and refers to the plant’s majestic growth. In pre-Columbian Mexico, dahlias were known to the Aztecs as acocotli, which can be translated as “water pipe” - the hollow stems of the tree dahlia were indeed used as simple pipelines and water containers. Dahlia tubers were eaten, and modern analyses indicate that they contain up to 13% sugars, making them a potential root crop. In Guatemala, the Q'eqchi' community of San Pedro Carchá still consumes the leaves of D. imperialis as a dietary supplement, while in folk medicine the plant was used for fever, kidney ailments and oral infections. Plants grown densely are also used as living fences. The species reached European gardens in the second half of the 19th century thanks to the Czech plant collector Benedikt Roezl, and became particularly popular in California, Australia, New Zealand and Mediterranean gardens.
Who this species is for
The giant dahlia is a choice for gardeners who enjoy a “wow” effect and have room for a plant several metres tall. It brings the most joy to gardens with an exotic character and to areas where autumns are long and mild. In cooler temperate climates, it requires a little more commitment - digging up the tubers every year and providing sturdy supports - but in return it offers a unique, tropical habit that no other perennial can match. It is not a good choice for small balconies or ordinary pots, however, and those expecting regular flowering in regions with early frosts should consider growing it under cover.
Summary
Dahlia imperialis combines rapid growth, a bamboo-like habit and late, bell-shaped flowers in a plant that can transform the character of a garden in a single season. It requires sun, fertile and moist soil, protection from wind and frost protection for its tubers, but is otherwise relatively easy to grow. In the wild, the species is threatened by habitat loss caused by deforestation, which is why it is listed as Near Threatened (NT) on the IUCN Red List - garden cultivation remains one way to preserve this remarkable plant in cultivation.
