Tree dahlia

Sun

Full sun; light afternoon shade is useful in hot climates. Shelter from strong winds.

Water

Water deeply during active growth, keeping the soil evenly moist but not waterlogged. Reduce watering as the stems die back.

Soil

Fertile, humus-rich, well-drained soil. Avoid sites that remain wet through winter.

Height

8-20 ft (2.4-6 m)

Spread

4-8 ft (1.2-2.4 m)

Flowers

Late autumn into early winter, with open, nodding lilac-pink flowers and yellow centres; flowering may be cut short or prevented by frost.

Foliage

Large, divided green leaves on thick, hollow, jointed stems; leaves and stems are killed by frost.

Growth

Fast; established plants can make their full height in one growing season.

Care needed

Medium

• Pollinators
• Heat tolerant
• Low allergen

Northern hemisphere months. In Australia, shift them by six.

Difficulty

Moderate

Toughness

Needs looking after

Pick your zone, country, light and whether you have pets.

1. Planting day

   1.7 ft (0.5 m)

2. After 1 year

   8 ft (2.4 m)

3. After 2 years

   14 ft (4.2 m)

   full size

About 2 years to full size.

From a typical nursery size, at an average rate for this kind of plant. A rule of thumb, not a forecast.

Cats and dogs

Mildly toxic: may upset a stomach

Growth rate

Fast

• Plant after frost danger has passed, leaving enough room for the mature clump.
• Install strong stakes early and tie the stems loosely as they lengthen.
• Water deeply during dry spells rather than giving frequent light sprinklings.
• Apply compost in spring; avoid excessive nitrogen, which encourages soft, leafy growth.
• Mulch the root area for winter where plants remain outdoors, keeping drainage clear.
• In colder regions, lift the tuberous clump after frost and store it somewhere cool, frost-free and barely moist.

Pruning. Cut dead stems down near ground level after flowering or once frost has killed them. If a shorter plant is needed, shorten shoots in late spring, but avoid late pruning because it can delay or remove the autumn flowers.

Early spring

Check stored tubers for rot and start them into growth under cover in cold regions.

Spring

Divide established clumps as buds begin to grow, keeping a section of crown with each division.

Late spring

Plant out after frost, add compost and install tall, sturdy supports.

Summer

Water during dry weather and check ties as the stems grow.

Autumn

Keep plants watered while buds develop; avoid pruning and unnecessary disturbance.

Late autumn

After frost kills the stems, cut them down and either mulch the clump or lift it for frost-free storage.

• Place at the back of a deep border, where the flowers rise above lower perennials.
• Use beside a sheltered wall or fence for support and a tall seasonal backdrop.
• Combine with cannas and large-leaved plants for a tropical-looking summer planting.
• Plant behind autumn-flowering salvias and asters to extend colour into the end of the season.

Garden styles it suits: Tropical, Modern tropical, Architectural minimal, Cottage garden, Autumn colour and Mexican hacienda.

Plant it with: Cannas, Ginger lilies, Mexican bush sage, Salvia 'Amistad', Japanese anemones and Asters.

Wildlife: The open flowers provide pollen and nectar for bees and other insects when weather remains warm enough for them to forage.

• Slugs and snails eat emerging shoots: protect new growth with barriers, hand-picking or locally approved iron-phosphate bait.
• Strong winds snap the hollow stems: choose a sheltered position and use substantial stakes with several loose ties.
• Winter rot destroys tubers in saturated soil: improve drainage or lift and store the clump frost-free.
• Frost prevents flowering: use a warm, sheltered site; in short-season climates, treat the foliage rather than the flowers as the main attraction.

Dahlias are mildly toxic to cats and dogs if chewed, potentially causing stomach upset; sap contact may also irritate skin. Keep pets from chewing leaves, stems or tubers. Tree-dahlia-specific evidence is limited, and it is not known to be seriously toxic to people or horses.

• Garden dahlia (Dahlia pinnata and garden hybrids)

  Most garden dahlias are much shorter and flower from summer onward, often with dense double blooms. Tree dahlia has towering, jointed stems and typically bears open pink flowers very late in the season.

Dahlia honours the Swedish botanist Anders Dahl, while imperialis means imperial and refers to the plant's imposing stature.

Propagation. Divide the tuberous clump in spring, ensuring each piece carries part of the crown with a live bud. Young basal shoots can also be taken as cuttings; sections of mature stem containing nodes can root with warmth and careful moisture control.

Accepted botanical name

Dahlia imperialis

Native range

Colombia, Costa Rica, El Salvador, Guatemala, Honduras, Mexico Central, Mexico Gulf, Mexico Northeast, Mexico Southeast, Mexico Southwest, Nicaragua and Panamá.

Sources: GBIF (CC BY 4.0) and Trefle (CC BY 4.0). The care and design notes are LandscapeGPT's own.