Bamboo-like stems climb to 13 feet, carrying pink or white blooms up to 8 inches across: this is the tree dahlia (Dahlia imperialis), a Mexican perennial grown for its towering late display. It gives up at 23°F (-5°C), so a sheltered, well-drained spot out of the wind decides whether you see it flower at all.
| Name | Dahlia imperialis |
|---|---|
| Family | Asteraceae |
| Type | bulb plant |
| Height | 6 to 13 feet (2 to 4 meters) |
| Exposure | full sun, part sun |
| Soil | enriched ordinary soil |
| Flowering | early autumn to late autumn |
| Hardiness to freezing | 23°F (-5°C) |
Sometimes called giant dahlia, it can reach a height of 13 feet (4 meters)!
Dahlia imperialis blooms in winter but cannot withstand temperatures below 23°F (-5°C). You must thus first check that your climate allows for planting before purchasing it.
Dahlia imperialis bulbs are best planted during the months of mid-spring and late spring, when the last frosts are past.
Plant them about 4 inches (10 cm) deep, and space the bulbs 40 inches (100 cm) apart, because dahlia imperialis requires sunlight and likes having space to spread out!
Dahlia imperialis is a flower that requires a little specific attention if you want to see it bloom year after year.
Dahlia imperialis is inclined to rather warm, rich and well drained soil.
Tree dahlia also appreciates being watered in summer but fears excess moisture that could destroy the plant.
If you fear particularly strong freezing:
After the blooming, at the beginning of winter, bring the tubers indoors when the first frost spells hit, and keep them in a dark, dry place until the following spring.
As mentioned above, the watering of tree dahlia is important because it must be regular but never excessive.
One of the most common problems that this plant meets is the rotting of its roots. It thus requires very well drained soil.
Water at the base in one slow, generous soak rather than a daily splash, so moisture reaches the tubers instead of evaporating from the surface.
Dahlia imperialis cannot survive freezing temperatures.
If you live in an area where the weather frequently drops below 23°F (-5°C), bringing your bulbs indoors before it freezes is your only hope.
This can be performed after the blooming and before temperatures have dropped under 23°F (-5°C).
In other regions, protect your tubers with dead leaf mulch for instance.
The major difficulty encountered with dahlia imperialis is the rotting of the tuber.
This happens when the soil is too moist and doesn’t drain well enough, thus preventing excess water from being evacuated.
This also leads to various fungus such as powdery mildew or gray mold.
As for parasites, the dahlia can occasionally fall victim to aphids.
Slugs and snails go straight for the soft young shoots as they push through, so check around the base of the clump after rain.
Trap earwigs in a rolled newspaper laid near the stems, as they chew the petals and foliage at night.
Inspect the lifted tubers while they are in storage and cut any soft brown patch away with a clean blade, discarding those that have gone mushy right through.
Tree dahlia asks for the right job at the right moment, from sowing under cover to lifting the tubers. Here is the sequence the article describes, gathered in one place.
| Period | Task | What to do |
|---|---|---|
| early spring | Sow under cover | Start the seed in a tray, then repot in mid-spring and plant out in late spring |
| mid-spring and late spring | Plant the bulbs | Set them 4 inches (10 cm) deep and 40 inches (100 cm) apart, once frosts are past |
| Spring | Propagate | Divide the tubers to increase your stock |
| Summer | Water | Regular watering, never excessive, preferably in the evening and never on the foliage |
| early autumn to late autumn | Flowering | Remove wilted flowers regularly to keep the display going |
| Beginning of winter | Protect or lift | Mulch the base, or bring the tubers indoors to a dark, dry place |
Follow that rhythm and the same clump will tower again year after year.
Native to Mexico, the dahlia imperialis or tree dahlia exhibits a very peculiar stem similar to bamboo.
Its flowers are as beautiful as ever, a cute pink that can also be white, and always very large. Tree dahlia flowers can grow to be 8 inches (20 cm) across.
In temperate climates, the foliage is evergreen.
It is also called Imperial Dahlia, tree dahlia, Lehmann dahlia or Bamboo dahlia because of the strong resemblance of its stem with bamboo.
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This is a plant that can grow quite large and will always be at its best in a sheltered spot where it can grow while being protected from wind.
