Updated: Apr 26, 2022
Imagine a plant lying dormant all through winter looking like a few old remnants of desiccated bamboo. Then in spring, little tuffs of bright green begin to emerge either at the former leaf nodes or from the ground. As they grow, we see they are obviously not bamboo but the lush green compound leaves of dahlias...only bigger. Yet, this is no ordinary dahlia, although most dahlias are far from ordinary. The seasons turn from spring to summer, and Dahlia imperialis is rocketing upwards growing up to ten, fifteen, even twenty feet or more in just one season. When it reaches its apex of growth as we (still) turn our clocks back, masses of flower buds emerge right at the top, slightly hanging downward, opening to look down upon us as I'm sure imperial nobility would do.
Tree dahlias bloom at a tenuous time. We've grown them for years in various locations here in Northern California, and every autumn season we are nearly holding our breaths waiting to see if the buds will avoid frost damage. Frost will turn this behemoth into withered and blackened sticks, the lavender blooms shriveled before they fully open. Not this year! Frost has not come yet, and its location looming over our pool has probably helped with maintaining just enough warmth through the cold mornings. The photos I am sharing below are from this year's growth and bloom. I look at it every morning, feeling blessed it has had time to bloom long enough to scatter pastel petals over still water, reinforcing that autumn is my favorite time of year.
Its bloom time makes planting in gardens so rewarding, and I am certain blooming will be better guaranteed in less frosty regions. Keep in mind imperial dahlias do not enjoy much heat either; they are native to higher altitudes of Mexico and nearby countries. They prefer persistently moist and fertile soil, otherwise their leaves easily turn yellow or brown as they continue their growth ascent. For California, they may be best suited in coastal gardens without much wind. In times of drought, this is the plant to allocate shower water collected as one waits for warm temperatures. Is it worth it? To the hobby gardener like me, most definitely, but for most landscape architects, clients will not be interested in this royal's tedious demands.
There is, however, another reward as a plant to share. Over time, each plant will continue to multiply requiring later divisions. As an oddity, this somewhat rare species are sought after by others that might like a division for themselves, as with so many other bulbs and perennials. Sharing is a great way to get to know other avid gardeners in your area, so consider adding to your own garden if you have the right conditions...and motivation. Oh, and great cut flower, too, especially if frost is eminent.
'Double White': Whereas the species produces lavender flowers with a single layer of rays, the 'Double White' has countless pure white petals looking similar to a fanciful chrysanthemum. The plant will not grow quite as large, topping out at twelve feet. When we had it, it grew to only eight feet, and I am not recalling if it ever bloomed for us. That's a reminder for me to try again.
An excellent video capturing the scale of the plant just before blooming.
Botanical Name: Dahlia imperialis
Dahlia: Honoree, botanist Anders Dahl
Common Name: Tree dahlia
Family Name: Asteraceae
Origin: Mexico, Guatemala, Panama
Garden Themes: Woodland margin, cottage, tropical, cutting, pollinator
Uses: Border, specimen, accent, floriculture, mass
Type: Herbaceous perennial grown from tubers (multi-stem)
Size: 20' tall and 6' wide
Outstanding Feature(s): Flower, size
Stem (Cane): Green but may turn purple when exposed to sun with prominent leaf attachments appearing similar to bamboo segments.
Type: Bipinnately to tripinnately compound
Shape: Deltoid with lanceolate leaflets
Margin: Serrate, lobed
Color: Medium green
Flower: Showy panicle of pendulous, large, daisy-like blooms with lavender rays and yellow disks. Long lasting bloom season if no frost.
Fruit: Brown achene, usually removed before maturity due to frost.
cultural requirements, tolerances & problems
Sunset Zones: 4-6, 8, 9, 14-24
USDA Zones: 8-10
Light: Sun to partial shade
WUCOLS SF Bay Area Hydro Zone: Moderate
Texture: Sand, loam, well composted
Moisture Retention: Even moisture without sogginess, well-drained
pH: Slightly acidic
Problems: Slugs, wind (limb/leaf breakage)
Branch Strength: Brittle
Insects: Leaf miners, spider mites, aphids
Disease: Rot, powdery mildew
citations & attributions
Bayton, R. (2019). The Royal Horticultural Society's the Gardener's Botanical: An Encyclopedia of Latin Plant Names. London: Mitchell Beazley.
Norris Brenzel, K. (Ed.). (2012). The New Sunset Western Garden Book. New York: Time Home Entertainment, Inc. (See Dicentra spectabilis, p. 287).
Water Use Classification of Landscape Species. "WUCOLS IV Plant List." University of California, Division of Agriculture and Natural Resources, Davis. Accessed on December 1, 2021, from https://ucanr.edu/sites/WUCOLS/Download_WUCOLS_IV_List/.
Photos by TELCS.