The Tree Dahlia

By: Mary Wilbur
Photo: Ted Pease

Mary Wilbur, a native of Wales, worked in the field of psychiatric care, and has gardened in England and New…

More From This Author
Tree dahlia (Dahlia imperialis) flowering in November in a San Francisco garden. Photographs by RGT

Tree dahlia (Dahlia imperialis) flowering in November in a San Francisco garden. Photographs by RGT

I saw my first tree dahlia a few years ago, as I was walking down a street in the north coastal town of Arcata, California, glancing into gardens on my way. There it was: a multi-stemmed tree in full bloom in mid-November, and close to twenty feet tall. Its light lavender pink, single flowers looked like large, delicate dahlia blossoms. It crossed my mind that someone had given a garden dahlia a dose of Alice’s “eat me” pills for it to develop into this arboreal beauty. I am no lover of the common garden dahlia, but this one captivated me.

My immediate search of local nurseries for a dahlia like this was unsuccessful. Eventually I found a small one at the Farmers Market and planted it with great expectations, only to have it quickly devoured by slugs. A second tree met the same fate, even though I took more care to protect it. I was prepared for failure with the third attempt, but, once started, the tree grew well and flowered in the following November.

When that plant died back after its first flowering, I feared it had perished, for the remaining stem looked like nothing more than a dead stick. We were well into summer before it showed signs of life again. It made up for its tardiness by growing at great speed. By its third year, it was fifteen feet high and full of flowers. The saucer-sized blossoms are unassuming, peering down modestly from the top of the tree, as they drift their pale lilac way above the handsome, heavily divided leaves.

Outfacing blossom of Dahlia tenuicaulis flowering in the New World Cloud Forest collection at San Francisco Botanical Garden in July

Outfacing blossom of Dahlia tenuicaulis flowering in the New World Cloud Forest collection at San Francisco Botanical Garden in July

Named in honor of Dr Dahl, a student of Linnaeus, the first garden dahlias brought into cultivation were also single-flowered and unpretentious, but they have suffered increasingly elaborate development (mainly in Europe) since the late nineteenth century. The petals of the today’s single-flowered dahlias seem stiff and rigid compared to the ethereal quality of a tree dahlia’s flowers.

Like the ancestors of common garden dahlias (Mexico’s national flower), the tree dahlia (Dahlia imperialis) originated in the mountains of Mexico and Guatemala. Botanists accompanying the Spanish conquistadors made collections of New World plant life, discovering tree dahlias in the process. (I imagine these sober scientists, eyes cast to the ground, tagging along behind the heedless soldiers galloping far ahead.) The Aztec name for the tree dahlia was acocotil, meaning water-cane. The hollow stems of the tree dahlia were, in fact, used for transporting water and as a source of potable water for hunters and travelers.

It took 200 years for the first dahlia seeds and plants to find their way to Europe by way of the Royal Botanical Gardens in Madrid. The first tree dahlia grown in Europe was at the Zurich Botanic Gardens in 1863; it was described as having white blossoms tinged with blood red. Most tree dahlias are now described with pale lavender flowers.

Dahlia excelsa, found in the valley of Mexico in 1834, is fifteen to thirty feet in height, like D. imperialis, but produces flowers of a pale rose purple. Some consider the two species to be one and the same; the flower colors on plants I have seen lend credence to that theory, often leaning more to rose purple than pink. A double pink form is common in cottage gardens of Mexican farmers.

The San Francisco Botanical Garden has grown tree dahlias since 1969. Around that time, the late Wayne Roderick, a well-known Bay Area plantsman, introduced about ten different clones. Currently, the botanical garden has about the same number of different forms of this plant. The most recent were collected in the wild in Guatemala. There are single whites, single lavenders, single pinks, and a double white with a pom-pom center. There is also a single pink that often throws a few double flowers.

The botanical garden also has another species of dahlia (Dahlia tenuicaulus), which is similar to D. imperialis but with smaller, darker lavender flowers; it blooms much earlier (August or September) and stays in bloom for a long period, sometimes year round. It also tends to have a shrubbier habit, with many more stems, and seldom exceeds about eight feet in height.

Whatever the name, I love my tree dahlia. Attractive to bees and butterflies, it does well in my acid soil, tolerates full sun or light shade, needs only average water, and, above all, it blooms in November when little else is in flower. That could be a mixed blessing, as tree dahlias are sensitive to frost. From November on, I anxiously watch the weather and look for rime on the grass each morning. I am lucky to live near the ocean where frost usually arrives later in the year than it does further inland. It is best to plant tree dahlias in a sheltered spot, as high winds can break off the slender stems. [Sunset suggests zones 4-6, 8, 9, 14-24]

Tree dahlias can be propagated by cuttings from any part of the stem or by dividing the roots. Don Mahoney, curator at the San Francisco Botanical Garden, routinely cuts up long trunks into twelve- to eighteen-inch segments with one or two nodes and sticks them upright in a gallon can of potting mix. He places them in moist shade until they start growing. Most of the cuttings handled this way root easily.

When several clones are grown in close proximity, bees cross-pollinate them, and a lot of seed can be produced. Mahoney has developed a new clone that flowers earlier and continues for a longer period than its parents. It will be tested at the botanical garden and will eventually be named. Good news for anxious frost watchers like me.

We had some exceptionally cold nights this past winter, but my tree dahlia survived and is thriving. Once its roots are established, a tree dahlia is fairly resistant to frost, though the flowers and foliage may be damaged. If I should lose this one, I shall try again; fortunately, they are fast growers. I would not want my garden to be without one.

Web
Extra
The Tree Dahlia Resource Guide