A few years ago, a friend in San Francisco mentioned he had pruned a "10 foot tall tree dahlia." I shot back in a know-it-all tone, "There's no such thing as a tree dahlia. That's insane. It must be some other genus that has dahlia-like flowers." One tends to become a skeptic after learning the hard way as a budding gardener that there's no such thing as an "air fern" or a "tree tomato". A quick flip of the Western Garden Book and I was eating tree dahlia crow. A tree dahlia really is a dahlia. Dahlia imperialis. It can grow to a whopping 12 feet. Who knew? Not even this someone who aced several plant i.d. classes in college.
Two years ago, I saw... and touched... and photographed... my first tree dahlias in Mendocino, CA. On a coolness scale of 1 to 10, tree dahlias are a 10. I had to have one.
Last December I put out a query on GardenWeb re: tree dahlia sources and on whether or not anyone grew them in hot-summer, cool-winter Sacramento. Towering tree dahlias, with their striking pink (or white) blooms, are not common in the Sacramento area. In fact, I've never seen them for sale at a local nursery and I've never seen one growing and about to bloom in a local garden. Until now. In my garden!
A generous gardener from Calistoga offered to swap with me. He got starts of my 'Tropicanna' Canna and I got hefty stem cuttings of pink Dahlia imperialis. I was encouraged by the fact that Calistoga gets nearly as hot in the summer as it does here and if this man's tree dahlias thrived despite summer heat, then they stood a chance in my garden. I found a sheltered spot in the yard where they'd get sun most of the day but would be spared in the late afternoon. Because they were up against the house on the south side, they'd also be protected from strong north winds.
A previous attempt to grow tree dahlias from cuttings shortly before my GardenWeb exchange was a failure. A dear friend and fellow plant addict with the most kick-ass garden on the planet gave me a couple cuttings from her brother via a friend in San Francisco. For some reason, I decided to start my cuttings in my greenhouse, in sand. Not successful. My friend planted her starts in containers using regular potting soil and her tree dahlias are now in their second season and are neck-achingly tall, but with no flowers yet. All I can guess about why hers haven't bloomed yet is that the cuttings she started with were small compared to the whoppers I got from Calistoga. Or... is it divine justice for all the flowers blooming in her yard that aren't in mine?
Nah... probably just
Bigger stalk cut closer to the base = more nodes rarin' to grow
When I saw buds on my plants, I figured it'd be a good idea to apply some Fox Farm Tiger Bloom since I couldn't remember the last time I fed them. In fact, I probably hadn't, at all, ever.
Stem cuttings of tree dahlia resemble bamboo. I was instructed to plant them horizontally in containers. Shoots arise vertically from horizontal nodes on the stems. My plants are still in 5-gallon containers but when it's time to cut them back in December, I intend to plant the potted tubers in the ground and will stick a few new cuttings in pots in case my soil proves to be too heavy for good in-ground growth.
I hope to be able to share cuttings with friends and family who haven't yet been invited to join the church of the tree dahlia. I always find myself saying, "This plant is so cool. You have to grow this." Plant evangelist, yep, that's me.
If you want to try growing tree dahlias in northern California or beyond, check your local nursery; Kudos to any nursery carrying collector plants like this. You might also try your local dahlia society, and I certainly recommend online plant swapping through GardenWeb's Plant Exchange or Dave's Garden! Annie's Annuals sells the double white form and the pink form, by the way. In fact, tree dahlias might not be considered such a big deal in the bay area and other parts of the coast. Still... a dahlia... up to 12 feet tall? Dang!
Just came across the following link while doing a search for photos of the stalks. It's an ebay dealer in San Francisco that sells all kinds of strange wonderful things, including Dahlia imperialis. Hey, guess what the business name is? StrangeWonderfulThings.com. They have a great photo of the base of a mature tree dahlia plant. Also, they say a tree dahlia can reach twenty feet! OMG.
Ok, here's the official Sunset Western Garden Book entry on D. imperialis--
D. imperialis. TREE DAHLIA. Zones 4-6, 8, 9, 14-24. Multistemmed tree grows each year from permanent roots to a possible 10-20 ft. tall, 4-6 ft. wide. Daisylike, 4-8-in.-wide lavender flowers with yellow centers bloom at branch ends in late fall. Leaves divided into many leaflets. Frost kills tops completely; cut back to ground afterward. If tree dahlia were longer blooming or evergreen, it would be a valued landscape plant, but anual dieback relegates it to tall novelty class. Available from specialists; seldom sold in nurseries. Grow from cuttings taken near stem tops (or from side shoots) in fall; root in containers of moist sand kept in a protected place over winter. Or dig root clump and divide in fall. Give full sun or partial shade. D. excelsa, D. maxonii are similar.