I’m one of those glutton-for-punishment people who like to try plants which will be virtually impossible for me in USDA zone 5. So, I indulged in what might be called "the height of folly" by purchasing seeds of Weeping Tree Dahlia (Dahlia campanulata) and a cultivar of Giant Tree Dahlia (Dahlia imperialis) last autumn and planting them this spring.
Tree Dahlia Details
Perennial in USDA zones 9 to 11, the weeping dahlia grows 8 to 10 feet tall. Its 7-inch single, orange-centered white blooms, which sometimes dangle like the bells implied in campanulata, wear cerise “eye-liner” at the bases of their petals. Perhaps those red-rimmed eyes explain the weeping nickname, but that also could be attributed to the flowers hanging their heads.
The giant king of the dahlias (imperialis) shoots up like Jack's beanstalk and can reach 20 feet in USDA zones 8 to 10. Its 4 to 8-inch orange-centered blooms usually are single lavender, pink, or white but—in this case—were supposed to be ‘Caerulea,’ a blue shade instead. Some imperialis cultivars reportedly can produce double flowers too.
Tree Dahlia Druthers
Because they originated in the mountains of Mexico, tree dahlias seem to prefer somewhat cool conditions with protection from afternoon sun. So, the vegetable garden where I set some of them probably was ideal due to the trees on its western side. The dahlias did grow quite tall there, well over my head, while those I planted in a sunnier bed in front of the house didn’t achieve the same height or vigor.
Unfortunately, though, the masses of buds I did get on one of the vegetable garden plants failed to open before our first killing frost, even though it came late this year—in early November. And even though I'd warned the dahlias frequently that they were dilly-dallying too long! There’s probably good reason why most of the information on them comes from California gardeners, since I’m guessing these plants will work best in coastal climates which are both somewhat cool and relatively frost-free. (It would be very difficult to cover a tree dahlia.)
Unfortunately, I failed to mark which type was which, though I noticed that one has smooth bamboo-like stems and the other ridged ones. When I dig the tubers, I may have to label them as either “groovy” or “un-groovy!”
There is yet another bushier tree dahlia called tenuicaulis, meaning “slender-stemmed,” which I have yet to get my greedy hands on. Also known as Everblooming Tree Dahlia, it probably would make a better choice for me, since it reportedly doesn’t wait until late autumn to perform but begins flowering in August or thereabouts. Also hardy in USDA zones 8 to 10, it can reach 15 feet with 3-inch orange-centered lavender blooms.
Tree Dahlia Derivation
And, yes, you can grow other dahlias besides tree dahlias from seed as well. Those smaller ones usually do bloom before frost, though all dahlias seem to be somewhat dilatory! As I mentioned in The Great Dahlia Experiment, some of the more common varieties may need shorter days to initiate bloom just as the tree dahlias do. To sow the seeds, simply cover them with about ¼ inch of damp seed-starting mix, keep them warm, and the resultant seedlings should pop up in 3 to 5 days. Tree dahlias root easily from stem cuttings too.
So, am I fazed by my failure? Not at all. My theory is that my tree types should bloom earlier next year now that I have tubers rather than seeds. And Longwood Gardens performed an experiment on Weeping Tree Dahlia once, proving that it could be forced into early bloom by subjecting it to short days earlier in the year than those days usually come. Now, if I could just figure an easy way to do that without people thinking I was out of my tree. . .
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Photos: The tree dahlia flower images are courtesy of AnniesAnnuals, from the Dave's Garden PlantFiles. The other photos are my own.