Tree Dahlias

Dahlia imperialis is trying to bloom for me. It will be a race against dampness and the possibility of frost. So far, so good, but some mild sunny weather would help. Maybe it will do better next year.

This is one of those completely herbaceous plants on a tree-like scale. It can easily hit 6 meters and I think I have actually seen taller than that. But each year all of that growth will be sacrificed, and the plant dies back to the fleshy roots. Like other Dahlias, its fat roots are not really tubers. If one breaks off it won't regrow, and if the stem dies or the buds die, the plant is dead.

What's the use of growing a perennial the size of a tree? Well, it's just different-looking in an appealing sort of way. If you ever wanted your own fairy woodland, this would be a good “tree” for it. Grow a bunch of them, at least a small grove. The stems are vaguely cane-like, and the ferny leaves big and twice-divided. The flowers are a pale shade of purplish-rose, and are reputedly fragrant like vanilla cookies. I'll have to take hearsay as a matter of faith, because they are far too high above my head to have any hope of smelling them. I've noticed vanilla cookie fragrance in other Dahlias, tho. This would be a fun plant to grow if you happened to have a terrace or balcony above it at just the right height to enjoy the flowers.

Not quite as big, but beautiful in perhaps an even more unusual way is Dahlia tenuicaulis. It's “only” about 4 meters tall, but it blooms more reliably than its bigger cousin, and the foliage has an unusual and striking light purple cast to it, especially on the newer leaves.

These tree-like Dahlias are native to the highlands of southern Mexico and northern Guatamala. The elevation and cloudcover keep summers cool, while being in the tropics keeps winters relatively mild for the elevation. They are adapted to the northwestern USA, some of the cooler parts of the southeastern USA, and much of maritime western Europe, wherever their hybrid cousins grow. They need good drainage to get through our wet winters, and vigilant protection from slugs in the spring when their shoots are trying to come up. Slugs are probably their biggest enemy, because they can actually kill the plants by destroying the new shoots. Otherwise, they are easy to grow in typical garden conditions, in full sun and fertile, well-aerated soil (a slightly sandy loam is good).