A tree dahlia isn’t actually a tree at all. It is a very tall dahlia that gets its name from its height. The tree dahlia can attain a height of 20 feet (6 meters) and has been reported to grow as high as 30 feet (9 meters) in its native Central American habitat. This tuberous perennial sports showy blooms around 6 to 7 inches (15 to 18 centimeters) in diameter that range from a pale purple to pink with yellow to orange centers. These flowers grow in stunning sprays a couple of feet across with dozens of blooms.
The flower sprays grow at the top of the stems, which are segmented and hollow, and which closely resemble bamboo. These long stems can attain a diameter of 4 inches (10 centimeters) and are so brittle that they can be broken if someone bumps against them. A strong gust of wind can destroy the whole plant by breaking every stem, so the tree dahlia grows best in a location sheltered from the wind. Some gardeners stake the plant every few feet to prevent breakage.
A tree dahlia blooms in late autumn, providing a gorgeous splash of color long after many other flowers have disappeared. In warm climates with mild winters, it has been known to bloom until early spring. It is not cold or heat tolerant, and will die back at the first frost unless it is sheltered. The tree dahlia generally thrives in full sun, but may require afternoon shade in regions where temperatures are hot. It is a fast grower that does well in tropical and subtropical regions, and can also thrive in colder areas.
The tree dahlia should be pruned back almost to the ground in the winter, unless it is in a mild climate, where it blooms throughout the cool season. The tops can be trimmed during the summer if a shorter, bushier plant is desired. Tree dahlias require well-drained soil and cannot tolerate extreme dry or wet conditions.
In climates where the temperatures drop below 25°F (4°C), it is recommended that the tubers be dug up for the winter and replanted in the spring. The tree dahlia is easy to propagate by cutting off part of a stem that has at least one node and planting it horizontally in the soil. It is also available at some nurseries, and an Internet search will provide a list of nurseries that will send tree dahlias through the mail.