Dahlias are native to Central America and are not hardy in cold winters.

Where winter temperatures drop below 20 F, gardeners have to dig up the tuberous roots of common dahlias and store them indoors. Here in the Bay Area, you don't really need to dig them every year, because they will survive our winters anyway, but some people still do it to divide the roots and make more plants. If you don't need more plants, you can leave common dahlias alone until they get crowded, which usually takes about three years, and divide them then.

Tree dahlia (Dahlia imperialis) is also from Central America, ranging into Colombia. Like common dahlias, it is not very hardy, so if you were trying to keep it alive over winter where winters are cold, you would have to dig it up and store it. However, it isn't grown in cold winter climates, because it doesn't bloom until November or December, and by that time it would have frozen to death. Here in the Bay Area, it is hardy in our mild winters. We enjoy the late bloom period, and we don't dig it up after it blooms.

However, the plants do look pretty bad by spring, so gardeners cut the cane-like stems to the ground, usually during March. The plant will grow fresh new stems in summer that will bloom next fall or winter. One of the definitions of "imperial" in my dictionary is "of superior or unusual size or excellence." It is clearly the definition the botanist thought of when he gave this species its name. This plant towers 10 to 20 feet tall, and then, at the tops of its stems, forms white or lavender flower heads that are 4 to 6 inches across. The swaying stems topped with large flowers command attention. If you like suspense in the garden, waiting all summer for the huge plant to put on its glorious winter show, tree dahlias may be for you.

But be sure to plant your imperial dahlia where strong winds can't reach it, because they will shred its leaves and flowers, turning the plant into a disappointing, broken mess. If you want to propagate tree dahlia, you can do it by planting some of the canes that were cut in spring. Cut pieces with at least three nodes (rings with buds). Keep them right side up and bury them in a container deeply enough to cover a couple of the nodes until you see shoots growing. Or you can dig the whole plant and divide it, but this is not necessary unless your clump has grown for many years and has become too large for its site.

http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2008/01/09/HOJCU7TQI.DTL