Tropical Trees - Up North?
The solution for having a tropical tree in your garden up north does not involve growing one from a tuber. Instead, you want to select a hardy tree that looks tropical enough to do the job. This can be done with a careful choice of your tree, or by training an otherwise non-tropical tree to look tropical. Since a number of tropical trees are legumes, or members of the bean family, you can choose a hardy species of legume tree as a substitute. An example is the Black Locust, Robinia pseudoacacia (see thumbnail picture at right), which has nice white clusters of blooms and ferny leaves that are plenty tropical looking for our purposes. Another example of a legume tree for your tropical garden is Cercis, the Redbud. You can also choose a "disposable" tree, of which the best example is the Castor Bean, Ricinus, with tropical palm-like leaves and the ability to grow pretty large during one growing season. I call it "disposable" because cold will kill it, but you can grow more from seed next summer!
As far as training a non-tropical tree goes, one that I used in this manner was the Staghorn Sumac (see photo at left). The technique is to prevent the plant from growing out more than one stalk and bud. You do this by pinching off all buds except the terminal one on a straight, unbranched stalk. What happens then is the one bud that does grow out will have larger than normal leaves, and thus it ends up making the plant look more like a palm than a sumac! This would work even better with Ailanthus altissima (Tree of Heaven). Plant and train them in a small group for a bold tropical effect, but don't forget to pinch off any flower stalks to prevent seeding.
Some hardy trees look fine and tropical just as they are. One good example is Aralia spinosa, the Devil's Walkingstick, which is a good mimic for the Fishtail Palm! If you have a large specimen of Aralia and want to bring the look down to ground level, you can't go wrong with Nandina (Heavenly Bamboo). Although Nandina is not true bamboo, you can also find several hardier varieties of real bamboo to add an oriental touch to your tropical planting. Somehow, the smooth ringed stalks of bamboo lend an extra air of tropicality to your landscape, even if the types you choose are hardy in snow and freezing temperatures!
Big Fuzzy Leaves (and Flowers, Too!)
A couple of other trees to consider for your "tropical" canopy are the Empress Tree (Paulownia tomentosa) and Catalpa. Both trees have large leaves and showy flowers that are not typical of northern trees. The Empress Tree is a very rapid grower that produces large soft fuzzy leaves, and pink to purplish blooms when mature. Catalpa trees have showy blooms from white to yellow and the northern Catalpa has larger leaves than the southern one does. Both Catalpas produce long bean-like pods, although these trees are not legumes.
Many of the trees I've presented in this article, such as Tree of Heaven, may be considered weedy or invasive in some areas; check with your local extension office to be sure that you choose the most beneficial and environmentally friendly ones for your area.
Consider placing your chosen trees either at the northern side of your planting, or in areas where you wish to have shade for other plants that need it. If your tropical planting has a northern exposure, as opposed to backing up to a structure or solid fence, you should also consider a protective windrow planting of hardy dense evergreen shrubs or trees behind your "tropical treeline" to break the wind and help form a microclimate area. This will help extend your growing time a little, both in the Fall and in the Spring.
Image credit: Public Domain and GNU Free Documentation License