The tree dahlia (Dahlia imperialis) gives Texas gardens plenty of bang for their buck. Tree dahlias grow to the size of small trees with huge blooms appearing in late fall and continuing through the winter and spring if no frost occurs, making it a true showstopper in winter gardens. While the tree does not flower if a surprise frost occurs, the thick, tall stems of the plant provide enough reason to include it in the garden.
Dahlia trees reach giant proportions, but the trees are actually tuberous perennials just like dahlia plants. The trees grow up to 30 feet in height in their native Central American home. But most reach 8 to 15 feet in height in Texas gardens before pruning each winter. The dahlia tree sports tropical-looking dark-green foliage and stems that resemble thick, hollow bamboo. The stems easily reach 3 to 4 inches in diameter. In early fall, the tree puts forth a profusion of pinkish-purple flowers with orange centers. The flowers reach 8 inches in diameter. In some gardens, early frosts stop the plants from blooming, but the thick stems make the plant interesting anyway.
Several different tree dahlias with different types of blooms give gardeners some choices as to what works best in their gardens. The California angel tree dahlia grows up to 8 feet tall, featuring big double white blooms that appear in mid-November. The double or nothing tree dahlia sports beautiful lavender double-flowers growing on top of 9-foot-tall stems.
Tree dahlias prefer full or partial sun in hardiness zones 8 to 10 where they stand the best chance of blooming before frost strikes. The trees prefer well-drained soil in areas protected from early frosts. After planting, the ground should be mulched to protect the tubers from any late spring frosts. As the plant gets established during the first four to six weeks, it requires thorough watering and protection from mid-day sun.
Once the plant establishes itself, it needs regular watering. Monthly feedings of vegetable fertilizer work well to encourage the plant's fast growth. In early fall, switch to a fertilizer with more potassium to encourage blooming. The somewhat delicate plant requires protection from the wind, especially when the flowers bloom. After the flowers fade away, the dahlia tree takes a good pruning right down to the ground. New shoots appear in the spring.
Aztec Indians used the bamboo-like canes of the tree dahlia to carry drinking water, thanks to the stem's hollow core. Nowadays, gardeners use the plant for a showpiece that becomes the focal part of the garden in late fall when the flowers start to bloom. The plant often continues blooming through the winter and into spring, making it a great garden specimen for color when everything else looks dry and brown.