As many as twenty species of trees may inhabit a hectare of temperate U.S. forest, whereas tropical forests often hosts more than 250 species per hectare. In fact, just fifty hectares of Borneo or Amazonia rain forest contain the diversity level found in the temperate zone forests of North America, Europe and Asia combined. While tropical biologists continue puzzling over the reasons for such diverse and successful coexistence, gardeners relish ornamental tropical trees.
Trees in the Tropics
The Equator lies exactly midway in the tropical band girdling the Earth between the Tropics of Cancer and Capricorn. In addition to the typically envisioned steamy jungles and lush rainforests, tropical habitats include deserts, savannas, wetlands and woodlands. Trees in more than two-thirds of the tropical belt must endure annual drought. Tropical trees include palm trees, conifers, broadleaf evergreen and deciduous trees, and trees with luscious fruits or flamboyant flowers. Tropical landscape trees thrive in the no-frost regions of U.S. Department of Agriculture plant hardiness zones 10 and 11, although many varieties are surprisingly cold tolerant and adapted to lower USDA zones.
Tropical Palm Trees
The shrubby, tropical needle palm (Rhapidophyllum hystrix) is considered the world's most cold-tolerant palm. Eventually reaching 6 to 8 feet high and wide, its black 4-inch-long needle-like spines form at the leafstalk bases of its fan-shaped leaves. The subtropical Chinese windmill palm (Trachycarpus fortunei) graces temperate landscapes with its tropical flair, reaching 30 feet high at maturity with a 10-foot-wide crown. Both palms survive down to USDA zones 6b to 7a. The Brazilian native queen palm (Syagrus romanzoffianum) matures at 50 feet tall with its elegant 20-foot-wide crown adorning a straight trunk. Graceful, slow-growing young palm trees adorn bright indoor rooms and patios as container plants.
Tropical Conifer Trees
Tropical evergreen conifers (Araucaria spp.) include the Norfolk Island pine (A. heterophylla), maturing at 100 feet tall, and the bunya-bunya tree (A. bidwillii), growing to 80 feet. The bunya-bunya survives down to USDA zone 9. The most cold hardy of the Araucarias is the monkey puzzle tree, a Chilean native (A. araucana) suited to USDA zones 7 through 10, which grows slowly to 90 feet tall and 30 feet wide in its native habitat, but usually remains under 30 feet tall in cultivation. The monkey puzzle tree's ropelike branches terminate in clusters resembling starfish. Mature tropical conifers regularly drop their 10- to 15-pound cones, making them ill advised as shade trees, but they are excellent long-term patio container plants.
Tropical Flowering Trees
The umbrella-shaped royal poinciana (Delonix Regia) tree graces cultivated tropical gardens throughout the world. Brilliant red flowers nearly obscure its ferny foliage in the spring and summer. The 25-foot-high-and-wide tree-of-gold's (Tabebuia aurea) brief but dazzling 2-week flowering display erupts in sunshine-gold among leafless spring branches. The Hong Kong orchid tree (Bauhinia blakeana) and the purple orchid tree (B. purpurea) bloom in autumn through winter with flamboyant lavender, orchid-like flowers on charming, 20-foot-high trees. Frangipani (Plumeria rubra) blooms more than half the year beginning in spring with fragrant flowers in pink, yellow, white or red in varying combinations according to variety. The 20-foot-high trees make excellent container plants in youth.
The sugar apple, or cherimoya (Annona cherimola x A. squamosa), naturally grows to 30 feet tall and wide. Long hairy leaves that drop in spring precede fragrant fuzzy brown or yellowish flowers that persist for up to 4 months. Harvest the heavy, warty fruits in late fall or winter to enjoy a creamy, pineapple-banana-flavored treat. Knobby red lychee (Litchi chinensis) fruits hang in decorative clusters among the narrow willowy leaves of 40-foot-high-and-wide trees. Small-pitted varieties such as "Groff" and "Kaimana" make it easiest to enjoy the luscious, perfumed fruits. Citrus varieties abound for low-frost to frost-free climates, producing especially sweet fruits in tropical regions. Dwarf varieties make excellent container or greenhouse plants anywhere.
- Floridata: Delonix Regia
- University of California Botanical Garden at Berkeley: Garden Sections
- United States Department of Agriculture Forest Service: Tropical Community Tree Guide
- Sunset Western Garden Book; Kathleen Norris Brenzel, ed.
- Virginia cooperative Extension: Introduction to Cold-Hardy Tropicals for Virginia Landscapes
- University of Florida IFAS: The Ten Most Popular Tropical-Subtropical Fruit Trees for the Home Landscape
- University of Florida IFAS Extension: Tabebuia Aurea
- Floridata: Plumeria Species
- Jack Hollingsworth/Photodisc/Getty Images