Tropical plants, those that are native to frost-free regions closest to the equator, add an exotic look to gardens, either in containers or when planted as summer annuals. In U.S. Department of Agriculture hardiness zone 7, winter low temperatures drop to 0 to 10 degrees F. annually. The cold winter limits the types of tropical plants that can be grown outdoors year 'round, but some plants do demonstrate an ability to overwinter in their underground roots and rejuvenate in late spring.
While the long summer growing season in zone 7 allows tropical plants to prosper, the key is choosing tropical plants that meet your garden and design expectations. If you merely want the tropical plant to survive alongside other plants and be killed by the fall frost, your choice of plant is limited only by budget and availability at a plant nursery. There are also several choices for enduring tropicals that act like perennials or shrubs and survive the winter.
Examples of Hardy Plants
While the ability of a tropical plant to survive the winter in zone 7 is also affected by soil types, rainfall, elevation and duration of subfreezing temperatures, several tropical plants are reliably hardy. Palms include needle palm (Rhapidophyllum hystrix), windmill palm (Trachycarpus spp.), dwarf palmetto (Sabal etonia) and jelly palm (Butia capitata) if shielded from cold, dry winter winds. Crinum lilies (Crinum spp.) that are proven to survive chilly winters, such as cultivars "White Prince" and "Infusion," are also worthwhile. Ginger lilies (Hedychium spp.) die back in fall and overwinter underground in their fleshy rhizome roots. Other tropical shrubs that will overwinter and re-sprout in spring are angel trumpets (Brugmansia spp.), canna lilies (Canna spp.), bananas (Musa spp.), candy corn plant (Cuphea micropetala) and purple heart (Tradescantia pallida 'Purpurea'). If winter is dry, many tropical or subtropical succulent plants will overwinter well in zone 7. Examples include some species of prickly pear (Opuntia spp.), beargrass (Nolina spp.) and century plants (Agave spp.).
Tropical plants that may be marginally hardy (those that may or may not survive the winter, depending on how cold it gets) can be grown in containers outdoors on the patio in summer and then, once winter comes, brought indoors to a sunny room. In winter, these plants do not need high heat and lots of water but merely to be kept cool and dry and free from cold. Rainforest tropical plants like orchids, ferns and bromeliads need a warmer winter, so keep these more cold-tender plants in the house where temperatures remain around 65 to 75 degrees F. In mid- to late spring, well after the threat of frost passes, these plants may be again brought outdoors.
Once fall frost kills back leaves and stems of the hardy tropical plants outdoors, placing a thick layer of loose, coarse mulch over the roots or underground bulbs helps insulate them from winter cold. Mulch 4 to 6 inches deep can shield roots from bouts of brutal cold in midwinter just enough to prevent the plant from dying. Once the warmth of spring returns, pull back the mulch to allow the sun to heat the soil.
Contact your county's Cooperative Extension office and speak with the horticulture agent or Master Gardeners well-versed in gardening information. Ask about tropical plants grown successfully in the area, where to obtain them and tips on how to overwinter them. Mail-order catalogs can supply plant materials, but make certain to look at the USDA hardiness zone rating for the plant; some plants may not survive in the chillier winter areas of zone 7, but grow fine where the winter is only 5 degrees F. warmer. Planting tropical plants on the warmer southeastern side of hills or buildings may also provide additional warmth to overwinter some tropical plants. These "warmer pockets" of your garden are known as "microclimates."