The plant genus Thunbergia, which includes the popular Black-eyed Susan Vine (T. alata) and the Blue Trumpet Vine (T. grandiflora), is often classified differently depending on its growing location. Whether this vibrant flowering vine is a perennial or an annual depends entirely on the climate. This tropical plant has the biological capacity to live for multiple seasons, but only when environmental conditions permit.
Understanding the Thunbergia Life Cycle
Species within the Thunbergia genus are technically classified as tender perennials or sub-shrubs, meaning they have the capacity to live beyond two years. This means they are genetically capable of completing their life cycle over multiple growing seasons. A tender perennial is defined by its inability to survive temperatures at or below freezing. For example, Thunbergia alata is a herbaceous perennial climbing plant native to tropical East Africa. Its roots are not adapted to withstand the sustained cold of temperate winters, meaning the entire plant will die back and fail to return if the ground freezes.
Climate as the Deciding Factor
The distinction between perennial and annual use is directly tied to the plant’s sensitivity to cold temperatures. Gardeners use the USDA Plant Hardiness Zone system, which maps regions based on their average minimum winter temperature, to determine suitability. Thunbergia species are reliably perennial only in USDA Zones 9, 10, and 11, where temperatures rarely drop below freezing. In these warm, frost-free regions, the vine acts as a true perennial, growing and flowering year-round. It can achieve its full potential height of up to 15 to 30 feet, depending on the species. In any climate colder than Zone 9, the plant is effectively an annual because winter frost kills the root system, requiring replanting each spring.
Growing Thunbergia Based on Location
Annual Care (Colder Climates)
Gardeners in colder climates must treat Thunbergia as an annual and focus on maximizing growth during the warm season. To get a head start, seeds are typically sown indoors six to eight weeks before the last expected frost. Plants should only be moved outdoors once all danger of frost has passed and night temperatures consistently remain above 50°F. For those who wish to keep a favorite plant, overwintering is a practical option. Potted plants can be brought indoors before the first hard frost and kept in a sunny, bright window as a temporary houseplant. Alternatively, gardeners can take softwood cuttings from the healthy vine in late summer and root them, ensuring new plants are ready for the following spring.
Perennial Care (Zones 9-11)
In the warmer climates of Zones 9 through 11, Thunbergia is a true perennial, and the focus shifts to managing its aggressive growth. These vines are vigorous and require a sturdy trellis or arbor for support. Regular pruning is necessary to control the size and shape of the vine. Because the plant’s self-seeding habit can be problematic in tropical conditions, spent flowers and seed pods should be removed to prevent unwanted spread.