The Thunbergia genus includes the popular Black-eyed Susan Vine (T. alata), a fast-growing, ornamental climber valued for its vibrant flowers with characteristic dark centers. Gardeners often seek out this tropical vine to quickly cover trellises, arbors, or to cascade from hanging baskets throughout the summer. The question of whether this plant returns year after year is a common point of confusion. The answer depends on the plant’s botanical classification and the specific climate conditions where it is grown.
Understanding the Thunbergia Classification
The Black-eyed Susan Vine (T. alata) is classified as a “tender perennial,” distinguishing it from true annuals and hardy perennials. A true annual completes its entire life cycle within a single growing season. A hardy perennial can survive for many years because it withstands freezing temperatures.
A tender perennial lives for more than two years but cannot tolerate frost or freezing conditions. Thunbergia originates from tropical East Africa, where freezing temperatures are not a factor. The plant will grow and flower indefinitely in consistently warm climates, behaving as a perennial. When grown outside these tropical zones, its life cycle is cut short by the first hard frost, leading most gardeners to treat it as a seasonal annual.
Climate Zones and Year-to-Year Survival
Whether Thunbergia survives the winter is determined by local climate, specifically the winter low temperatures. Plant hardiness is mapped using the USDA Hardiness Zones, which categorize areas based on the average annual minimum winter temperature. The Black-eyed Susan Vine is reliably perennial only in USDA Zones 10 and 11, where temperatures rarely drop below freezing. In these frost-free regions, the vine can grow vigorously and may even become invasive due to its rapid growth and self-seeding.
In cooler zones, such as Zone 9 and below, the plant’s foliage and stems are killed when the temperature falls below 32°F. The plant cannot survive the winter outdoors without intervention. Although survival is technically possible in Zone 9 with heavy mulching and protection, it is typically grown as an annual, requiring replanting each spring.
Methods for Bringing Thunbergia Back
Gardeners in colder climates have two main strategies for ensuring their Thunbergia returns the following season.
Seed Collection
One method is to collect and save the seeds produced by the mature plant. Seeds should be harvested from the seed pods, allowed to dry, and stored in a cool, dark, and dry location over winter. They can be started indoors six to eight weeks before the last expected frost. Soaking the seeds in warm water overnight before planting helps break dormancy and speeds up germination, which typically takes 10 to 21 days.
Overwintering Live Plants
The second strategy involves overwintering the living plant indoors, either by taking stem cuttings or moving a potted plant. Before the first frost, terminal cuttings several inches long can be taken from a healthy vine, with the lower leaves removed. These cuttings root readily in a glass of water or directly in moist potting mix. Alternatively, a container-grown plant can be pruned back by about one-third and brought indoors to a bright location with temperatures above 60°F. This method retains the exact characteristics of the parent plant, avoiding the genetic variability of seeds.