Thunbergia alata, commonly known as the Black Eyed Susan vine, is a climbing vine from the Acanthaceae family (Heywood et al, 2007)1, which produces large showy yellow flowers with a black spot in the centre. These large bright flowers and relative ease of cultivation (Smithies, 2007 )2 are the reason why it is a widely grown ornamental species.
There is a specimen growing in the tropical greenhouse on campus, and despite only being young (the greenhouse was only planted a little over a year ago!) – this plant was grown from seed in August this year, it’s already reaching the roof, climbing on several greenhouse structures to get there. This is a good demonstration of its speed of growth, and its ability to grow over most things, both good indicators of easy ornamental horticulture use.
Its natural range is tropical Southern Africa, around the eastern coast, but due to its extensive ornamental use, it’s no longer possible to determine its natural range with ease (Smithies, 2007 )3. Its wide ornamental use has led to it becoming naturalised in other areas of the world, such as tropical areas of the USA (USDA, 2013)4.
Identification of this species is relatively easy, especially if it is in flower, as it is for much of its life. When it is in flower, its flower is distinctive, and while other species in the genus Thunbergia have a similar flower and leaf shape, only wild types of T.alata have the bright yellow flowers with a black spot in the centre. The flowers are held all over the plant, so there are usually at least one or two at eye level. Identification to cultivar level, however, is more difficult, as cultivars with a range of flower colours are available, for example, T.alata ‘Susie’ (Suttons seeds, 2013)5, can have white flowers which can look similar to several other Thunbergia species.
The flowers can be held anywhere on the plant. You can see the bulk of the plant climbing in the background.
Identification to family level is fairly easy, despite a high level of variety of forms within the family. Acanthaceae, the family which contains Thunbergia, contains trees, shrubs, perennial herbs, and climbers, which might lead to the assumption that familial identification would be tricky, but they all reliably share characteristics which can be used for identification. All members of the family have opposite leaves, none have stipules, along with the flowers being gamopetalous (the petals are fused at the base), and zygomorphic (have only one plain of symmetry) as well as large bracts (Heywood et al., 2007 6). Whist this does leave a large range of forms, identification is possible.
Heywood V, Brummitt R, Culham A, Serberg O, (2007), Flowering Plant Families of the World, Firefly Books.1, 6
Smithies S, (2007), Thunbergia alata Bojer ex Sims, accessed 22/10/2013.2, 3
Suttons seeds, (2013), Thunbergia alata Susie Mix Seeds, accessed 03/11/20135
USDA, (2013), Thunbergia alata Bojer ex Sims blackeyed Susan vine, accessed 22/10/2103.4
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