@RyanSA - The article on Wikipedia that you linked to in your post answers your question. It says; "
Gomphocarpus physocarpus is an undershrub perennial herb, that can grow to over six feet."
Key word is HERB, the use of the word herb in this context means there is no true wood that develops in the stems. They refer to it as a species of milkweed.
Now there is nothing wrong with growing plants in pots to look attractive. At 6 feet tall this will not normally qualify as a companion planting for bonsai (kusamono) as the accent or companion plantings for bonsai usually are proportionate to represent the understory plants in the forest.
Sounds like a cool, showy species for the garden. It just doesn't have any bonsai possibilities. Feel free to share pictures in our "Tea House sub Forum" as it is our place for things that are interesting but not really bonsai related.
There are many South African trees and shrubs that do make decent bonsai. Look into your native thorn trees. The South African members of the genus Diospyros like the crocodile bark jackalberry. Any of the many members of the guava family, native or imported for fruit production. Olives, even Kooboo berry can work. Many members of the Malpighia family make good bonsai. Grewia asiatica and Grewia occidentalis can make good bonsai. Also Natal plum, Carissa macrocarpa makes a good flowering bonsai.
So there are many more species that make good bonsai than the ones I listed. Look around and see what woody trees or shrubs you have that have medium to small leaves, and interesting flowers, or fruits or seeds or nuts. There are many possibilities.
We have a fair number of members from South Africa, perhaps they will help you find interesting material.
