The common name Milkweed is often used for plants of the Apocynaceae family that produce milky sap. It is most frequently applied to members of the genus Asclepias. The genus Asclepias is a cosmopolitan genus that at different times has been split up into numerous smaller genera. Currently three species are known from Phillipskop, split across two genera: the tuberous Asclepias crispa and the shrubby Gomphocarpus species, Gomphocarpus cancellatus and Gomphocarpus fruticosus. The difference between the two genera is based predominantly on growth form: Asclepias having tuberous roots and annual unbranched stems, Gomphocarpus having fibrous roots and woody branched stems (see Goyder & Nicholas 2001). As molecular work progresses, it may be that we see the species of Gomphocarpus return to Asclepias or Asclepias split up further. Indeed the confusion has so reigned over the years that not only were both Gomphocarpus species originally described as Asclepias but Asclepias crispa was once included in Gomphocarpus itself.
Fortunately, despite the problems of the genera, the three species are easy to identify. The commonest one is Gomphocarpus cancellatus, a plant of dry rocky slopes throughout the Cape Floral Region. Gomphocarpus cancellatus produces stout stems with broad oval tough leaves. When these leaves break off the milky white sap oozes out profusely. In May, the large heads of flowers appear. Like all members of Asclepias and its relatives, the flowers are complex in their structure. The sepals are almost hidden by the swept-back petals, the petals therefore end up looking like the sepals. What looks like folded petals is the corona, which is derived from outgrowths of the stamens. This corona holds the nectaries and directs the insects towards where the pollen is stored in pollinaria. The ovary is hidden below a star-shaped flat and thick stigma head. This ovary develops into an inflated bristly pod, with a beaked point and bent neck. The pointed pod on a curved neck with a fanciful allusion to a goose’s head, gives the species one of its common names: Gansiebos (Little Goose Bush).
The other species of Gomphocarpus in the Klein River Mountains, Gomphocarpus fruticosus, has very similar flowerheads and the fruit too are inflated with a beaked point. However, the leaves are not only much narrower and longer but they are also not as thick or stiff. It also forms taller more branched shrubs. Gomphocarpus fruticosus is found in disturbed areas not just in the Cape but through Tropical Africa, as well as naturalised around the world. It is not really a true element of fynbos, as Gomphocarpus cancellatus is. A very similar species, Gomphocarpus physocarpus, has also been introduced into the area from tropical Africa. This has not yet been found in the Klein River Mountains but is found on neighbouring farmland. It grows in a similar habit to Gomphocarpus fruticosus but the fruit are inflated into large round balls, not tapering into a beak like Gomphocarpus fruticosus. There is also a difference in the shape of the corona: Gomphocarpus fruticosus has a very clear tooth, while in Gomphocarpus physocarpus it is barely developed.
Asclepias crispa differs from Gomphocarpus in producing a swollen tap root and the above ground stems dying back each season. The flowerheads though are not dissimilar in shape and size from Gomphocarpus and weigh down the weak branches, so that they appear at ground-level. The leaves are narrowly lanceolate like Gomphocarpus fruticosus but have crisped wavy edges, which give it the species name. Like Gomphocarpus cancellatus it is found throughout the Cape Floral Region, as well as slightly beyond, into the Eastern Cape.
The Milkweeds in North America are renowned for being the food plant of the Monarch Butterfly (Danaus plexippus), which has the amazing migration and roosting sites in Mexico, Florida and California. The South African milkweeds also have a close relationship with the African Monarch (Danaus chrysippus). The African Monarch is a relative but does not form the same mass roosting sites as their American cousins. Both species of Monarch feed on members of the Apocynaceae on account of the milky sap. By sequestering the toxin in their bodies it makes them distasteful to predators. Other insects also make use of the sap of milkweeds to protect themselves from predation. One of these is the Green Milkweed Grasshopper, Phymateus leprosus. The adult stage is rather drab but the nymph has the most remarkable coloration, an almost futuristic neon green and black. The bright coloration of both the grasshoppers and the butterflies is there to warn predators of their poisonous nature – a term known as aposematism. At Phillipskop, Phymateus leprosus can be found feeding on Gomphocarpus cancellatus, often swarming and decimating the plants.