Joanna and Alisdair,
Thanks for your thoughts.
It is not due to artificial chemicals such as weedkillers. The plant has been growing steadily ever since I first noticed it, which was well after the last time I weedkillered the area. I have no reason to suspect other abnormal chemicals in the location. Weeds pop-up in the same spot and around it, showing no sign of abnormality.
The plant is perhaps even more vigorous than the red flowered sort.
I shall give pieces to friends, so we have clonal plants in different locations to monitor.
I shall be interested to see if any fruit appear, noting:
"flowers are followed by a fruit of variable shape to 4 inches (10 cm) across"
[http://www.plantoftheweek.org/week553.shtml]
Notice, too, that the photos on that page show flowers including hints of orange.
...Makes me wonder, while we're grasping at straws, if there is a red-flowered A. haeckeliana, and I happened to have some of it, which reverted to being yellow-flowered. Seems far-fetched.
There are two other species of Aptenia:
- A. lancifolia (smaller purple flowers)
- A. geniculiflorais (white to pale yellow flowers, erect deciduous perennial)
[http://www.plantzafrica.com/plantab/apteniacord.htm]
My plant fits with neither of those.
Further research...
"many of the cultivars of the widely cultivated A. cordifolia are hybrids, probably between A. cordifolia and A. haeckeliana, these often have narrower leaves and reddish - instead of bright pink - flowers (acc. to Gerbaulet)."
[Joachim Thiede, in
https://www.researchgate.net/post/How_can_you_distinguish_South_African_Aptenia_cordifolia_from_Aptenia_lancifolia]
So one or more of my red-flowered plants could be a hybrid, which allows the possibility of reversion to a A. haeckeliana parent.
Mike