I have been growing this plant since I received it in October 2020 under the name Trichocaulon flavum. That is an old name, that plant is now usually called Hoodia flava. It flowered last week and I now know that it is definitely not Hoodia flava. It appears to be a hybrid between two species of Hoodia.
Though it looks very like a cactus, Hoodia is very different and only very distantly related. They are found growing in the south of Africa, in Angola, Botswana, Namibia, South Africa and Zimbabwe. There are about 13 species recognised by Kew Gardens. As with some species of Euphorbia, they have converged through evolution to have similar characteristics to the family Cactaceae which is found in the Americas. All have been selected by their environment to have similar solutions to the challenges of living in arid areas.
In the picture below, on the ribs between the flower and flower bud you can see that the mature spines become detachable. If they stick in your flesh they will stay with you and come free of the plant. I recently used a cushion to help repot the plant without puncturing myself. I spent a while removing spines from the cushion. The spines are about 4 to 5mm in length. You can also see that there are more flowers on the way.
None of the stems are taller than 15cm (6 inches) and the flower is about 3cm (an inch and a bit) across. It only had one stem when it arrived and has increased a lot in bulk and number of stems. The flower does not have a very strong smell like some of the species, though if you get close it is a bit stinky. This is to attract various species of flies as pollinators.
I went through Ulrich Meve's Illustrated Handbook of Succulent Plants: Asclepiadaceae and this plant seemed to have some quality of every species of section Trichocaulon and several from section Hoodia. It is likely that the mother was Hoodia flava, which would explain why they were sold by that name. Hoodia are usually raised from seed rather than cuttings. They hybridise easily if a fly carries the pollen from one species to another. They are difficult to identify before flowering.
I am a little annoyed that it was not a pure species but this is a common problem when plants are raised from seed. It is a very attractive plant. Artificially pollinating asclepiads is very fiddly and labour-intensive and requires a microscope and skill.
This plant is probably easier to grow than the pure species as it will have hybrid vigour. The flower is a lot bigger than Hoodia flava. It has certainly been a very easy plant to grow in my conditions. It has not experienced temperatures below 20°C and has a heat mat underneath to ensure that. In summer it has got up to 40°C. For most of the last year it has been under strong LED lights with a 14 hour daylength. It is potted in a pure mineral mix, some Seramis, some perlite and mostly pumice in the recent repotting. (I am no longer using Seramis, I much prefer pumice.) I crushed some of the lumps to make a muddy but well-draining mix to resemble the soil that Hoodia flava prefers. I water about every four days with a very dilute high phosphorus, high potassium, medium nitrogen feed.
The supplier Uhlig Kakteen got back to me when I emailed them and suggested the plant could be Hoodia flava x Hoodia pilifera or Hoodia flava x Hoodia ruschii. They got it from an Italian supplier. It was a very nice-looking and healthy plant when I received it.
Hoodia flava x Hoodia ruschii looks very possible, it explains the size and shape of the flower and the papillae (tiny bumps) that cover the flower. Those darker-coloured bumps have little bristles at the top, a characteristic of several species of Hoodia. These can be seen in my microphotographs below. They can just be made out in the photo of the flower at the top of this page, if you open the image in a new window and expand it to see maximum detail.
I missed the centre when I cut through the flower to photograph a cross-section. I should get plenty more flowers to try again. Explanations of the flower structure of asclepiads and other cross-sections can be found in my other articles, for Frerea, Hoya, Huernia and Stephanotis.
This appears to be the best picture I got of the bud.
