Shy mangrove relative tucked away in shady border

Some plants are hard to name. Yes I know all plants are hard to name if you don't have a passing interest in gardens or botany, but some are more obvious than others. I don't mean identifying exactly which species of Eucalyptus a gum tree is, or what cultivar of rose. Sometimes it's hard to get your bearings at all.

That was the case with this tree growing in a shady border on our northern boundary, between Alexandra Parade and the Ornamental Lake. It's Cassipourea gummiflua, which doesn't tell you much either.

It came to my notice when one of our horticultural team leaders, Therese Turner, asked me if I knew what the odd looking tree was with 'cauliferous' flowers - that is, flowers sticking out of the trunk and stems (although in this case, mostly the near terminal stems).

In April most of the petals had dropped from the flowers and I was about to apologise for the images in this post. But I see from other web images that even at their best, the flowers of this species are on the drab side. Not obvious here, but the petals that were remaining had bit of fringe to them.

Anyway, I didn't know what it was so I plugged it into a couple of my plant identification apps (these days I swap between iNaturalist and Pl@ntnet). Back in the office I did a little googling but without any luck. Therese said she had sent some flowers and leaves to our Identifications Botanist, Val Stajsic.

That's where things sat between my photographing of the flowers in April, and mid-June. Then, as he almost always does, Val was able to track down the name Cassipourea gummiflua, probably subspecies gummiflua. I expect he had to go back to basics and key out the family, then check against floras and herbarium specimens. That is, he didn't just click his phone a few times like me.

From our records, it seems we brought seed of this species into the Gardens from Zimbabwe in the 1980s. The connection between this particular plant and that propagation had been lost, but is now (tentatively) reconnected.

Val told me later that the developing fruits were helpful in confirming his identification. There were some fledgling fruits in late June (above and below), although they all seemed to be aborting once they got any bigger than those in the picture above. You can see them a little more detail in the next image, but as far as I know there was not fertile seed produced.

I wish our plant was subspecies verticillata which bears the uninviting but memorable common name, Large-leaved Onionwood. Given there seems to be a some difficulty differentiating and applying the subspecies, let's go with that for the common name of the species as a whole.

Both subspecies (and perhaps others if you split this further) grow in the wet forests of tropical and subtropical eastern Africa, and Madagascar. In Zimbabwe, bark of at least the verticillata variant is in high demand for traditional medicine. Unsustainable harvesting has left trees ring-barked, sometimes destroying local (sub)populations. Deforestation more generally has also led to its decline.

Botanically this is a curious one. It's in the plant family Rhizophoraceae, which includes many mangroves. The Large-leaved Onionweed is not, as you might have gathered, a mangrove. Nor are the nearly 70 others species in the genus Cassipourea - a tropical genus of the Americas, Asia and Africa. A lot of mangroves - and Rhizophoraceae - do have caulifery, so that's a helpful trait.

Val's parting words to the horticultural team were that given the rarity of this species in cultivation and in the wild we should propagate some extra material just in case. Wise words.