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When is Giant Swan Plant (GSP) NOT GSP? An interesting question…
I have just learned that there are more than two types of milkweed (or
swan plant) from Africa here in NZ.
Some years ago (25?) a Kerikeri lady gave me some seeds of the "Giant
Swan Plant", which I then began distributing to others to help establish
more Monarch butterflies. It is hard to distinguish GSP from ordinary
Swan Plants (SP) as seedlings, and people ask me which are which. I had
also noticed ‘another’ two milkweeds which look like they’re close
relatives growing here in Russell.
Mona Miller in the USA provided me with surprising information from ‘The
Monarch Butterfly: International Traveler’, 1987, 1998, Fred A.
Urquhart (Chapter 7, Range Extension, Page 176)
"Three species of the genus Asclepias have been reported in New
Zealand: curassavica, semilunata, and tuberosa. Larvae have also
been reported feeding on three species of Gomphocarpus, a close
relative of the genus Asclepias: arborescens, physocarpus, and
fruticosus.
A. fruticosus is referred to as the ‘swan plant’ because its seed
pods resembles a swan; it is grown in many gardens, often for the
expressed purpose of attracting monarch butterflies."
In the year 2000 David Goyder, botanist at Kew Gardens, put together a
paper on the various species and sub-species of milkweed (Gomphocarpus)
growing on the African continent, of which there are 27! At this time
the swan plant etc was reclassified from Asclepias to Gomphocarpus.
Now I have to see if I can identify the ones we’ve got, but it’s a slow
process as I have to remember some words when describing plant parts -
it’s a long time since I did School Certificate Biology. For instance
follicles… I now ecall they are the seed pods.
I also need a good Latin dictionary. The name ‘semilunatus’ was a
mystery. It means crescent-shaped (half moon). Arborescens I believe is
tree-like, fruticosa I think means ‘many fruit’. But why is it used as
semilunatus sometimes and semilunata others?
Here’s some excerpts from David Goyder’s paper… I’ve only quoted the
bits which are useful to us here.
Habit: Most species are short-lived perennial subshrubs. /G.
fruticosus/, as its name suggests, is much branched, generally from
near the ground, whereas species of the /G. physocarpus/ group have
more scattered branches arising higher up the main stem.
Follicles: Follicles of the /G. physocarpus/ group are both strongly
inflated and ornamented with soft flexible processes. In /G.
physocarpus/ itself, the follicle is more or less globular, and has
no beak, whereas in /G. semilunatus/ … the fruit is more ovoid.
/G. fruticosus/ typically has somewhat smaller ovoid follicles with
an acute apex.
From my initial reading /G. semilunatus/ also can be distinguished by a
different ‘corona’. This is part of the flower best seen on the website
below (note that this is an Asclepia, which is a close relative but from
America.)
http://www.microscopy-uk.org.uk/mag/artfeb05/bjmilkweed.html
Now I realise that I have been sometimes selling G. semilunatus as GSP!
I don’t think it’s a real issue, though, as people like the plant for
its vigorous growth, and both species have the similar strong-growing
habit. Beverley has noticed that the trunks of hers are grey, while SP
is green.
Nowhere can I find reference to the fourth plant I have seen, which has
foliage so dense that you can’t see the branches. The grwoth is more
upright and overlaid somewhat (which almost makes me feel like it is
adapting to dry conditions. It almost looks like a Euphorbia (but it is
not). I cannot recall seeing seedpods or flowers, and the site where it
was growing is now under development. I will have to go and take another
look sometime and see if there are some of the plants around the edge of
the site or growing next door.
I hope to find a botanist who will work with me on this, so if anyone is
interested, please contact me privately, jacqui@monarch.org.nz. Perhaps
a Latin scholar can explain why the difference in the Latin word for the
species sometimes, e.g. physocarpUS and physocarpA, semilunata and
semilunatus. I can’t remember my Latin! If anyone would like a copy of
Goyder’s paper, please email me.
Of course, one thing is worth remembering:
Monarch butterflies and their caterpillars, don’t care much about all this!
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