Vachellia and Senegalia species are not Acacias anymore!

To South Africans the word Acacia is quintessential with Africa, and the word evokes an iconic image of a beautiful sunset with an expansive African savannah studded with one or more flat topped thorn trees protecting the untamed land.

Well, as we sometimes do in rugby we lost to the Australians at the 17th International Botanical Congress in 2011.  But this time the loss of the name was forever.  By that stage the taxonomic debate over the genus “acacia” had already been going on a long time so we really only have ourselves to blame.  The genus had an enormous number of species that occurred in Australia, Asia, North & South America and Africa.  As molecular (genetic) and biochemical tools advanced it became clear that many of the species should not be grouped together in a single genus.  The debate was emotional but when all was said and done, Acacia was reserved for the 900 Australian species that retain this name.

As distressing as it is for us, we need to understand the importance of moving on and accepting, if not actually embracing, the change.  We did in fact refer to two quite diverse plant ancestries as Acacias causing confusion of some of the most important tree species in Africa.  The two new genera are separated by structural (morphological), anatomical and biochemical characteristics. The easiest way to differentiate them is when they are in flower as all the Vachellia species have round flower heads whereas the Senegalia have flowers in spikes (spicate).  There is a difference in whether the stipules are spinescent (Vachellia ) or non-spinescent (Senegalia) but to me this is academic -all their ‘thorns’ are sharp and prick!

In the latest Approved Plant List for Simbithi we have included the old names of all the plants that have been reclassified including the Acacias, with a reference to the new name to assist homeowners.

The various species allowed on Simbithi include:

Flame Thorn (Sengalia ataxacantha formerly Acacia ataxacantha)

It is called the Flame thorn due to the conspicuous bunches of deep, purple-red seed pods. More a scrambling shrub than a tree, it is multi-stemmed and seldom grows above 5 meters.  The many creamy yellow flower spikes are conspicuous at the ends of branches and appear between mid-summer and autumn.  The dark green leaves are the larval food for the caterpillars of many butterflies and birds enjoy the many insects attracted to these trees.  The seed pods contain between 6 and 8 brown or olive-green flat seeds that need to be soaked in hot water before sowing. Viable seeds will germinate within two weeks and although growth rate is initially slow it does speed up.

Sweet Thorn (Vachellia karroo formerly Acacia karroo)

This is a beautiful tree with a rounded crown. The flowers, a mass of bright yellow pompoms are a stunning sight in early summer.  The seed pods are crescent shaped and flat. The thorns are definitely not sweet but the tree gets its common name from the sweet gum that exudes from wounds in the bark.  Lesser bushbabies, other animals and even people enjoy the taste of the gum.  No less than ten butterfly species use this tree as a larval host tree, birds love to make nests in its branches and the leaves provide good fodder for buck.  It is also a good bee plant and Acacia honey is produced by bees feeding on the nectar of sweet thorn flowers.  It a very useful tree to have on Simbithi.  As the roots can be invasive do not plant it next to the verge or too close to your home.  This tree has a fast growth rate of up to a metre a year so do provide it with super phosphate or bonemeal and compost.  Seeds need to be soaked in hot water overnight and germinate between 3 and 12 days. The seedlings can be transplanted once the second leaves unfurl.  Incidentally, the spelling of karroo is correct in this instance. It is an old spelling of karoo but due to conventions of scientific nomenclature it cannot just be changed to the more modern spelling with a single ‘r’.

Scented-pod Thorn (Vachellia nilotica subspecies kraussiana, formerly Acacia nilotica)

This is a medium to large tree and can grow to 10m.  The mature bark is dark and has deep, longitudinal grooves and it is easily recognised by the seedpods which are described as resembling bead necklaces as there are constrictions between each seed.  These pods are sweetly scented when crushed. The flowers are round, golden yellow and sweetly scented.  They appear mainly between September and January.  The gum from the bark of this tree is also edible and has the dual purpose as a glue.  After soaking overnight, seeds are best planted singly in bags as they quickly develop a long tap root. The trees are slow growing.

Broadpod Robust thorn or Splendid Thorn (Vachellia robusta formerly Acacia robusta)

One of the most popular thorn trees for gardens as it is small to medium and fast growing.  In addition, it does not take up too much space as is has an upright form.  Not sure why it is called an ‘enkeldoring’ in Afrikaans though, as the thorns are long and paired.  This tree usually loses its leaves at the end of winter and after a short time the bright new green leaves appear at the same time as the creamy-white pompom flowers.  The trees look spectacular in flower.  The flowers are strongly scented and appear between July to October.  They are followed by straight, broad seedpods.  Both the leaves and the seedpods are browsed by buck.  Like other Vachellia species birds like to build their nests in the tree as the thorny branches provide protection from many predators.  The strongly scented flowers also attract many insects which in turn attract insectivorous birds.  The tree is also a larval host plant to the Hutchinson’s Silver-spot and several other butterfly species.  Should you wish to grow this tree from seed it is best to collect the seed from on the tree and not off the ground.  Give the seed a light rub with sandpaper to scarify it, then soak in hot water.  Once the water cools leave the seed to soak for a further 12 to 24 hours before planting in a mixture of river sand and compost. 

Paperbark Thorn (Vachellia sieberiana formerly Acacia sieberiana)

These are the trees that have the iconic shape we associate with “acacia’ – beautiful, wide spreading flat crowned trees.  Easily identified by their shape and the papery bark that flakes and peels off in strips showing a yellow under bark.  The flowers are pale creamy-yellow balls of fluff that appear anytime from September to November.  Near riverbanks this tree can reach 15 meters.  As do all thorn trees, this tree attracts lots of birds when in flower and wood-hoopoes visit the trees to search for insects in the bark.  Birds nest in the thorny branches and Pied and Crested Barbets make nesting holes in the branches.  The seed pods which are initially covered in velvety hairs are cylindrical, woody and smell very musty.  Paperbark Thorns trees are easily grown from seed that has been immersed in hot water and left to soak.  They are only suited to a large garden as they require a lot of space to reveal their beautiful shape.

Umbrella thorn (Vachellia tortilis formerly Acacia tortilis)

Both the common and botanical names give clues to identifying this species. The canopy is umbrella shaped and tortilis means twisted referring to the seed pods which resemble a coiled spring or may be rolled up into a tight circle.  The seed pods are highly nutritious and loved by our buck when they fall to the ground, provided that is, that the monkeys haven’t eaten all the green pods!  The flowers are fairly sparse, sweetly scented, globe-shaped and either white or cream.

Umbrella thorns are not fussy about water or soil-type and grow in a wide range of habitats. Depending on amount of water and soil quality they stay shrubby or can grow into tall trees of 20 metres.  Once again, the seeds need to be immersed into hot water and left to soak, but in this instance, soak until they actually swell before planting in a mix of river sand and compost.  Germination time varies from 4 to 10 days and they are generally slow growing, especially when young.

There is another Vachellia growing on Simbithi.  It is not on the approved plant list as it usually only occurs north of the Tugela River, but several specimens can be found on the Tamboti trail and this is Vachellia kosiensis or the Dune Thorn.  This species was originally considered to be a coastal form of Vachellia karroo and the easiest way to tell them apart is when they are in bloom. V.karroo has flowers at the branch ends whereas the flowers of the Dune Thorn are hidden among the leaves on short branches.  The spines on the Tamboti trail trees are large and long and there are masses of them.  This species of thorn tree grows tall and upright.

The bright yellow pompom flowers are sweetly scented and their nectar attracts lots of bees, butterflies and other insects and consequently insectivorous birds.  Vervet monkeys and thick-tailed bushbabies love the gum from the damaged bark and game browse the leaves and eat the fallen pods.

In thinking about name changes maybe we need to take advice from Jerry Spinelli who said I’m not my name. My name is something I wear, like a shirt. It gets worn. I outgrow it, I change it.”  After all, those horrible wattle trees that head up South Africa’s  Alien Invasives list are Australian Acacias!

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