SA 172 Acacia Karroo = Vachellia Karroo
“Just 25mls of rain and the sweet thorn bursts into bloom”
A sweet thorn acacia requires just 25 millimetres of rain for it to turn all gold with its glorious fragrant yellow, puffball flowers. The sweet thorn is one of the last of the Acacia species to flower in summer. It can flower intermittently throughout the summer and sometimes as many as six times in the course of a year and particularly after good rains. If conditions are generally favourable it is just a beautiful mass of yellow flower-balls. The colour of the developing buds is yellow-green prior to full bloom. The flowers are sweetly scented, pollinated mainly by insects, which undoubtedly are attracted by the strong colour of the flowers. Other pollinators include beetles, flies, moths and butterflies. Caterpillars of ten species of butterflies are dependent on the tree for their survival and two of the more common butterflies found on it are the club-tailed charaxes and the topaz-spotted blue.
Our Acacia karroo trees are certainly one of the most widespread and common trees in South Africa. They are found in a variety of habitats but prefer open woodland and wooded grassland. They grow on most soil types, preferring deep, blackish nutrient-rich clay soils to the sandy soils. In dry areas the sweet thorn is known to be a good indicator of underground and surface water. The taproot of the Acacia karroo is long enabling it to use water and nutrients from deep underground. For the early explorers and naturalists out exploring or collecting specimens across the vast arid landscape these Acacia trees were a very welcome sight. Water was near.
The name acacia comes from the Greek word ‘’akis’’ which directly translated is a point or barb referring to the thorns found on African Acacia species. Karroo is the old spellings for the Karoo where the species was first described by the early naturalists. The name cannot be changed because of the laws governing the botanical naming of plants but the origin of the name is from the Karoo region of the former Cape Province of South Africa, where the Acacia karroo is often the only tree found.
WHY THE COMMON NAME SWEET THORN?
The common name sweet thorn possibly refers to the sweet smell of the flowers, or to the fact that the presence of the species often indicates ‘sweet’ veld, which is prized for its good grazing and fertile soils or even possibly so called from the copious amounts of sweet gum which oozes from wounds in the tree’s bark. The tree yields a clear, golden brown or red, pleasant tasting gum which is edible and suitable both for confectionary and for use as an adhesive. The gum is an important part of the bush baby’s winter diet.
So whatever it is you choose to believe regarding the origin of the common name for the sweet thorn it is a truly remarkable tree. The flowers attract bees in search of pollen and nectar and it is a good source of honey for the bee farmers. Apart from the many varieties of butterfly dependant on the tree for their survival many varieties of insects in turn are attracted and consequently this brings birds, lizards and snakes. Birds just love to build their nests in them as the thorns offer them some sort of protection from predators. In the bushveld the tree is something of an ‘all-in-one supermarket’ for browsers and grazers. The trees leaves are nutritious ,the grasses and other plants thriving in the shade of the tree are a lot more palatable because of the increased fertility of the soil from the nitrogen- fixing fungi attached to the trees roots helping cover otherwise bare ground and then in winter it drops its protein-rich pods.
THE FEATURES OF A SWEET THORN
The sweet thorn acacia tree has a rounded crown and is variable in shape and size. It reaches a height of about 15 meters where there is good water and is usually a single stemmed tree, but you will notice that the trees are prone to branching fairly low down on the trunk so can also be multi-stemmed. The bark is red-brown on young branches, darkening and becoming rough with age. Sometimes this attractive reddish colour is visible in the deep bark fissures. The main stems of young trees resemble the colour on these younger branches. Due to its high tannin content, the bark was used in years gone by for tanning, imparting a red colour to the leather, but a disagreeable odour.
The seed pods occur in bunches forming a tangled mass of pods sometimes. They are narrow, sickle-shaped and flat with minor constrictions between the seeds. The young pods are green and then as they mature they become dark brown and dry. When the pods split open from January onwards a profusion of dangling olive green to light brown seeds are released that fall to the ground. The seeds can be roasted as a coffee substitute, but it is an acquired taste.
The paired and straight conspicuous greyish to white spines, occur especially on the lower branches and on younger trees to protect them from browsing animals. On the mature older trees the thorns are a lot shorter or occasionally thornless. So sharp are these distinctive white thorns that they have been used for sewing needles and were even used by the early naturalists to pin the insects they collected.
The white spine is a significant factor in the new classification of the Acacia karroo to the group Vachellia. All Acacia trees with long white stipular spines are now to be known as Vachellia while Acacia with short and curved prickles more like rose thorns, are classified as Senegalia.
On the Greater Woodmead Estate we have observed what a true pioneer species the Acacia karroo is and how it is able to seed and rapidly establish itself as a tree species without shade, shelter or any care or protection. The survival rate with this self-seeding process is very successful. Transplanting a sweet thorn has rarely worked on the Estate. Like most pioneer plants, Acacia karroo is short lived usually dying after 30-40 years.
The sweet thorn acacia really is a tree Full of Life. Lichens and mosses grow happily on the tree and just, when you think it isn’t alive it is in fact supporting an incredible number of other fascinating creatures. Several fungi are associated with this tree and occasionally the crown of mature trees may be parasitized by various mistletoes which can lead to the tree’s decline. When an Acacia karroo tree dies its rotting timber attracts thousands of different little insects and microorganisms which in turn attract and feed the different insect eating birds and smaller animals. In the end this remarkable tree has fulfilled a role of making a difference.
So, whichever name you prefer and know it by, Acacia karroo, Acacia vachellia, Sweet thorn, Soetdoring Mookana, Mooka or umuNga this late flowering Acacia of the family Fabaceae of the pea, bean and legume family is about to burst into flower and brighten your December days with a display of bright yellow puffball flowers. The trees are a glorious sight in full flower, but after one of our Highveld storms the flowers become matted and spoilt by the rain and it can be ten days or more before new blooms appear.