The Common Tree of the Year to plant for Arbour Week in 2021 is the Sweet Thorn

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Vachellia karroo - Picture courtesy Bernard DUPONT see his flickr pageVachellia karroo - Picture courtesy Bernard DUPONT see his flickr pageThe lovely Sweet Thorn, Soetdoring, mookana, mooka, umuNga (Vachenalia karroo) is tough as nails, and an adaptable pioneer which is able to establish itself without shade, shelter, or protection from res. It is very adaptable to soil types, and tolerates long periods of drought. Read more about growing it below.

Known for many years as “Acacia karroo”, this indigenous thorn tree is widely spread throughout South Africa; growing in both the summer and winter rainfall regions. This beautiful tree is an integral part of our history and known for its many practical uses. In fact, it was given the name “sweet thorn” for several reasons: The gum which exudes from wounds in the bark is surprisingly very pleasant tasting and is eaten by both people and animals, including the lesser bushbaby which feeds exclusively on gum from trees, particularly Vachellia species, and insects. It was also believed that where sweet thorns were found growing the grazing was sweeter, and the soil more fertile; and sweet thorns do indeed love growing near surface water, and if they are found growing in arid regions with no visible surface water, they are a good indicator of underground water.

Vachellia karroo (SA Tree No: 172) may be found from the Western Cape through to Zambia and Angola, occuring in a variety of habitats, from low lying areas to the Highveld. It is not usually found in mist belt and montane ecosystems like the Drakensberg, which are found on the slopes of mountains, because the alpine climate in these regions strongly affects the ecosystem, with significant falls in temperature as elevation increases. This causes the ecosystem to stratify, with different plant types growing in layers, at the various elevations

The Sweet Thorn has a variable growth habit, depending on the climate in which it is grown, and there are different forms in some places, which can be confusing. In regions with good rainfall it can reach a height of approximately 12m, but generally it develops into a wonderful small to medium sized shade tree, 4 to 7m in height, with a spreading, rounded crown, and which branches fairly low down on the trunk. The tree is evergreen, except for cold and dry areas where it is deciduous. The dark green leaves are finely textured, and the straight, greyish to white "spines" or thorns, grow in pairs, and are held at 90° to the stem, or raked forward slightly. They vary in length from 1 to 7cm long, and on mature trees the thorns may be quite short.

The bark on the young branches is red, darkening and becoming rough and deeply fissured with age, and sometimes an attractive reddish colour can be seen in the deep fissures. Between October and February it bears such a profusion of sweetly-scented, golden-yellow balls of flowers that the leaves can hardly be seen. The flat and crescent shaped seed pods start out green and ripen from January to April, becoming brown and dried out before splitting open to release their seeds.

Vachellia karroo flowers - Picture courtesy Bernard DUPONT see his flickr pageVachellia karroo flowers - Picture courtesy Bernard DUPONT see his flickr pageUses:

The sweet thorn is a most useful tree for small holdings, farms and game farms, as it is a particularly good fodder tree because both stock and game feed on the leaves, flowers and pods, and the trees also provide valuable shade. If sweet thorns are planted fairly closely together, and are pruned occasionally, they will form an almost impenetrable barrier, and a wonderful windbreak. 

The flowers produce lots of nectar and pollen for bee-farming, and the honey produced has a pleasant flavour.

The seeds are roasted as a coffee substitute, and the gum is eaten by people and monkeys, and had commercial value in the past when the gum was exported as "Cape Gum", to be used for the making of confectionary.

Traditionally the sweet thorn is used medicinally for many ailments ranging from poultices for wounds, to eye treatments, and a gargle for a sore throat.

During colonization the wood was widely used for fuel, fencing posts, and furniture and wagon wheels. The heartwood is heavy and hard but susceptible to attack from borer. However, the "dune forest" form of the sweet thorn, which is found along the coast of Kwazulu-Natal and northwards of the Tugela river, has soft wood which is not be suitable for woodworking.

The inner bark was used for making strong twine, and contains tannin which was used to tan leather red. The stout white thorns were used to make sewing needles, and were even used by early naturalists to ‘pin’ the insects they collected.

In the Garden:

The sweet thorn makes a beautiful garden specimen, and is fantastic to include in wildlife gardens because the thorny branches make ideal nesting sites for birds, and offer them some protection from predators. Because many insects visit and pollinate the flowers, they will attract many types of insect eating birds.

Caterpillars of 10 species of butterflies are dependent on the tree for survival. These include the club-tailed charaxes (Charaxes zoolina zoolina) and the topaz-spotted blue (Azanus jesous). The silver striped hawkmoth caterpillar also feeds on the leaves.

Because this  thorny tree branches low-down it makes an excellent barrier plant, and is planted extensively on farms to provide food, shelter, and shade for stock, and cattle kraals are still made from the branches. 

Although this tree has a long taproot which enables it to use water and nutrients from deep underground, it still has an aggressive root system. It can also grow fairly large, so it should not be planted close to buildings, paving, walls, etc. Its long tap root, combined with its ability to fix nitrogen, allows for grasses and other plants to thrive in its shade.

Cultivation/Propagation:

The sweet thorn is tough as nails, and an adaptable pioneer plant which is able to establish itself without shade, shelter, or protection from grass fires, and saplings only one year old will most often re-sprout after a fire.

This pioneer is also very adaptable to soil types, and tolerates poor, sandy soils and long periods of drought, but if planted in good garden soil and watered regularly it will grow surprisingly fast, up to 1m per year, and the tree can reach anything from 4 to 7m in height. Ensure that you plant it in a place where it can be left to grow undisturbed for the next generation to come, as the sweet thorn can live for up to 40 years.

The sweet thorn is very hardy, and will grow where summers are very hot and dry, and winter temperatures drop below freezing. It can be deciduous in cold areas, and it drops its leaves if stressed for water etc. If it is watered regularly, it will remain largely evergreen in the garden. 

Plant with plenty of compost, bonemeal or superphosphates, and water well and deeply, as frequent but shallow sprinklings of water only encourage shallow root growth. 

The tree can be pruned after flowering to give it a more compact appearance and to develop more width than height.

The sweet thorn is easy to grow from seed which should be soaked in hot water overnight, or until the seeds swell up. Seedling trays, black bags or pots can be filled with seedling mix, and the seeds are only covered lightly with soil. Do not allow the soil to dry out until germination takes place, usually within 3 to 12 days after sowing. The seedling will transplant well in spite of their long tap root, but wait until they unfurl their second leaves before transplanting.

Problems, Pests & Diseases:

Sweet thorns are resistant trees that are not usually attacked by pests and diseases. Several fungi are associated with this tree and the crown of mature trees may be parasitized by various mistletoes, leading to the tree's decline.