Large Acacia karoo or Sweet Thorn tree

Acacia karroo

(presently Vachellia karroo)

Description

A truly exceptional tree, with a plethora of benefits for the gardener, farmer and the environment. There are three distinct growth forms divided into several subspecies with the largest (and the one we grow) being the Dune Forest Form. It has a tall, single, upright trunk, sometimes branching low down to create a multi-stemmed appearance with attractively peeling bark, and a dense, somewhat rounded crown that provides lovely dappled shade. The tree will often flower prolifically up to 4 times per year, depending on the rains, and the cheerful, bright yellow puffball blooms light up the entire tree, bringing life and beauty to its surroundings.

SA Tree

Family

Fabaceae or Leguminosae (Legume or pod-bearing family).
This family is also divided into 3 subfamilies; mimosa or acacia subfamily, cassia and pea.
Regarded as one of the largest, as well as economically significant families, its members have been cultivated since early times for their plethora of uses – as food, medicine, fodder, for practical use (tannins) and ornamental displays.

Habitat

Arguably one of the most widespread trees in Africa, it occurs in nearly all parts of the country, from temperate to almost desert-like conditions.
They are found mainly in savannah grassland, where the soil is alkaline, but also grow in coastal shrub, woodland, and along streams and rivers.
The trees are however not normally found growing in overly acidic soils, or where the winters are cold and wet.
In the wild, the trees are usually a good indicator of water and nutrient-rich soils

Bark

Deep rusty-brown, sometimes darker often flaking to expose the reddish under bark.
The bark has a somewhat leathery texture, and young branches are a lovely olive-brown or rusty-red colour, with a layer of conspicuous beige dots.

Foliage

Semi-deciduous.
The leaves are twice-compound, oblong, light and feathery-textured, dark-green, and typically hairless.
The main leaves comprise of about 5 pairs of leaflets, and each of these is again split into 8 or more pairs of smaller leaflets of about 6 x 2.5mm long.
They occur singly or in pairs of up to 8 in the axils of thorns.
The leafstalks are 5-11mm long.
The thorns grow in pairs, are straight, stout, white with darker tips, and usually measure between 2.5 to 10 cm long.
On younger trees they appear to be more prominent.
Typically, they will be longer and thicker towards the base of tree, whereas at the top they are smaller and fewer in between.

Flowers

Small, golden-yellow balls (13mm diameter) in groups of 1-7, growing on short branches at the ends of stems, not amongst the leaves.
They are sweetly scented., and usually occur from October to February, but flowering can be sporadic, depending on the seasons rains.

Fruit

Dark-brown, slender pods, (100-260 x 6-10 mm).
The pods are woody textured, usually slim and sickle-shaped, and are borne in clusters that split open on the tree.
January to April.

Height

The tree size can be extremely variable, depending on conditions, but typically they grow between 3-15m, occasionally reaching 15 – 17m

Spread

Medicinal

The bark, leaves and gum of the tree are mostly used, less often the roots.
Crushed leaves are used as poultices for open wounds, and as a gargle for sore throats.
The gum is used as a remedy for oral thrush.
The bark and leaves are used as emetics in the treatment of dysentery, while concoctions of both of these and the gum are used as ointments to treat colds and conjunctivitis.
A substance found in the heartwood of this tree is currently being researched for its blood pressure lowering properties.

Practical

The nutritious and versatile gum is often used in the food industry as an additive and makes a good water- soluble glue.
Seeds have been roasted to make a good alternative to coffee, and the gum is also consumed raw by people and monkeys.
The wood is hard, heavy, thick grained and durable, and makes a good quality firewood.
It has also been used extensively for making furniture, fencing posts, yokes and in turnery.
The early European settlers used the wood to construct their wagons.
A lovely red dye can be obtained from the tannin-rich bark, and it has been used in the tanning process to give leather a rich, even colour.
The bark also produces a good twine, as it is slightly fibrous.

Warnings

The nutrient rich wood is prone to attacks from borers, and mature trees have been known to suffer relentless attacks from both these and several fungi species.
The tree also has an invasive root system and should be planted well away from permeant structures.

Wildlife

The tree provides a high quality, nutritious fodder for livestock and game and is known to be a favourite of the Black Rhino.
Monkeys, baboons and bush babies eat the sweet gum, as well as many insect species.
The nectar and pollen rich flowers attract a multitude of honeybees (very good honey tree) and butterflies, many of which have been known to use this as a food tree for their larvae.
Birds also favour the tree as a nesting site, as the thorny branches provide shelter and protection from predators.
The flowers are also readily eaten by many animals, and the multitude of insect life it attracts will inevitably lure a host of insectivorous bird species.

Advantages

Like many other Acacia species, the tree has nitrogen fixing abilities, and is able to harness the element directly from the air with the help of symbiotic bacteria living in its roots.
It then effectively converts it and thereby enriches the surrounding soil.
The tree is thus an asset in any garden or park, as it will not only provide shelter from the elements to sensitive plants planted in its wake, but also helps with the rehabilitation of the soil.

Tolerance

An amazingly hardy tree, that, once properly established, can withstand extended periods of drought and cold.

Growth Rate

Normally a fast grower, 1- 1.5 m per year, but this may be less if the habitat is unfavourable.

Lighting

Soil & Water

Prefers an alkaline, nutrient rich soil, but will tolerate a wide range of soil types, from clay to loam, but typically does not like sandy soils.
Low to moderate water needs.

Propagation

Soak seeds in warm water overnight and use only ones that have swollen in size.
They can then be directly sown into large individual bags, as transplanting them at a later stage might stunt the saplings growth due to the presence of the long, sensitive taproot.
The growing medium should be a mixture of river sand (to ease drainage) and compost (2:1).
Cover the seeds with a thin layer of mulch and keep moist.
Germination is usually good and will occur within 2 weeks.
Place the containers in a warm, bright area.
It is recommended to provide the saplings with adequate trunk support, as they grow rapidly.