Acacias/Vachellia/Senegalia are Africa’s trademark and (apart from the Baobab), the most recognised tree in Africa. No one will ever forget the silhouette of a Thorn tree against the setting sun.
They are known for being hardy and thorny. Many a bonsaist will tell you that they regret working on this tree without gloves (and who works with gloves?).
Acacias are normally tall and semi-evergreen, shrubs or small trees. Their leaves are feathery, divided leaflets and generally a soft green colour. The stems are generally rough, vary in colour and very often possess long, sharp, or hooked multiple thorns. Some acacias have a very nice fragrant flowers. Flowers, ranging from white to yellow in colour appear in late winter. Some fruits have glossy coats and contain seeds preferred by birds and other wildlife. Acacias have a tendency to grow up and out, so you have to pay attention to training.
There are very many different species and not all are suited to bonsai. Popular species (but not limited to) are: Senegalia burkei (Black Monkey Thorn) ; Vachellia erioloba (Camel Thorn); Senegalia erubescens (Common Hook thorn); Senegalia galpinii (Monkey Thorn); Vachellia karoo (Sweet Thorn); Vachellia robusta (Enkeldoring) and Vachelia sieberiana (Paperbark Thorn).
Soil & pH
Always use a well-draining basic bonsai soil mix . Any fast draining soil is good. Acacias can tolerate low pH, but are sensitive to saline/salts in soil, so special care should be taken along coastal areas. It also helps to spray down the leaves late in the afternoon to wash away any salt deposits. (Thanks, Thian – this may not be necessary if you are inland! – Ed). I will also add that one should spray the leaves to wash away dust.
Acacia prefer slightly drier conditions; so allow the soil to dry between waterings. Do not let the soil dry out too much, as the leaves will drop if the soil is allowed to get too dry. You can water twice a day in summer (morning and night) depending on your location and once a day in cooler months. Acacias are very hardy, so don’t stress too much if you miss an odd watering!
Acacias are generally pruned in early Spring when the buds break. They not only tolerate a second heavy pruning around the middle of Summer, but actually thrive. Regular maintenance during the growing season is necessary. Always seal all cuts.
Repot only when necessary and take care not to disturb the roots too much. Acacias hate having their tap root cut, so do that in stages, and always using an anti-fungal agent.
Light and Temperature
Acacias prefer full sun, but will do well in dappled shade. Acacias need at least 4 hours of sun a day.
Feed every 2 – 3 weeks during the Summer growth period. You can use a standard bonsai fertilizer of your choice, or look for one with low nitrogen content to encourage flowering. (Although I have never seen or heard of a flowering Acacia bonsai.
They germinate very easy from seed and are quick growers. It is best to plant the seeds in pots with little soil so the tap root bends and curls early on. It will make it easier to fit in a pot later on.
Pests & Diseases
Aphids, flies, mites etc., hard shelled insects like scale and wooly aphids. Occasionally anthracnose can infect leaves.
Different species flower at different times of the year, mainly April to November. Do not over water in winter and do not trim at all after summer (December) to encourage flowering.
Acacia bonsai have a tendency towards growing upwards which can destroy lower branches and harm the overall look of your tree, keeping the top branches shorter in order to give the lower branches more light. Ensure you prune regularly at the top of the tree to encourage lower growth. Pay attention to branch pruning as leaf pruning will follow. Wiring Acacias are a tough job and they lend themselves more towards clipping and growing. You can even opt to tie branches down, so as to minimise damage. Don’t let moss grow on the main stem. It will destroy the bark and rot can set in. You can treat damage with a dusting of fungicide.
I have seen the usual Black Monkey Thorns (Senegalia burkei) (I find it really difficult to use the new names Vachellia or Senegalia – and will lovingly refer to them as Acacias, but use their proper names🙂 as bonsai, and I have seen the Knob Thorns (Senegalia nigrescens – they are notoriously difficult to have in a proper bonsai style), and Paperbark Thorns (Vachellia sieberiana). What is rare as bonsai, is the Fever Tree (Vachellia xanthophloea), Sweet Thorn (V.karoo) and my favourites, the Enkeldoring (V.robusta), Camel Thorn (V.erioloba) and Driehaakdoring (S.senegal). These trees are my favourites because their leaves are naturally small. What other types of Acacias do you have?