Overview:
Acacia robusta Burch. subsp. usambarensis (Taub.) Brenan, commonly known as Usambara, is a plant species that belongs to the legume family Fabaceae. It is a native plant to East Africa, particularly in Tanzania, and it is commonly found in the forest regions at altitudes of up to 2,000 meters above sea level.
Appearance:
The Usambara plant is a tree that can reach up to 30 meters in height, although it commonly grows to around 10-15 meters. It has a straight trunk with gray to brown bark, and its branches are sturdy and often bear thorns. The leaves of the plant are bipinnate, meaning they consist of multiple leaflets arranged in a feather-like structure. The flowers are yellow and arranged in clusters, while the fruit is a straight, flat pod that can reach up to 25 cm in length.
Common Names:
The plant is commonly known as Usambara acacia or Usambara pea tree in English. In Swahili, it is known as Mkojo or Mkwaa. Other common names for the plant include the Umbrella thorn and the Flat top acacia, referring to the tree's flattened crown.
Uses:
The Usambara plant has a variety of uses. The wood is known to be hard and durable, making it ideal for construction purposes, including furniture and building poles. The tree's bark is a rich source of tannins, which can be used to tan leather. In addition, the plant's pods and bark are often used for medicinal purposes, particularly in the treatment of respiratory and digestive issues. The Usambara plant is also used in traditional African medicine as a treatment for fever, malaria, and other ailments.
Overall, the Usambara plant has significant cultural, economic, and medicinal value in East Africa, making it an important and cherished species in the region.Light requirements
The Acacia robusta Burch. subsp. usambarensis (Taub.) Brenan typically grows in areas with full sunlight exposure. This plant can tolerate some shade but performs best when exposed to bright light. In a shaded environment, the plant can grow spindly, producing a weak stem.
Temperature requirements
The Acacia robusta Burch. subsp. usambarensis (Taub.) Brenan thrives in hot and arid areas, such as those found in subtropical and tropical regions. It can withstand high temperatures of up to 50 degrees Celsius during the day and slightly cooler temperatures at night. Optimal growth occurs within a temperature range of 20 to 35 degrees Celsius. This plant is intolerant of frost and freezing temperatures, and it cannot survive in areas with prolonged cold weather.
Soil requirements
The Acacia robusta Burch. subsp. usambarensis (Taub.) Brenan grows best in well-draining soils with high nutrient content. The plant prefers sandy or loamy soils with pH values ranging between 6 and 7.5. This plant can tolerate slightly acidic or slightly alkaline soils but performs best within this pH range. Acacia robusta is susceptible to root rot, and as such, it requires soils with good drainage to prevent waterlogging. The plant can also tolerate slightly saline soils, making it an excellent option for coastal planting.
Cultivation Methods
The Acacia robusta Burch. subsp. usambarensis (Taub.) Brenan can grow in various types of soils, including sand, loam, and clay. It prefers well-drained soils with a pH range of 4.5-7.5. It requires ample sunlight and grows well in areas with an average annual temperature of 20°C or above. This plant can be propagated by both seeds and stem cuttings. However, it is easier to propagate from seeds, which should be sown in a well-draining medium.
Watering Needs
The Acacia robusta Burch. subsp. usambarensis (Taub.) Brenan requires moderate watering. It can tolerate some drought but should be watered during dry spells to prevent the loss of leaves. Avoid overwatering as it can cause root rot.
Fertilization
Fertilizer application to the Acacia robusta Burch. subsp. usambarensis (Taub.) Brenan should be done during the growing season, which is typically in spring and summer. A balanced fertilizer with equal amounts of nitrogen, phosphorus, and potassium is ideal. Apply the fertilizer at the base of the plant, and do not over-fertilize.
Pruning
Pruning the Acacia robusta Burch. subsp. usambarensis (Taub.) Brenan should be done during the late winter or early spring. Prune any dead, damaged or diseased branches. Also, prune any overgrown branches to control the size and shape of the plant. Trim the branches just above the leaf node and avoid removing more than one-third of the plant's total foliage.
Propagation of Acacia robusta Burch. subsp. usambarensis
The propagation of Acacia robusta Burch. subsp. usambarensis can be done through several methods, including:
Seed propagation
Seeds can be used for propagating Acacia robusta Burch. subsp. usambarensis. The seeds need to go through pre-treatment before sowing. This can be achieved by soaking them in hot water for 24 hours or nicking the seed coat. After pre-treatment, the seeds are sown in a tray containing a mixture of sand and soil in equal proportions. The tray should be placed in a well-lit area, and the seeds should be watered regularly. The seedlings can be transplanted to a permanent location once they have grown big enough.
Cutting propagation
Propagation through cuttings is another method that can be used to propagate Acacia robusta Burch. subsp. usambarensis. Semi-hardwood cuttings of about 15cm long are taken during the growing season. The lower leaves of the cuttings are removed, and they are dipped in a rooting hormone before being planted in a pot containing a mixture of sand and potting mix. The pot should be placed in a warm and humid area, and the cuttings should be watered regularly. The cuttings will start to show new growth within a few weeks and can be transplanted to a permanent location once they have developed roots.
Air layering propagation
Air layering is another propagation method suitable for Acacia robusta Burch. subsp. usambarensis. A mature stem is selected, and a ring of bark about 2cm wide is stripped from the stem. The exposed area is covered with a rooting hormone and wrapped with moist sphagnum moss. The moss is then covered with a plastic wrap to create a humid environment. The moss is kept moist, and after a few months, roots will develop from the exposed area. The rooted section can be cut from the parent plant and potted in a well-draining potting mix.
Disease Management
The Acacia robusta Burch. subsp. usambarensis (Taub.) Brenan plant is known to be susceptible to a few diseases that might affect its growth and productivity. The following are some of the common diseases:
- Acacia leaf spot: This disease is caused by a fungus that appears as dark spots or patches on the leaves, and it might lead to leaf drop or defoliation. To manage this disease, it's advisable to prune out affected branches and leaves and dispose them in a burn pile or landfill. Applying a fungicide might also help to suppress the spread of the disease
- Canker diseases: Canker diseases are caused by various fungi that attack the bark of the tree, leading to sunken areas, dead patches or scabby spots. To manage this disease, it's recommended to prune out affected branches and use a fungicide to protect the remaining parts of the tree. Also, ensure that the tree is healthy and free from wounds to prevent the infestation of fungi.
- Powdery mildew: Powdery mildew appears as a white powdery substance on the leaves, flowers, and stems. It might lead to stunted growth and deformity of the tree. To manage this disease, spray the tree with a fungicide early in the season, ensure good air circulation, and avoid overhead watering.
Pest Management
Like all plants, the Acacia robusta Burch. subsp. usambarensis (Taub.) Brenan plant is susceptible to attacks from various pests that might cause damage and reduce the productivity of the plant. The following are some common pests:
- Borers: Borers are insects that bore into the trunk or branches of the tree, leading to wilting, branch dieback, and eventually death. To manage this pest, use pesticides that contain insecticides, remove infected branches, and ensure that the tree is healthy to resist the infestation.
- Mites: Mites are tiny creatures that suck sap from the leaves, causing them to become yellow or stippled. To manage this pest, it's advisable to use acaricides, spray the tree with neem oil, and ensure that the tree is well watered and fertilized.
- Mealybugs: Mealybugs are insects that appear as white fluffy masses on the leaves, causing them to become yellow and wilted. To manage this pest, use a systemic insecticide, prune out infected branches, and avoid overwatering or over-fertilizing the tree.
