Grevillea Robusta: A versatile tree to plant

Grevillea Robusta, commonly known as Silky Oak or Silver Oak, has gained widespread popularity among agroforestry enthusiasts, originally as a shade tree for tea and coffee and more recently as an agroforestry tree.

The tree provides economically valuable products including timber, poles, firewood and leaf mulch; it is easy to propagate and establish and is relatively free of pests and diseases; its root system helps it grow in low-fertility soils; it does not compete strongly with adjacent crops; and it tolerates heavy pruning of its roots and branches.

With its fern-like leaves and attractive orange flowering display, it is also popular as an ornamental.

While the species is alien to Uganda, it has been grown in the country for a long period of time and has proved to be an appropriate agroforestry species. It is now a naturalised exotic species without any negative tree-crop interactions reported.

Grevillea can grow as tall as 30 metres in height and 80cm in diameter in its natural range and erect posture.

Why Grevillea?

Growing trees as a business in Uganda, whether for timber or other wood products, can be a profitable and deeply satisfying business venture but only if it is carefully planned and well managed.

While Grevillea Robusta farmers focus mainly on construction timber production, it can provide other benefits. Generally, forest plantations are a cost-effective way of producing wood-based products. In Uganda, the forest cover is slowly decreasing.

With demand already exceeding supply, there is a fuel wood and a timber crisis. Planting Grevillea Robusta is one way to help restore the disappearing forest cover.

It makes economic sense too as the estimated sustainable yield of exploitable trees in natural forests in Uganda is still very low at around 2m3/ha/yr., where the average annual yield can easily be 10-15 times. Grevillea Robusta, in particular, can help solve fuel wood and construction timber challenges at both domestic and industrial levels.

Dr Isaac Kiyingi, the programme leader for forest conservation and management at National Forestry Resources Research Institute (NaFORRI) in Mukono District, says that they promote agroforestry campaigns for different reasons.

“Many people in agroforestry normally plant eucalyptus and pines but we are trying to diversify. Grevillea trees rejuvenate the soil and can be intercropped effectively,” says Dr Kiyingi.

He explains that has been successfully intercropped with bananas and beans.

Agronomy

Used as intercrops, 208 trees per ha can be planted in a spacing of 4m by 12m. Other spacing recommendations are 312 trees/ha in 4m x 8m and 625 trees/ha in 4m x4m.

Dr Kiyingi says that Grevillea Robusta is a preferred cover tree in coffee to act as windbreaks. Previously, it has been present on most of the tea farms.

Shading systems, Dr Kiyingi says, minimise the exposition of plants to climatic risks such as radiation, winds and high temperatures, which contributes to increased sustainability of the coffee.

Previous studies of Grevillea’s versatility have been conducted in the coffee-banana agro-ecological zones of Uganda for Trees for Global Benefit (TGB) - an afforestation/reforestation carbon management scheme for rural communities in the Albertine rift (Rubirizi, Mitooma, Kasese, Hoima and Masindi) and Mt. Elgon (Mbale, Manafwa and Bududa districts).

Maintenance

The main costs associated with maintenance of Grevillea Robusta, if planted as a monocrop, include labour costs of tillage operations, tree and crop planting, weeding, harvesting of crops, thinning and pruning, crop harvesting and timber harvesting.

Tillage, which involves removing the weeds that would otherwise compete with the trees, is normally done while the trees are young and vulnerable to competition. At planting, holes should be deeper than the root-ball and should be at least three times wider. This creates an opportunity for settling of the root and decreases the chance of root ball suffocation. Planting stock should come from seeds or wildlings of high quality mother trees. Seedlings should be healthy, non-deformed, and of the recommended height of 1 foot (30cm).

Before the trees mature, a farmer can reap some money by selling firewood for cooking. After maturity, which happens after about 12 years, the farmer is able to sell the trees for firewood and make a hefty profit in constriction wood.

Dr Kiyingi explains that young trees fetch about Shs6,000 per tree but at maturity each goes for at least Shs100,000. In an intensive plantation system, one can earn up to 62.5m from a hectare of 625 trees. “It is definitely worth the wait,” he says.

Key considerations

Dr Kiyingi cautions against the excitement this profit can create.

Dr Kiyingi explains that forestry is not an investment that suits everybody. It is long-term and requires a lot of investment in the first few years.

“Establishing high yielding plantations is not cheap. The investor must have sufficient funds available for the field activities - especially in the critical period immediately before and after planting,” he says. He estimates the costs associated with commercial tree establishment at around Shs1.5m per ha.

Access

Dr Kiyingi stresses that good access to the planting site is important to be able to carry out the required establishment activities as well as subsequent management operations such as thinning, pruning and fire protection.

“When the trees are mature (and even when thinning operations are yielding utilisable material), good access is required to transport the logs to the mills. It pays to plan the internal road network early on,” he says.

Site selection

According to Dr Kiyingi, the main components of site quality are soil depth and drainage; soil physical and chemical composition; the competing vegetation; the amount and distribution of rainfall, and the general climate of the area.

Although trees can often do well on degraded unfit for most agriculture practices, he says that shallow soils especially on hilltops will not produce very productive tree growth.

Species

An important factor is that improved genetic material performs better than locally available seeds.

He says that huge gains in productivity have been realised through using genetic material that has been improved through rigorous selection and testing. These seeds are available at the National Tree Seed Centre in Namanve.