Agroforestry is a practice that is quickly gaining popularity among Kenyan farmers due to multiple benefits that farmers receive. When planting trees on the farm, farmers need to grow them in a way that minimizes competition between the plants and ensures they benefit from each other. For example, millet farmers who have livestock may plant acacia trees on the same farm as the millet. During the millet growing season, acacia trees shed their leaves and pods, which are rich in proteins, and also good provide feed for cattle.
One advantage of incorporating trees on farms is that soil fertility is maintained. Crops like maize take up a lot of soil nutrients and can exhaust the soil after a few years, leading to decline in yields. Production costs increase as farmers attempt to increase yields quickly using chemical fertilizers, with subsequent reduction in profits.
Grevillea robusta, also known as silky oak, is a high value tree in Kenya, which provides timber for furniture, plywood, veneer and poles for the building and construction industries. It also provides shade and fodder, while controlling soil erosion and improving soil fertility through leaf litter decomposition.
Initially, the tree was used in tea and coffee farms for shelter and windbreak, but quickly became popular for its wood and timber.
Planting Grevillea robusta
Research by the Kenya Forestry Research Institute (KEFRI) shows that most farmers plant grevillea along the farm boundaries although others plant them as woodlots in terraces/alleys and also scattered among tea, coffee, banana and bean plantations.
The trees grow in altitudes of 0-2300m above sea level and establish well in loam soils.
When planted along farm boundaries, KEFRI recommends a minimum spacing of 2 by 2 metres between single rows. Planting at a spacing of less than 1.5m is discouraged as such trees grow slowly and are smaller in diameter than those planted at wider spacing. Leave a space of 3m-4m between seedlings.
For woodlots and plantations, a spacing of 2.5m by 2.5m is recommended.
Because of its deep rooting system, Grevillea robusta does not interfere with shallow rooted crops and can be intercropped successfully with bananas, tomatoes, maize, beans and other crops.
Caring for young trees
The trees grow fast in areas with suitable climate and medium to fertile soils and free of weeds. In such conditions, the height increases by at least 2-3m every year while the diameter increases by at least 2cm.
Farmers should prune lower branches of the trees repeatedly after the first year to provide wood and prevent excessive shade to the farm and competition with crops.
Pest and disease control
Seeds of grevillea sometimes are attacked by moulds caused by the mold fungi. They also suffer nfrom rots caused by Fusarium and may sometimes harbor seedborne pathogens such as the canker fungi.
At the young stages, the seedlings may be attacked by the above fungi and many others which may cause various disease symptoms that include root colar rots, stem rots, leaf spots and blights among others.
Seed diseases are controlled by dusting the seeds with fungicides such as Trichotech®, Eco-T® and Harzianum® or other common seed treatment fungicides before sowing. Once sown in the seed bed, it is important to spray the young seedlings with fungicides such as Copper Oxychloride to protect them from fungal attacks.
Insects such as cut worms may attack young seedllings and they are easily controlled using common insectides used to control cut worms infestation in crops.
In the field, grevillea is attacked by termites and canker and die back disease (caused by canker fungi), which greatly reduce timber quality. The disease is characterized by dieback (slow death of branches or shoots), spots and blights on leaves, and cankers on stems (trunks) with moderate to severe resin flow.
It is recommended that infected branches are removed as soon as dieback symptoms are noticed.
Although not common, termites can be a problem especially in the dry areas and can be difficult to locate since they mostly build their nests underground.
The following are some methods that are useful in the control of termites:
- Using plant extracts: Such as garlic (bulb), cashew (seeds and oil), tea leaves, pawpaw (fruit, fresh leaves and roots), basil and teak (wood or pulp) and black jack. To prepare the mixture, grind up the 100-200g of the relevant parts of the plant, place in boiling water, stir and leave to soak for 24 hours. Spray immediately, early in the morning or late in the afternoon.
- Adding organic material to the soil: Compost or well-rotted manure increases organic matter in the soil, which provides termites with dead plant material for food. This prevents them from feeding on living plants.
Avoid having bare and dry soil around the trees. You can also mulch the trees with hay, wood shavings, wood ash or threshed maize cobs to decrease termite attacks. Using inorganic fertilizer is not recommended as it encourages fast growing soft tissue, which is susceptible to termite attack.
- Natural predators: Organisms like spiders, beetles, flies, wasps and ants as well as frogs, reptiles, birds, monkeys and humans feed on termites, which are a rich source of protein. Ensure you maintain natural habitats like bushes and trees, which are home to most of the predators.
- Transplant only healthy seedlings: Weak plants are more susceptible to termite attack than healthy ones. Grevillea seedlings should be transplanted at the beginning of the wet season to give them a chance to establish properly and remain healthy in the field.
- Breaking up mounds and removing the queen is effective as is regular digging and ploughing of the soil.
- Metarhizium anisopliae ICIPE 69, an insect-killing fungus manufactured by Real IPM. The biopesticide is effective in controlling termite colonies and can be injected into the nest at the rate of 200ml per hectare.
These methods of control are more effective if used in combination rather than one alone. When seriously attacked by termites, the eco-friendly termiticide Metarhizium anisopliae ICIPE 69 is effective in keeping the termites under control to avoid huge losses especially in the warmer areas.
Harvesting and marketing trees
Mature Grevillea robusta trees are usually ready for harvesting once they are 6-7 years old. These can be used for wood and poles. Such young trees are not good for timber as the wood is easily attacked by pests like wood borers and rot fungi.
Mr. John Magoma, a farmer from Mosocho, Kisii County, says he usually harvests his trees once they are about 15 years old.
The demand for timber is huge and he sells sells a tree for between Kshs 16,000 – 30,000 depending on the tree’s size and quality. Each tree yields about 50 pieces of timber each 14 ft long. “I have planted the trees around the farm (boundary planting) and this does not affect the crops.
The Grevillea trees improve the soil around it and do not compete with the crops for water or nutrients. We also cut the branches for firewood which provides good money for our daily needs.”
Although KEFRI recommends that trees less than 10 years should not be harvested for timber, Mr. Magoma notes that this is difficult to adhere to since he has many household needs.
“Waiting for 15-20 years to get timber is good but because my family needs money, we harvest some trees when they are less than 10 years old for poles and firewood, which has a high demand and fetches good prices in the local market.
I planted 50 trees around my 1 acre homestead in 1984 and I harvested half of them after 15 years. I replanted seedlings and
I am now almost harvesting the second lot for timber.
Facts about grevillea
- Grevillea grows best in moderately fertile to fertile and well drained soils. These are mostly found in the medium and high potential areas in Kenya.
- Grevillea grows well with crops and does not compete with them for nutrients or water. Its ability to withstand regular pruning provides farmers firewood throughout the year.
- Trees that are aged over 30 years are the best for good quality and durable timber.
- The trees can be repeatedly pruned and pollarded. As the tree matures, the volume of wood produced increases.
- Hot conditions such as those in the semi-arid areas appear to promote growth of fungus that causes canker and die back symptoms.
- Grevillea produces hard timber with beautiful grain structure, which is used to make attractive furniture.
This article draws on the research and published work of Dr. Jane Wangu Njuguna, Chief Forest Pathologist /Deputy Director, Forest Productivity and Improvement, Kenya Forestry Research Institute (KEFRI).