Grevillea is a member of the Protea family (Proteaceae) and its close relatives include Banksia, Hakea, Isopogon and Telopea (the Waratah). Grevillea is named after Charles Francis Greville who was one of the founders of the Royal Horticultural Society in 1804. There are over 300 species in the genus.
Scientific name:Grevillea robusta
Common name: Grevillea, Silky oak
The flowers of Grevillea species are quite small but they occur in clusters (an inflorescence) which, in some species, may consist of perhaps 100 or more individuals. The sequence of opening of each flower is similar to other members of the Proteaceae and goes through several stages:
- In bud, each flower appears as an elongated narrow tube (the perianth) comprising four segments each having an anther containing pollen at its tip.
- As the flower opens, the perianth segments separate to reveal a narrow style. Just before the flower fully opens the anthers transfer their pollen to the tip of the style (the stigma)
- Finally, the style separates from the perianth. At this stage the style and stigma, with attached pollen, is called a 'pollen presenter' (i.e. it is 'presenting' the pollen to a pollinator, usually a bird or small marsupial, which acts as the agent to transfer pollen from one flower to another for fertilization
Grevilleas are propagated by three principal methods; seed, cuttings and grafting. Tissue culture has also been used with a few species and cultivars but this is a more specialist method which is not of practical interest to most amateur growers. To maintain desirable characteristics of a particular plant, vegetative propagation (e.g. cuttings or grafting) must be used. This also applies to propagation of named cultivars.
As seed is shed annually, plants need to be kept under observation and seed capsules collected when they first begin to open.
Germination of seed of grevilleas can be slow and sometimes difficult. To assist germination a variety of seed pre-treatments have sometimes been attempted. Some of these work on some species but not on others.
The most usual method of pre-treatment is to 'nick' or peel off the seed coat with a sharp blade to allow moisture to reach the embryo. This needs to be done with care to avoid damage to the embryo. For seed with thin walls, pouring hot (not boiling!) water over the seed and allowing it to soak for a day or so is sometimes successful.
Seed can be sown in normal seed raising mixes and seedlings could be expected to appear in anything from 2 weeks to a year after sowing, depending on the species and the time of sowing. Those species native to temperate areas may not germinate in the heat of summer (this may be an ecological factor to enhance the chance of survival of the seedling in the wild). These species are best sown in autumn or early spring.
Propagation of Grevilleas from cuttings is generally a reliable method and is preferred over seed because of both the scarcity of seed and problems in germination. In addition, cutting-grown plants will usually flower at an earlier age than seedlings.
Cuttings about 75-100 mm in length with the leaves carefully removed from the lower half to two-thirds seem to be satisfactory. "Wounding" the lower stem by removing a sliver of bark and treating with a "root promoting" hormone both seem to improve the success rate. No special propagating mixes or treatments are required.
In conclusion Grevillea robusta is mainly propagated through seeds and wildings from natural regeneration. A kilogram of seed may contain 70,000-100,000 seeds. Seeds can be stored in air-tight containers in a cool dry place for up to two years without significant loss of viability. Seeds germinate within 8 – 20 days with expected germination of 55,000 seedlings from 1 kg of seed
Grevillea robusta originated from Eastern Australia. The species was introduced in Kenya as a coffee shade and is now naturalized in the country. Grevillea robusta grows well in areas between 850 to 2500 metres above sea level with a mean annual rainfall of 600 to 1500 mm and mean annual temperatures of 13 degree Celsius to 21 degree Celsius.
Grevillea robusta trees can be planted; along boundaries, as woodlots, on terraces, in alleys, and scattered among crops such as tea, coffee, maize, bananas and beans. Planting along farm boundaries is done in single rows at 2–2.5 m spacing but in small farms it is can be planted closely spaced at about 1.5 m between trees. A spacing of 2.5 x 2.5 m is recommended for plantations and woodlots. The species performs best on well drained fertile soils but also grows moderately well on medium textured soils (loam, clay-loam to light sandy soils). However, it does not tolerate water logged soils. The species is widely grown on farms in the coffee and coffee-tea zones of central highlands eco-region with high populations of the species in; Meru, Embu, Kirin-yaga, Muranga and Kiambu counties. Grevillea robusta is a fast growing tree. On suitable sites, Grevillea can attain a height of 20 m and diameter of up to 25 cm in 15 to 20 years.
Grevillea is used for sawtimber, firewood, poles, in agroforestry applications, as fodder and bedding for livestock, and as shade for tea and coffee.
In conclusion Grevillea has deep roots and tolerates heavy pruning and pollarding, meaning it doesn’t compete for water, nutrients or sunlight with surrounding crops. It is easily propagated from seed and grows well even without fertilizer and in soil prepared by hand implements, meaning farmers can plant it without great cost or labor. Most people typically plant grevillea around their homes, to demarcate fields, as wind breaks between fields, as single trees, and in small woodlots.