Grevillea ROBUSTA

Grevillea ROBUSTA - Silky oak, Silk oak, Australian silver oak, Grevillea robusta.

Can be trained as bonsai!

Grevillea is a large genus of evergreen shrubs and trees in the protea family (Proteaceae), numbering nearly four hundred species. Almost all of them are endemic to Australia, with only a few occurring in New Guinea, on the islands of Sulawesi and New Caledonia.

Leaves in grevilleas are very diverse: from narrow entire to repeatedly pinnatisect, often with recurved margins and silvery pubescence on the underside, causing the crown to shimmer in the wind.

Flowers lack typical petals: their perianth is tubular, splitting into four lobes and releasing a long curved style. They are gathered in dense one-sided racemes resembling either a toothbrush or a spider. The flowers produce a great deal of nectar, and in Australia they are pollinated not only by insects, but also by birds. The fruit is a leathery follicle containing two flat winged seeds.

Silky oak is the largest species of the genus and the only one that succeeds well in ordinary indoor culture. It originates from eastern Australia, where it grows along riverbanks, in subtropical and dry rainforests, growing into a single-trunked tree from 5 to 40 m (16 to 131 ft) in height.

Leaves resemble fern fronds: large, bipinnatisect, dark green above, silvery beneath from velvety pubescence. Flowers are honey-yellow and orange, gathered in dense one-sided racemes with protruding long styles. Grevillea grows very quickly: in 3-4 years, a tub specimen develops from seed.

Grevillea seeds are flat, with a thin membranous wing along the edge, ripening in leathery follicles with two per pod. Soaking such flat seeds is impractical, so the substrate itself is sanitized: before sowing, it is drenched with a weak potassium permanganate solution and dried. Scarification and stratification are not required.

Sowing is carried out with an interval of 2-3 cm (0.8-1.2 in) between seeds, with minimal embedding. The substrate is composed of 1 part leaf mold, half a part turf soil, half a part humus, and 1 part sand, or a universal potting soil with added river sand and vermiculite is used. The sowing is moistened with a spray bottle using a growth stimulant solution: SUPERthrive, HB-101, Aminozol, Epin, or Zircon, covered with plastic wrap or placed in a propagator, and germinated in the light at a temperature of no lower than 18-20 °C (64-68 °F).

Under favorable conditions, the first sprouts emerge in as little as 2-3 weeks; main germination extends over a month to a month and a half, while individual seeds may delay longer: they germinate noticeably unevenly. Sowings are ventilated regularly; seedlings are kept in a bright spot with shade from harsh rays. Pricking out is performed as they grow, one seedling per individual pot about 7 cm (2.8 in) in diameter, into a mixture of equal parts turf, peat, humus soil, and sand.

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The plant needs bright diffused light. From April through September it is protected from direct sun, while in winter it is kept in the brightest spot. In summer it is beneficial to take grevillea outdoors to a spot protected from direct sunlight: it appreciates a fresh air supply.

In spring and summer, the optimal temperature is 19-24 °C (66-75 °F). In winter, the plant needs a genuine dormant period in a cool, bright room at 6-12 °C (43-54 °F).

From spring to autumn, watering is regular as the top layer of substrate dries out, using soft settled water. Starting in autumn, watering is reduced; in winter, it is watered moderately, without letting the root ball dry out completely. Grevillea appreciates high air humidity: it is regularly misted with soft room-temperature water or the pot is set on a tray of moist expanded clay or peat without the bottom touching the water.

The substrate must have an acidic reaction: a mix of 1 part leaf mold, 2 parts conifer soil, 1 part peat, and half a part of sand with added brick rubble is suitable. Good drainage is placed at the bottom of the pot. A shallow, not overly spacious container is chosen: grevillea grows more poorly in excessive soil volume.

Fertilizer is applied during the period of intensive growth, from April through October, twice a month with a balanced fertilizer.

Pruning is essential for the plant: without it, it stretches and quickly outgrows the room, while after shortening shoots it readily branches and becomes a dense bush. Specimens 2-3 years old are repotted annually in spring, those over three years old every two years, and tub plants as needed, top-dressing with fresh substrate annually.