Silky oak (southern silky oak) Grevillea robusta
Silky oak (southern silky oak) (Grevillea robusta) is an evergreen perennial tree in the Proteaceae family, native to Subtropical rainforest of south-east Queensland and north-east New South Wales. It grows 18–30 m (59–98 ft) tall and 8–15 m (26–49 ft) wide, wants full sun and low water, and is hardy to about -6 °C (21 °F). Space plants about 10 m (33 ft) apart.
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Silky oak (southern silky oak) at a glance
| Type | Perennial tree, evergreen |
|---|---|
| Family | Proteaceae |
| Mature height | 18–30 m (59–98 ft) |
| Mature spread | 8–15 m (26–49 ft) |
| Spacing | 10 m (33 ft) centre to centre |
| Sun | Full sun, at least 7 h of direct sun a day |
| Water | Low; drought tolerance high, waterlogging tolerance none |
| Soil pH | 5–7.5 |
| Cold hardiness | About -6 °C (21 °F) once established (roughly USDA zone 9) |
| Heat stress | Likely above about 42 °C (108 °F) |
| Flowering | about 313–521 weeks after planting, for 4–7 weeks |
| Wind tolerance | Low |
| Pollinator value | High |
| Edible | Not grown as food |
| Native to | Subtropical rainforest of south-east Queensland and north-east New South Wales |
| Common in gardens of | 🇦🇺 Australia, 🇮🇳 India, 🇰🇪 Kenya, 🇪🇹 Ethiopia, 🇿🇦 South Africa, 🇺🇸 United States, 🇧🇷 Brazil, 🇮🇱 Israel |
| Invasive in | Hawaii, South Africa, Jamaica, New Zealand: check local rules before planting |
Is Silky oak (southern silky oak) toxic?
- People: mildly toxic
- Cats: mildly toxic
- Dogs: mildly toxic
- Parts: leaves, flowers, seed pods
Causes allergic skin rashes in some people (from tridecyl resorcinol, related to poison ivy’s urushiol); the flowers and seeds may release cyanide.
If a child or pet may have eaten part of a plant, call a poison centre or vet straight away.
Companion planting: what to grow with Silky oak (southern silky oak)
No well-documented helpful pairing. Group it with plants that like full sun and low water.
Questions about Silky oak (southern silky oak)
- How far apart should I plant Silky oak (southern silky oak)?
- About 10 m (33 ft) apart, centre to centre. A mature plant spreads 8–15 m (26–49 ft).
- Does Silky oak (southern silky oak) need full sun?
- Full sun: at least 7 hours of direct sun a day.
- How much water does Silky oak (southern silky oak) need?
- Low water. Drought tolerance is high and waterlogging tolerance none.
- How cold-hardy is Silky oak (southern silky oak)?
- It tolerates about -6 °C (21 °F) once established, roughly USDA zone 9.
- When does Silky oak (southern silky oak) flower?
- About 313–521 weeks after planting, for 4–7 weeks.
- Is Silky oak (southern silky oak) poisonous to cats and dogs?
- For cats it is mildly toxic; for dogs mildly toxic; for people mildly toxic. Causes allergic skin rashes in some people (from tridecyl resorcinol, related to poison ivy’s urushiol); the flowers and seeds may release cyanide.
- What grows well with Silky oak (southern silky oak)?
- There is no well-documented companion pairing for Silky oak (southern silky oak). Group it with plants that like the same sun (full sun) and water (low).
Sources and evidence
Evidence grade A (A: strong evidence).
- Plants of the World Online (taxonomy, native range), Royal Botanic Gardens, Kew
- PlantNET: the NSW Flora Online, National Herbarium of New South Wales, Botanic Gardens of Sydney
- CABI Invasive Species Compendium, CABI
Notes and assumptions
- Like other Proteaceae it has cluster roots that make it very sensitive to phosphorus: feed only with a low-phosphorus native-plant fertiliser, or not at all.
- The tallest grevillea: a fast tree with fern-like silver-backed leaves and one-sided brushes of golden-orange flowers in spring. Planted worldwide for timber and as a shade tree over tea and coffee in India and East Africa; in Hawaii and South Africa it has gone wild.
Reference values for planning, not horticultural, veterinary or medical advice. Varieties, soils and local climates differ: check with a local nursery or extension service.
Related plants
- Macadamia Macadamia integrifolia
- King protea Protea cynaroides
- Pincushion protea Leucospermum cordifolium
- Rosemary grevillea Grevillea rosmarinifolia
- Coast banksia Banksia integrifolia
- Waratah Telopea speciosissima
- Silver tree (witteboom) Leucadendron argenteum
- Oleander-leaf protea Protea neriifolia