Silky oak

Sun

Full sun; young plants tolerate light shade.

Water

Water regularly while establishing; established trees tolerate seasonal drought but benefit from deep watering during prolonged dry spells.

Soil

Well-drained, acidic to neutral soil; tolerates sandy and loamy soils but dislikes waterlogging and strongly alkaline ground.

Height

50-75 ft (15-23 m), sometimes taller in favourable conditions

Spread

25-40 ft (7.5-12 m)

Flowers

Golden-orange, toothbrush-like flower clusters in spring to early summer; young trees may take several years to flower.

Foliage

Evergreen. Deeply divided, fern-like green leaves with silky, silvery undersides; some leaves may fall before flowering or during drought.

Growth

Fast

Care needed

Medium

• Pollinators
• Drought tolerant
• Heat tolerant
• Wildlife

Northern hemisphere months. In Australia, shift them by six.

Difficulty

Hard

Toughness

Needs looking after

Pick your zone, country, light and whether you have pets.

1. Planting day

   7 ft (2.3 m)

2. After 1 year

   12 ft (3.6 m)

3. After 2 years

   16 ft (4.9 m)

4. After 3 years

   20 ft (6.1 m)

5. After 5 years

   29 ft (8.7 m)

About 13 years to full size.

From a typical nursery size, at an average rate for this kind of plant. A rule of thumb, not a forecast.

Cats and dogs

Toxic

Native to

Australia

Invasive in

the US

In the garden

Spreads vigorously: plan to contain it.

Growth rate

Fast

• Plant well away from buildings, drains, paving and overhead wires, allowing room for a large mature tree.
• Water deeply through the first two growing seasons, letting the surface soil begin to dry between waterings.
• Spread organic mulch over the root area, keeping it clear of the trunk.
• Avoid high-phosphorus fertilisers; if growth is weak, use a light application of low-phosphorus native-plant fertiliser.
• Stake only if the root ball moves in wind, and remove supports once the tree is firmly rooted.
• Protect young trees from frost; in most of the UK and Canada, grow only as a container foliage plant brought into frost-free shelter.

Pruning. Form a single main trunk and remove competing leaders while the tree is young, in late winter or early spring after frost risk has passed. Avoid topping or heavy crown reduction; have a qualified arborist assess large, damaged or poorly attached limbs.

Spring

Plant after frost risk has passed and water the root ball thoroughly.

Late spring

Check young trees for competing leaders and make only small formative cuts.

Summer

Water establishing trees deeply during dry weather and check that mulch stays clear of the trunk.

Autumn

Remove unwanted seedlings, especially near natural areas where the tree can escape cultivation.

Late autumn

Move container plants into bright, frost-free shelter before cold nights arrive.

Winter

Check exposed trees after storms and arrange professional inspection of cracked or hanging branches.

• Use as a tall specimen in a large lawn where its divided foliage and flowers can be seen.
• Provide high shade above drought-tolerant planting in a spacious, warm-climate garden.
• Combine with Australian shrubs and grasses for a layered native planting within its native range.
• Use its upright young form as a temporary container foliage feature, with a plan for its rapid growth.

Garden styles it suits: Australian native, Architectural minimal, Tropical, Wildlife garden, Grand estate and Mediterranean.

Plant it with: Bottlebrush, Coastal rosemary, Mat rush, Kangaroo paw and Flax lily.

Wildlife: Nectar-rich flowers attract birds, including honeyeaters in Australia, as well as bees and other insects. Its value to wildlife does not outweigh its escape risk where it is invasive.

• Root rot in waterlogged soil: improve drainage before planting and avoid frequent watering of established trees.
• Yellow leaves on alkaline soil: check soil pH and drainage; choose a better-suited tree if the soil is strongly alkaline.
• Phosphorus injury from unsuitable fertiliser: stop high-phosphorus feeding and use only a suitable low-phosphorus product if needed.
• Storm damage or weak branch attachments: train the tree while young, avoid topping and have mature trees inspected by an arborist.

Leaves and flowers contain cyanogenic compounds and should not be eaten. Sap, foliage and especially wood dust can cause contact dermatitis in people, sometimes severe; wear gloves and avoid inhaling sawdust. Reliable species-specific poisoning information for cats, dogs and horses is limited, so prevent chewing or browsing and seek veterinary advice after ingestion.

Where it spreads. Invasive in Hawaii, South Africa and parts of East Africa, including Kenya, Tanzania and Uganda. It can seed into disturbed ground and natural vegetation; check local restrictions before planting.

• Fern tree (Filicium decipiens)

  Has pinnate leaves made of separate, glossy leaflets and small, inconspicuous flowers, rather than deeply divided leaves and large orange flower clusters.

• Bull oak (Allocasuarina luehmannii)

  Despite the shared 'oak' name, it has fine, jointed, needle-like branchlets and woody cone-like fruits, not fern-like leaves.

Grevillea honours Charles Francis Greville, robusta means robust or strong, and silky oak refers to the silky-looking grain of its timber.

Propagation. Sow fresh seed in a free-draining seed mix and keep warm and lightly moist. Semi-ripe cuttings are possible but generally less straightforward than seed.

Accepted botanical name

Grevillea robusta

Native range

New South Wales and Queensland.

Measured

Soil pH 5.3 to 7. Average height 1480 cm.

Sources: GBIF (CC BY 4.0) and Trefle (CC BY 4.0). The care and design notes are LandscapeGPT's own.