How to grow silky oak
Overview
Silky oak (Grevillea robusta) is a large, fast-growing tree from eastern Australia, grown for its fern-like foliage and spectacular orange flower spikes in late spring. It provides valuable shade and habitat. Key fact: it is frost-sensitive and requires well-drained soil to avoid root rot.
Soil Preparation
Best in slightly acidic to neutral soil (pH 5.5–7.0). Prefers deep, free-draining loam. In clay soils, mound the planting site to improve drainage.
Growing Guide
Plant in full sun with well-drained soil. Space at least 8 m from buildings to allow for its large canopy. Dig a hole twice the width of the root ball, backfill with native soil and organic compost. Water deeply weekly for the first year. Avoid high phosphorus fertilisers and heavy clay soils.
Sowing
Sow seeds 1–2 cm deep in a seed tray of coarse sand and peat moss. Keep at 20–25°C; germination occurs in 4–6 weeks. Transplant seedlings into 10 cm pots once they have two sets of true leaves.
Care
Apply a low-phosphorus native fertiliser each spring. Prune annually to shape the canopy and remove dead wood. Keep a thick layer of organic mulch over the root zone, but maintain a gap around the trunk.
Companion Planting
Underplant with native groundcovers such as Dichondra repens or Kennedia prostrata. Good canopy companions include Banksia integrifolia and Callistemon viminalis. Avoid planting near shallow-rooted shrubs that need frequent liquid feeding.
Common Pests & Risks
Leaf miner damage appears as serpentine trails on leaves; prune heavily infested foliage and use neem oil if severe. Root rot (Phytophthora) occurs in waterlogged soil; improve drainage and reduce watering. Scale insects may attack young stems; treat with horticultural oil.
Troubleshooting
Yellow leaves: often waterlogging or phosphorus toxicity; stop high-P fertiliser, improve drainage. Drooping foliage: usually a sign of overwatering; allow soil to dry. Few flowers: may be due to too much shade or nitrogen; prune for light and reduce N fertiliser. Leaf miner trails: cosmetic; use neem if severe. Slow growth: check for compacted soil or insufficient water.