How to grow old man banksia
Overview
Old man banksia (Banksia serrata) is a large evergreen tree native to eastern Australia, celebrated for its rough bark, serrated leaves, and tall lemon-green flower spikes. It provides long-term structure and shade in coastal and bushland gardens. Growers need excellent drainage and full sun for success.
Soil Preparation
Prefers sandy, well-drained acidic soil with a pH of 5.5–6.5. In clay soils, plant in raised beds or mounds to improve drainage. Mix in coarse sand or gravel if needed; avoid adding lime or phosphorus-rich amendments.
Growing Guide
Plant in full sun in sandy, well-drained soil. Space trees at least 4–5 metres apart to accommodate their eventual size. Dig a hole twice the width of the root ball, backfill without disturbing roots, and water in well. Avoid heavy clay or sites prone to waterlogging. Best planted in autumn or spring; young plants benefit from staking in windy locations. Once established, water deeply only during prolonged dry spells.
Sowing
Seeds require scarification – nick the seed coat or soak in hot water for 24 hours before sowing. Sow 5 mm deep in seed-raising mix in spring. Keep moist but not wet; germination occurs in 2–6 weeks. Transplant seedlings into individual pots when 10–15 cm tall.
Care
Apply a low-phosphorus native fertiliser in early spring each year. Prune only to remove dead, damaged, or crossing branches; never cut into old wood. Mulch with organic material such as bark chips around the root zone, keeping mulch away from the trunk to prevent collar rot.
Companion Planting
Ideal companions include other coastal natives such as Callistemon, Grevillea, and Lomandra. Low-growing grasses like Poa labillardieri help retain moisture and suppress weeds. Avoid planting near thirsty border plants that require constant irrigation, as excess moisture can harm Banksia roots.
Common Pests & Risks
Root rot (Phytophthora) is the biggest risk in poorly drained soil; ensure free drainage and avoid overwatering. Scale insects and sap-sucking bugs may appear on stems and leaves – treat with horticultural oil or introduce ladybirds. Leaf spot fungi can be minimised by improving air circulation and not overhead watering.
Troubleshooting
Yellowing leaves: usually iron chlorosis in alkaline soil – apply iron chelate. Brown leaf edges: wind scalding or heat stress – provide shelter or mulch. Stunted growth: phosphorus toxicity – stop high‑P fertilisers and use native blend. Mushroom growth at base: root rot – dig up and improve drainage. Poor flowering: too much shade or nitrogen – move to full sun and reduce feeding.