How to grow blackwood
Overview
Blackwood (Acacia melanoxylon) is a large, long-lived Australian wattle grown for its dense foliage, upright form, and creamy-yellow flower clusters in late winter to spring. It provides excellent shade and habitat, and is valued for its durable timber. This tree thrives in part shade when young and tolerates a range of well-drained soils.
Soil Preparation
Blackwood prefers well-drained loam or sandy loam with a pH of 5.5–7.0. It tolerates clay if drainage is improved with gypsum or organic matter. Before planting, incorporate a handful of slow-release native fertiliser and ensure the site does not become waterlogged.
Growing Guide
Plant blackwood in a position with bright, filtered light and protection from harsh afternoon sun, especially when young. Space trees at least 5–8 m apart to allow for their mature canopy spread. Prepare a deep planting hole and backfill with native soil; avoid heavy clay or waterlogged sites. Water deeply every 7–10 days during the first two summers to establish a robust root system. Mulch generously with organic matter to retain moisture and suppress weeds.
Sowing
Scarify seeds by pouring near-boiling water over them and soaking for 12–24 hours before sowing. Sow 5 mm deep in seed trays or direct at a spacing of 10–15 cm apart. Germination takes 10–21 days at 20–25°C. Transplant seedlings when 10–15 cm tall.
Care
Feed with a low-phosphorus native fertiliser in early spring (e.g., 50 g per m² of canopy). Prune to shape when young, removing lower branches to encourage a clear trunk and strong central leader. Once established, no heavy pruning is needed; remove only dead or crossing wood.
Companion Planting
Blackwood makes an excellent shade or habitat tree for a mixed native garden. Underplant with shade-tolerant species such as Lomandra longifolia, Dianella revoluta, or Correa alba. Avoid planting near shallow-rooted vegetables or annuals that cannot compete with its dense root zone.
Common Pests & Risks
Borers (especially longicorn beetles) can attack stressed or damaged trunks; maintain tree health and avoid wounding. Scale insects may appear on branches; treat with horticultural oil in early spring if infestations are heavy. Leaf-eating caterpillars are occasional but rarely require control.
Troubleshooting
Yellow leaves: often caused by waterlogging or poor drainage; improve soil aeration. Stunted growth: may indicate phosphorus toxicity from high-P fertilisers; switch to native-specific low-P feed. Leaf drop: common after transplant shock; reduce sun exposure and keep soil consistently moist. Borer damage: look for small holes or sawdust on trunk; maintain tree vigour and avoid wounding. Scale: sticky honeydew on leaves; treat with horticultural oil in early spring.