Overview
Summary
Grevillea robusta, known as silky oak or silver oak, is a large tree species in the Proteaceae family. Native to eastern coastal Australia, it's widely cultivated globally in warm temperate, subtropical, and tropical highland regions. Valued for timber, ornamental qualities, and agroforestry, it's also considered invasive in some areas like Hawaii.
Key Features
Grevillea robusta grows rapidly, reaching 20-40 metres tall with a 6-7.6 metre spread. It's an evergreen tree with a single main trunk covered in dark grey, furrowed bark. The tree's distinctive fern-like foliage and golden-orange flowers make it visually striking and attractive to wildlife.
Natural Habitat
In its native range along Australia's eastern coast, Grevillea robusta thrives in subtropical rainforests and wet sclerophyll forests. It's commonly found in New South Wales and Queensland, adapting well to various soil types. The tree prefers full sun exposure and well-drained soils, demonstrating drought tolerance once established.
Growing Guide
Cultivating Grevillea robusta successfully requires attention to specific environmental conditions. Here's a comprehensive guide to help you grow and maintain this striking tree.
Requirements
Grevillea robusta thrives in full sun and well-draining soils with a pH of 4.5 to 7.5. It prefers sandy loam, loam, or clay loam textures. The tree tolerates temperatures down to -8°C and moderate winter frosts. While drought-resistant, it benefits from regular watering during dry spells. Avoid planting in areas with persistent strong winds, as this can damage branches.
Planting Tips
Choose a sunny location with well-draining soil for your Grevillea robusta. Dig a hole twice the size of the root ball and mix compost into the backfill soil. Plant at the same depth as it was in its container. Water thoroughly after planting and mulch around the base, keeping mulch away from the trunk to prevent rot.
Maintenance
Prune Grevillea robusta annually to maintain shape and remove dead or damaged branches. Water deeply but infrequently to encourage deep root growth. Apply a slow-release, low-phosphorus fertiliser in spring. Monitor for pests like scale insects and treat if necessary. Remove any suckers that appear at the base of the tree to maintain its form.
Uses & Benefits
Grevillea robusta offers numerous benefits and uses in various applications. This versatile tree serves multiple purposes, from timber production to ornamental landscaping.
Garden Design
Grevillea robusta's striking appearance makes it an excellent focal point in garden designs. Its fern-like foliage creates a soft, elegant backdrop, while the golden-orange flowers add vibrant colour. Use it as a standalone specimen tree or incorporate it into mixed borders for height and texture. Its fast growth rate quickly establishes privacy screens or windbreaks.
Wildlife Benefits
The tree's nectar-rich flowers attract a diverse array of wildlife. Bees, butterflies, and nectar-feeding birds flock to Grevillea robusta, enhancing garden biodiversity. Its dense foliage provides shelter and nesting sites for various bird species. In its native range, it's a crucial food source for honeyeaters and other nectar-feeding animals.
Special Features
Grevillea robusta boasts unique characteristics that set it apart. Its timber resists wood rot, making it valuable for external joinery and furniture. In agroforestry, it serves as a shade tree for crops like coffee and tea. The tree's deep root system helps control soil erosion and improves soil fertility, making it beneficial for land restoration projects.
Problems & Solutions
Grevillea robusta, while resilient, can face certain challenges in cultivation. Understanding these issues and their solutions is crucial for maintaining a healthy tree.
Common Issues
Grevillea robusta is susceptible to fungal infections, particularly in poorly draining soil. Root rot, caused by pathogens like Cercospora and Phyllosticta, can be fatal. Over-fertilisation, especially with high-phosphorus fertilisers, leads to toxicity, causing burnt and yellowing leaves. Poor drainage and overwatering exacerbate these problems, resulting in wilting and stunted growth.
Care Tips
Plant Grevillea robusta in well-draining soil and avoid overwatering to prevent root rot. Use slow-release, low-phosphorus fertilisers specifically formulated for Australian natives. Prune regularly to maintain shape and remove any infected plant material. Ensure adequate air circulation around the tree to reduce fungal growth. Monitor for signs of stress, such as leaf discolouration or wilting.
Seasonal Notes
In spring, apply a light layer of mulch, keeping it away from the trunk to improve moisture retention. During summer, increase watering frequency but avoid waterlogging. In autumn, reduce watering as the tree enters dormancy. Protect young trees from frost in winter by covering them with horticultural fleece. Prune in late winter or early spring before new growth begins.
Additional Information
Grevillea robusta, commonly known as silky oak, offers additional aspects worth exploring. Here's more information about its varieties, where to purchase it, and resources for further learning.
Varieties
Grevillea robusta has no recognized subspecies or varieties. It's a distinct species within the Grevillea genus, with no recorded hybrids. The tree's characteristics remain consistent across its native range and in introduced regions, maintaining its iconic fern-like foliage and golden-orange flowers.
Where to Buy
You can purchase Grevillea robusta from various sources:
- Local nurseries specialising in native Australian plants
- Online plant retailers with a wide selection of tree species
- Botanical gardens with plant sales programmes
- Specialty seed suppliers for those interested in growing from seed
Check availability and shipping restrictions, as some regions may have limitations on importing this species.
- Australian Native Plants Society (Australia): Offers comprehensive guides on native species
- Royal Horticultural Society: Provides detailed cultivation advice
- Local botanical gardens: Often have educational materials and expert staff
- University extension services: Offer region-specific growing information
- Scientific journals: For in-depth studies on the species' ecology and uses