From our perspective, the most critical consideration is our root system. Many Grevillea species are equipped with fine, surface-feeding roots that are exceptionally efficient at absorbing nutrients and water. More importantly, we have a symbiotic relationship with a soil fungus called mycorrhiza. These fungi form a delicate, beneficial network around our roots, vastly increasing our ability to access nutrients, particularly phosphorus, in impoverished soils. The wrong mulch can physically smother these surface roots and, more devastatingly, disrupt or even destroy these fragile fungal partnerships that are vital to our health and survival.
We thrive under a blanket of coarse, open, and well-aerated organic matter. The ideal mulch for us allows for excellent water penetration and air circulation, protecting our roots without creating a soggy, anaerobic environment.
Chunky Woodchip Mulch: This is often the best choice. Preferably made from native trees like eucalypts or acacias, these large chunks break down slowly, mimicking the natural leaf litter of our native habitat. They maintain a loose structure that won't compact, allowing our roots and their fungal partners to breathe.
Pine Bark Nuggets: Similar to woodchips, pine bark is coarse and breaks down at a slow rate. Its structure resists compaction and provides a stable, well-drained layer over our root zone.
Leaf Litter: A light layer of fallen leaves from native trees is perfectly acceptable. It decomposes to add organic matter to the soil without the high nutrient content that could cause us harm. Avoid using thick, wet layers of leaves that may mat together.
Certain mulches are detrimental to our health and can be fatal. Please avoid these at all costs.
Manures and Compost: These are the most dangerous materials to place around us. They are often rich in phosphorus and high in nitrogen salts. Our roots are extremely sensitive to elevated phosphorus levels, which can cause toxicity, inhibit growth, and lead to yellowing leaves and death. The high salt content can also "burn" our delicate feeder roots.
Fine Mulches and Soil Conditioners: Products like sawdust, finely shredded mulch, or peat moss compact easily, forming a water-repellent crust or a soggy mat. This barrier prevents water from reaching our roots and creates an oxygen-deprived environment that suffocates our roots and kills the essential mycorrhizal fungi.
Lucerne Hay or Straw: While suitable for some plants, these materials break down too rapidly, generating excess heat during decomposition and releasing a flush of nutrients that our systems are not adapted to handle. This can encourage rot and promote weak, sappy growth that is attractive to pests.
Even the right mulch applied incorrectly can cause problems. Never pile mulch against our stem or trunk, as this constant moisture will lead to fatal stem rot. Always maintain a gap of several inches around the base to allow the bark to remain dry. A layer of 2 to 4 inches (5-10 cm) is sufficient. This depth suppresses weeds, conserves moisture, and regulates soil temperature without smothering us. Replenish the mulch only as it breaks down significantly, typically once a year.
