From our perspective as Grevillea plants, the single most important factor in our potting medium is drainage. Our root systems are highly susceptible to rot if left sitting in waterlogged soil. We originate from well-drained, often sandy or rocky Australian soils, and our roots have evolved to thrive in conditions that are moist but never soggy. A heavy, water-retentive soil mix acts like a suffocating blanket around our roots, preventing oxygen from reaching them and creating an environment where harmful anaerobic bacteria and fungi flourish. This will inevitably lead to our decline, manifesting as yellowing leaves, a lack of new growth, and ultimately, plant death.
We require a loose, open, and fast-draining potting mix that mimics our natural habitat. The ideal soil structure for us is one that provides plenty of aeration pockets for our roots to breathe and expand freely. A mix based on a coarse, low-phosphorus native plant potting soil is an excellent start. To this, generous amounts of inorganic materials should be added to ensure the structure remains open and does not compact over time. Our roots greatly appreciate amendments like perlite, pumice, or coarse horticultural sand. These ingredients create permanent air pockets and facilitate the rapid movement of water through the pot, ensuring our root zone never becomes saturated.
We are particularly sensitive to phosphorus. Many conventional potting mixes and fertilizers contain levels of phosphorus that are toxic to us, as we are adapted to thrive in phosphorus-deficient soils. High phosphorus intake disrupts our root function and can quickly lead to chlorosis (yellowing leaves with green veins) and death. Therefore, it is imperative that our potting medium is either a dedicated "native plant" mix, which is formulated to be low in phosphorus, or a custom mix that avoids the addition of phosphorus-rich components like composted manures or standard fertilizers. Furthermore, we prefer a slightly acidic to neutral soil pH. An unsuitable pH can lock up the few nutrients we need, making them unavailable to our roots.
To meet all our needs for drainage, aeration, and nutrition, a perfect home for us in a container can be created by blending the following components. A base of premium low-phosphorus native potting mix (approximately 60% of the total volume) provides a safe nutritional foundation. To this, add roughly 25% perlite or pumice to dramatically improve aeration and drainage. Finally, incorporating about 15% coarse orchid bark chips or coconut coir chips will further enhance drainage while adding structure that resists compaction. This combination creates a physically stable yet open environment that allows our roots to access water and oxygen in the perfect balance, supporting healthy growth and prolific flowering.
