From our perspective as Grevillea plants, the single most critical factor for our survival and prosperity in your US garden is soil pH. We have a fundamental, non-negotiable requirement for acidic soil conditions. Our root systems are highly specialized and have evolved to thrive in a pH range between 5.5 and 6.5. This acidity is crucial because it dictates the availability of essential nutrients in the soil. In overly alkaline (sweet) soils, key micronutrients like iron, manganese, and phosphorus become chemically locked away and inaccessible to our roots. This quickly leads to a condition called chlorosis, where our leaves turn a sickly yellow while the veins remain green, severely stunting our growth and preventing us from producing our spectacular blooms.
Beyond pH, the physical structure of the soil is paramount. We abhor "wet feet." Our roots are exceptionally susceptible to root rot fungi (like Phytophthora) that thrive in waterlogged, heavy clay soils. Therefore, our ideal soil type is sharply draining, sandy, and loamy with a gritty, open structure. Think of the well-aerated, nutrient-poor soils of our native Australian landscapes. A perfect mix for us would be a native sandy soil heavily amended with gravel, pumice, coarse sand, or perlite. Incorporating organic matter like composted pine bark is also beneficial as it helps maintain acidity while improving structure. The goal is to create an environment where water flows through freely, providing moisture to our roots without ever allowing them to sit in saturation.
Many common gardening practices in the US are detrimental to us. Please avoid amending our planting area with products that contain lime or phosphorous-rich fertilizers. Lime is used to raise soil pH (make it more alkaline), which is fatal to our long-term health. Similarly, high-phosphorus fertilizers (often marked by a high middle number, e.g., 10-50-10) are toxic to our specialized proteoid roots, which are designed to scavenge nutrients in low-phosphorus environments. To create and maintain our ideal acidic conditions, we appreciate being mulched with acidic organic matter like pine needles or oak leaves. If you must fertilize, use a product specifically designed for acid-loving plants like azaleas or camellias, and ensure it is low in phosphorus.
Our success varies across the different US climate zones. In the arid Southwest and similar Mediterranean climates in California, the naturally drier conditions and often more acidic, sandy soils are much more to our liking. Here, the primary task is ensuring excellent drainage. In the humid Southeast, the challenge is greater due to heavier clay soils and higher rainfall. Here, you must plant us in raised mounds or entirely in raised beds filled with a custom, fast-draining acidic soil mix to overcome the native soil conditions. In cooler northern regions, our main concern is cold, wet winter soil, making exceptional drainage and a sunny, sheltered location even more critical for our survival.
