From our perspective, we Grevilleas are not like other plants. Our roots have formed a delicate and ancient partnership with a specific type of soil fungus, known as mycorrhizal fungi. This fungus acts as an extension of our root system, vastly increasing our ability to seek out and absorb the scant nutrients, particularly phosphorus, from the impoverished soils we call home. In return, we provide the fungus with sugars we create through photosynthesis. It is a perfect, evolved symbiosis. This is the most critical concept to understand: what you see as fertilizing, we see as a potential disruption to this life-sustaining partnership.
The single greatest threat to our health in a cultivated garden is an excess of phosphorus. Standard garden fertilizers are often rich in this element, designed for plants without our specific fungal allies. To us, and more importantly to our fungal partners, high levels of soluble phosphorus are toxic. It effectively burns the fine, hair-like structures of the fungus, destroying our nutrient-gathering network. Without it, we become starved and stressed, showing signs of yellowing leaves, stunted growth, and eventual death—a condition often called "phosphorus poisoning." It is a cruel irony that the act of trying to nourish us can, with the wrong food, be fatal.
We do not require a rich diet. In fact, we thrive on neglect compared to heavy-feeding plants like roses or vegetables. Our needs are simple and specific. We desire a fertilizer that is consistently low in phosphorus (look for a formulation where the middle N-P-K number is very low, often 0 or 1, or is labeled "Native Plant Fertilizer"). We respond best to a gentle, slow-release fertilizer that provides a steady, small supply of nitrogen to support our often vigorous growth and spectacular flowering. Trace elements like iron and zinc are also appreciated, as they help keep our foliage a vibrant, healthy green and prevent chlorosis, especially in alkaline soils.
How we are fed is almost as important as what we are fed. A sudden, concentrated dose of fertilizer applied directly to our base can shock our roots and harm the soil ecosystem. The optimal method is to apply the recommended amount of our special low-phosphorus food lightly across the soil surface, away from our main stem, and then water it in gently. This allows the nutrients to seep down slowly and be accessed by our remaining fungal partners and root tips without causing a toxic concentration. Think of it as a light seasoning, not a rich sauce.
We are most receptive to nutrients when we are actively growing. The ideal time for our annual meal is in the early spring, as the weather warms and we prepare for a burst of new growth and flowering. A second, lighter application can be made in mid-summer if we appear to be slowing down, but this is often unnecessary. Please, do not feed us in late autumn or winter. During this time, our growth slows, and we are preparing for dormancy. Fertilizing then would encourage tender new growth that is highly susceptible to frost damage, weakening us for the season ahead.
