As a genus of plants native to Australia, we Grevilleas find the climates of the United States, particularly in USDA zones 9-11, quite agreeable. Our intricate, spider-like flowers and hardy foliage make us popular choices in ornamental landscaping. However, our resilience does not make us immune to all threats. While we are largely trouble-free, certain pests and environmental conditions can compromise our health and vigor. Our main vulnerabilities stem not from complex diseases but from simple cultural missteps and a handful of persistent insect invaders.
Our greatest threat is not an insect, but a condition born from excessive kindness: overwatering. We are exceptionally drought-tolerant shrubs and possess a strong aversion to poorly drained, heavy, or constantly wet soil. When our roots are suffocated in waterlogged conditions, opportunistic soil-borne fungi from the Phytophthora and Pythium genera attack. These pathogens cause root rot, which manifests in you, the caretaker, as a sudden wilting of our foliage, a general decline in health, and often our eventual collapse. The roots will appear brown, black, and mushy instead of firm and white. Prevention is the only true cure, requiring well-draining soil and infrequent, deep watering.
Several insect pests seek to feed on our sap, which is the very lifeblood that carries nutrients throughout our structures. The most common are aphids. These small, soft-bodied insects cluster on our tender new growth and flower buds, sucking sap and excreting a sticky substance called honeydew. This honeydew then attracts sooty mold, a black fungus that further inhibits our photosynthesis by coating our leaves. Scale insects are another concern; they appear as small, immobile bumps on our stems and leaves, also feeding on sap and weakening us over time. While rarely fatal, heavy infestations can cause leaf yellowing and drop, stunting our growth.
Beyond sap-suckers, we face pests that consume our physical tissue. Caterpillars of various moths and butterflies may find our foliage palatable, chewing irregular holes in our leaves. While unsightly, their damage is usually minor and we can often outgrow it. A more serious, though less common, threat comes from borers. These are larvae of certain beetles or moths that tunnel into our woody stems and branches. This activity disrupts our vascular system, causing dieback of entire branches and creating entry points for secondary fungal infections. An affected branch will often show sudden wilting and can break off easily.
Many issues perceived as disease are actually physiological responses to environmental stress. A common sight is leaf scorch, where our leaf tips and margins turn brown and crispy. This is typically a reaction to high winds, reflected heat from pavement or walls, or a lack of water during extreme heat. Similarly, chlorosis, a yellowing of the leaves while the veins remain green, is frequently a sign of iron deficiency. This often occurs in alkaline soils where the pH is too high for us to properly uptake iron from the soil, not a pathogen attack. Amending the soil or using chelated iron supplements can correct this.
