Grevillea, a genus of evergreen shrubs and trees native to Australia, is prized for its vibrant flowers and hardy foliage. However, yellowing leaves can signal underlying issues affecting the plant's health. Understanding the causes and remedies from the plant's perspective is essential for maintaining its vitality.
From a plant's viewpoint, nutrient imbalances disrupt metabolic processes. Grevilleas are sensitive to phosphorus and often suffer from iron or nitrogen deficiencies, leading to chlorosis (yellowing).
Iron Deficiency: Young leaves turn yellow with green veins, as iron is crucial for chlorophyll synthesis. Alkaline soils exacerbate this by reducing iron availability.
Nitrogen Deficiency: Older leaves yellow first, as nitrogen is mobile and reallocated to new growth. Poor soil fertility or excessive mulch can trigger this.
Fix: Use a low-phosphorus, iron-rich fertilizer. For nitrogen, apply organic compost or a balanced, slow-release formula suited to acidic soils.
Grevilleas evolved in well-drained soils, making them vulnerable to water stress.
Overwatering: Soggy roots suffocate, reducing oxygen uptake and causing yellow, wilted leaves. Root rot may follow.
Underwatering: Drought stress limits nutrient transport, leading to dry, yellow foliage, often starting at the tips.
Fix: Ensure soil drains freely. Water deeply but infrequently, allowing the topsoil to dry between sessions. Mulch to retain moisture without waterlogging.
Grevilleas thrive in slightly acidic to neutral soils (pH 5.5–6.5). Alkaline conditions lock up iron and manganese, inducing yellowing.
Fix: Test soil pH. Amend alkaline soils with sulfur or organic matter like peat moss. Avoid lime or ash, which raise pH.
Infestations or infections drain the plant's resources, manifesting as yellow foliage.
Root-Knot Nematodes: These microscopic worms damage roots, impairing nutrient absorption. Leaves yellow and growth stunts.
Fungal Pathogens: Phytophthora or other root fungi cause similar symptoms. Wet conditions favor their spread.
Fix: For nematodes, introduce beneficial microbes or solarize soil. For fungi, improve drainage and apply fungicides if severe.
Abiotic factors like extreme temperatures or poor light disrupt photosynthesis.
Frost Damage: Cold snaps kill tender foliage, turning it yellow or brown.
Excessive Shade: Insufficient light reduces chlorophyll production, causing pale leaves.
Fix: Plant in full sun and protect from frost with cloth covers. Prune damaged growth to redirect energy.
Newly planted or repotted Grevilleas may yellow as roots adjust. This is a temporary stress response.
Fix: Minimize root disturbance during planting. Water with a seaweed extract to reduce stress and encourage root growth.
Older leaves naturally yellow and drop as part of the plant's renewal cycle. This is normal if limited to lower foliage.
Fix: No action needed unless excessive yellowing occurs. Regularly prune dead leaves to maintain plant hygiene.
