What's Wrong with My Rosemary?

Rosemary's fine, woody roots evolved in fast-draining rocky soil where they dry out completely between rains. In heavy soil or containers without drainage, those roots suffocate and rot quickly. The plant often looks normal right up until the roots fail completely, then collapses with little warning.

Indoor rosemary is caught between two contradictory stresses. Dry heated air pulls moisture from the needle-like leaves faster than the roots can supply it, but watering more often keeps the soil wet long enough to rot the roots. The plant browns from both ends at once, which is why rosemary kept indoors fails far more often than it does outdoors.

Rosemary needs full sun outdoors and the equivalent of a very bright window indoors. In dim conditions, the stems grow thin and pale, then begin to die back from the tips inward. The plant cannot photosynthesize enough to maintain all its stems and sheds the weakest ones first.

Powdery mildew spreads as white or gray powdery patches across rosemary's needle-like leaves, especially on the upper surface. The disease thrives when warm days and cool nights combine with poor airflow around the dense, upright stems. Rosemary's tight branching habit traps humid air inside the plant, giving the fungal spores a surface to colonize.

Rosemary is a woody subshrub that converts its lower stems to hard, non-productive wood each year. Without regular pruning back into the green growth, the blooming and leafy zone retreats higher up the plant. After two or three years without pruning, the base becomes a mass of bare brown wood with sparse green growth only at the tips.

Soft green or gray aphids cluster on rosemary's tender new shoot tips and emerging flower buds, sucking sap and stunting new growth. Rosemary's aromatic oils repel many insects but do not deter aphids, which target the fresh non-aromatic tissue at stem tips before oils concentrate in the mature needles.

Spider mites thrive in hot, dry conditions and are a common problem on indoor rosemary kept in low humidity. They feed on the needle-like leaves, leaving stippled or bronzed patches, and produce fine webbing between stems. Rosemary's densely packed needles give mites sheltered spots to build up before webbing becomes visible.

Whiteflies are small white-winged insects that lift off in a cloud when the plant is disturbed. They feed on the undersides of rosemary's needles, draining sap and leaving sticky honeydew that can lead to sooty mold. They are more common on indoor or greenhouse-grown rosemary where natural predators are absent.