The herb known as Rue, or Ruta graveolens, is a hardy, aromatic perennial native to the Mediterranean region, valued for its ornamental blue-green foliage. When this plant begins to turn yellow, it is exhibiting chlorosis, meaning it is not producing enough chlorophyll, the pigment responsible for its green color. Chlorosis is a visible symptom of underlying environmental stress or cultural problems that inhibit the plant’s ability to create this pigment. Understanding the specific needs of Rue, which thrives on neglect and poor conditions, helps diagnose why its vibrant color is fading.
Overwatering and Poor Drainage
The most frequent cause of yellowing in a Rue plant relates directly to overwatering and inadequate soil drainage. Ruta graveolens is highly drought-tolerant and requires sharply draining soil to mimic its native, rocky habitat. When the root zone remains saturated, water displaces the oxygen pockets that roots need to respire and absorb nutrients.
This lack of oxygen suffocates the root system, quickly leading to root rot, where roots turn soft, brown, and mushy. Damaged roots are unable to efficiently transport water and nutrients, causing the foliage to display stress signs like wilting and yellowing. This yellowing often starts with the older, lower leaves. To check for this, insert your finger two inches into the soil; if it feels wet, you are watering too often.
Correcting this requires attention to the soil structure and watering routine. If the Rue is in a container, ensure the pot has ample drainage holes and consider repotting it into a gritty, fast-draining mix amended with sand or perlite. For in-ground plants, reduce the watering frequency and allow the soil to dry completely between waterings. Rue tolerates dry conditions far better than consistently soggy soil.
Insufficient Sunlight or Extreme Heat
Rue thrives in bright conditions, requiring a minimum of six hours of direct sunlight daily to support chlorophyll production. Inadequate light exposure halts photosynthesis, resulting in a general pale or yellowing appearance across the plant. If the plant is located in a shaded area, the yellowing may be uniform and accompanied by spindly, weak growth.
Conversely, exposure to extreme, intense afternoon sun, especially when reflected off walls or concrete, can also induce stress. In temperatures exceeding 90°F, the leaves may show signs of scorching or rapid nutrient depletion, leading to yellowing edges or tips. This stress is often more pronounced in container-grown plants where the root system heats up quickly.
For plants receiving less than optimal light, prune nearby obstacles or relocate container plants to a spot with full, direct morning sun. If the issue is extreme heat, providing partial afternoon shade during the hottest part of the day can help mitigate heat stress and prevent leaf scorch.
Nutrient Deficiencies and Soil pH
The Rue plant is adapted to poor soils and typically requires no supplemental fertilizer. However, yellowing can signal a nutrient deficiency, usually caused by factors that prevent the plant from taking up existing nutrients, rather than a lack of them in the soil. Rue prefers a neutral to slightly alkaline soil pH, ideally between 6.0 and 7.0.
When the soil pH is too high or too low, certain micronutrients become “locked out” and inaccessible to the roots, even if they are present in the soil. For instance, high soil pH prevents the uptake of iron and manganese, leading to interveinal chlorosis. This specific pattern of yellowing shows the tissue between the leaf veins turning yellow while the veins themselves remain green.
A general, overall yellowing, particularly starting on the older, lower leaves, indicates a nitrogen deficiency. Nitrogen is a mobile nutrient, meaning the plant moves it from old leaves to new growth, causing the older leaves to yellow and drop first. Before applying any fertilizer, it is important to conduct a soil test to confirm the pH and nutrient levels, as fertilizing a plant that prefers poor soil can lead to further imbalances.
Identifying Pest and Disease Infestations
Due to the pungent oils in its leaves, Rue resists most common garden pests, making infestations a less common cause of yellowing than cultural issues. However, a few sap-sucking insects may still target the plant, causing localized damage that mimics chlorosis.
Spider mites cause a speckled or stippled yellowing on the leaves as they extract chlorophyll, often accompanied by fine webbing on the undersides of the foliage. Aphids may cluster on new, tender growth, causing distortion and yellowing as they feed. Fungal diseases like powdery mildew may also appear as a white, powdery growth that eventually causes the infected leaves to yellow.
A thorough inspection of the leaves and stems is necessary to confirm the presence of pests. If an infestation is detected, a simple rinse with a strong stream of water can dislodge many insects. For persistent issues, a horticultural oil or insecticidal soap can be applied to the affected areas.