Citronella plants (Pelargonium citrosum) are known for their lemony fragrance and mosquito-repelling qualities. While generally easy to care for, yellowing leaves can be concerning. This symptom indicates an underlying issue that needs attention to restore the plant’s health.
Understanding Yellowing Leaves
The green color of plant leaves comes from chlorophyll, a pigment essential for photosynthesis, the process by which plants convert sunlight, carbon dioxide, and water into energy. When leaves turn yellow, a process called chlorosis, it signifies insufficient chlorophyll production. This allows underlying yellow pigments, like carotenoids, to become visible, indicating the plant isn’t receiving necessary elements or conditions to produce energy.
Common Reasons for Yellowing
Watering Imbalance
Both overwatering and underwatering can cause yellowing leaves. Overwatering leads to soggy soil and root rot, causing yellowed, wilting leaves, often starting from the bottom. The soil feels excessively moist, and roots may become mushy. Citronella plants are susceptible to root rot if soil remains waterlogged.
Underwatering causes leaves to yellow, dry, and become crispy. The soil will be noticeably dry and hard. Leaves may droop or wilt before yellowing. Prolonged dry conditions stress the plant.
Inadequate Light
Citronella plants need ample sunlight, ideally at least six hours of direct sun daily. Insufficient light causes yellowing and a leggy appearance as the plant stretches for light.
Conversely, too much intense direct sunlight, especially in hot climates, can also cause yellowing, leading to scorched or bleached leaves. Some afternoon shade is beneficial in very hot regions.
Nutrient Deficiencies
A lack of essential soil nutrients can cause yellowing. Nitrogen deficiency often causes older leaves to turn uniformly yellow as the plant reallocates this mobile nutrient to newer growth. Iron deficiency, an immobile nutrient, typically results in interveinal chlorosis: yellowing between the veins of newer leaves while the veins remain green. Iron is crucial for chlorophyll production. Other nutrient imbalances or incorrect soil pH can also hinder nutrient absorption.
Temperature Stress
Citronella plants are sensitive to extreme temperatures. Sudden cold or frost can cause leaves to yellow and die back. They are hardy in USDA Zones 9-11 and often grown as annuals or brought indoors in colder regions.
Excessive heat also stresses the plant, leading to wilting and yellowing. While they appreciate warmth, scorching heat should be avoided.
Pests and Diseases
Certain pest infestations can cause yellowing. Pests like spider mites and aphids feed on plant sap, causing speckled yellowing, curled leaves, or leaf drop. Spider mites attack the underside of leaves, while aphids can cause damage if infestations become large. Root rot, often from overwatering, causes yellowing and wilting as damaged roots cannot absorb water and nutrients. Fungal issues can also lead to yellow spots or widespread chlorosis, especially in poorly drained conditions.
Natural Aging
It is normal for older, lower leaves to yellow and fall off. As the plant prioritizes new foliage, it naturally sheds less productive older leaves. This shedding is typically limited to a few lower leaves and does not indicate an overall health problem.
Restoring Health and Preventing Future Issues
To restore health, first address watering imbalances. For overwatered plants, allow the top inch of soil to dry completely before rewatering. Ensure pots have drainage holes and avoid standing water. For underwatered plants, provide thorough watering until it drains from the pot bottom.
Adjust light exposure. If light is insufficient, move the plant to a location with at least six hours of bright, indirect sunlight daily. If yellowing is from too much direct sun, provide afternoon shade.
Correct nutrient deficiencies with a balanced, water-soluble fertilizer applied regularly during the growing season. For suspected iron deficiency, a foliar application of ferrous sulfate can help. Ensure appropriate soil pH for optimal nutrient uptake.
Protect the plant from temperature extremes. In colder climates, bring potted citronella plants indoors before the first frost. During heatwaves, ensure shade and adequate moisture.
Regularly inspect for pests. If spider mites or aphids are present, manage them with insecticidal soap or horticultural oils. Pruning affected leaves helps contain spread. For root rot, improve drainage and allow soil to dry between waterings.
Preventative care is key. Use a well-draining potting mix, ideally with organic matter, for proper aeration and moisture retention. Consistently monitor soil moisture by checking the top inch. Regular pruning of spent flowers encourages new growth and maintains plant shape.